UNIVERSITY OF NIGERIA, NSUKKA

DEPARTENT OF METALLURGICAL AND MATERIALS

FACULTY OF ENGINEERING

TOPIC:HEAT TREATMENT OF TOOL STEELS

A TERM PAPER

WRITTEN IN PARTIAL FULFILLMENT FOR THE COURSE: MME 521

(TOOL STEELS: METALLURGY, MANUFCTURE AND APPLICATIONS)

BY EZEONU LOTANNA L.

REG. NO. 2010/173196

LECTURER: ENGR. P.O. OFFOR

FEBRUARY, 2015

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TITLE PAGE

HEAT TREATMENT OF TOOL STEELS

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DEDICATION

I dedicate this work to the Almighty God.

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PREFACE

This term paper is succently divided into four chapters. The Chapter one is

an introduction to tool steels and their heat treatment. The chapter two gave some

applications of tool steels. The Chapter three handled heat treatments. The chapter

four focused heat treatments of tool steels.

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ACKNOWLEDGEMENT

I express my utmost gratitude to Almighty God for his divine enablement to

write this term paper.

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TABLE OF CONTENTS

Title page - - - - - - - - - - i

Dedication - - - - - - - - - - ii

Preface - - - - - - - - - - iii

Acknowledgment - - - - - - - - - iv

Table of contents - - - - - - - - - v

Chapter One 1.0 Introduction - - - - - - - - - 11.1 What are tool steel? - - - - - - - - 11.2 Categories of tool steel - - - - - - - 1

1.2.1 Water-hardening group - - - - - - - 2

1.2.2 Cold-work group - - - - - - - - 3

1.2.3 Oil-hardening - - - - - - - - 3

1.2.4 Air-hardening - - - - - - - - - 3

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1.2.5 High carbon-chromium, d-type - - - - - - 4

1.2.6 Shock-resisting group - - - - - - - 5

1.2.7 High speed group - - - - - - - - 5

1.2.8 Hot-working group - - - - - - - - 5

1.2.9 Special purpose group - - - - - - - 5

Chapter Two2.0 Applications of tool steels - - - - - - - 7

2.1 A typical heat treatment of a screw driver blade as a case

study in a brazing hearth - - - - - - - - 7

2.2 Pictures of some tool steels - - - - - - - 10

Chapter Three

3.0 Heat treatment - - - - - - - - - 11

3.1 Heat treatment of steels - - - - - - - 11

3.1.1 Annealing - - - - - - - - - 13

3.1.2 Quenching - - - - - - - - - 13

3.1.3 Normalising - - - - - - - - 13

3.1.4 spheroidising - - - - - - - - 13

3.1.5 stress relieving - - - - - - - - 13

3.2 Tool steel analysis and characteristics - - - - - 14

3.3 Heat-treat considerations - - - - - - - 14

Chapter Four

4.0 Heat treatment of tool steels- - - - - - - 15

4.1 Classification and nominal compositions of selected tool steels - 28

4.2 Normalizing and annealing temperatures of tool steels - - 30

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4.3 Heat treatment of specific classes of tool steels - - - 64

4.3.1 Water-hardening tool steels - - - - - - 64

4.3.2 Shock-resisting tool steels - - - - - - 70

4.3.3 Oil-hardening cold-work tool steels - - - - - 77

4.3.4 Hot-work tool steels: Medium-alloy air-hardening, and

high-carbon high-chromium, cold-work tool steels - - - 81

4.4 Recommended heat-treating procedures based on steel

group and type - - - - - - - - - 108

4.4.1 Hardening of specific machine tools - - - - - 122

4.4.2 Low-alloy special-purpose tool steels - - - - 127

4.4.3 Carbon-tungsten special-purpose tool steels - - - - 130

REFERENCES

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INTRODUCTIONTool steels are high-quality steels made to close compositional and physical tolerances; they are used to make tools for cutting, forming, or shaping a material into a part or component adapted to a definite use. The earliest tool steels were simple, plain carbon steels, but beginning in 1868, and to a greater extent early in the 20th century, many complex, highly alloyed tool steels were developed. These complex alloy tool steels, which contain, among other elements, relatively large amounts of tungsten, molybdenum, vanadium, and chromium, make it possible to meet increasingly severe service demands and to provide greater dimensional control and freedom from cracking during heat treating. Many alloy tool steels also are widely used for machinery components and structural applications where particularly severe requirements must be met, such as high-temperature springs, ultrahigh-strength fasteners, special-purpose valves, punches and dies, wear-resistant liners, and bearings of various types for elevated-temperature service. This article discusses procedures and process control requirements for heat treating the principal types of tool steels. It also provides a review of heat-treating processes that are applied to tool steels and the specific applicability of these processes to the various types of tool steels.

What is tool steel?Tool steels are high-quality steels made to controlled chemical composition and processed to develop properties useful for working and shaping of other materials.

A variety of steel exist for a variety of application. Tool steel is one of these varieties of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion and deformation and their ability to hold a cutting edge at elevated temperatures. As a result tool steels are suited for their use in the shaping of other materials.

With a carbon content between 0.5% and 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The presence of carbides in their matrix plays the dominant role in the qualities of tool steel. The four major alloying elements in tool steel that form carbides are: tungsten, chromium, vanadium and molybdenum. The rate of dissolution of the different carbides into the austenite form of the iron determines the high temperature performance of steel. Proper heat treatment of these steels is important for adequate performance. The manganese content is often kept low to minimize the possibility of cracking during water quenching.

CATEGORIES OF TOOL STEEL

The choice of group to select depends on, cost, working temperature, required surface hardness, strength, shock resistance, and toughness requirements. The more severe the service condition (higher temperature, abrasiveness, corrosiveness, loading), the higher the alloy content and consequent amount of carbides required for the tool steel . The AISI-SAE grades of tool steel is the most common scale used to identify various grades of tool steel. Individual alloys within a grade are given a number; for example: A2, O1, etc.

There are six groups of tool steels:

water-hardening, cold-work,

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shock-resisting, high-speed, hot-work, and special purpose.

Water-hardening group

W-group tool steel gets its name from its defining property of having to be water quenched. W-grade steel is essentially high carbon plain-carbon steel. This group of tool steel is the most commonly used tool steel because of its low cost compared to others. They work well for small parts and applications where high temperatures are not encountered; above 150 °C (302 °F) it begins to soften to a noticeable degree. Its hardenability is low, so W-group tool steels must be subjected to a rapid quenching, requiring the use of water. These steels can attain high hardness (above HRC 66) and are rather brittle compared to other tool steels. W-steels are still sold, especially for springs, but are much less widely used than they were in the 19th and early 20th centuries. This is partly because W-steels warp and crack much more during quench than oil-quenched or air hardening steels.

The toughness of W-group tool steels are increased by alloying with manganese, silicon and molybdenum. Up to 0.20% of vanadium is used to retain fine grain sizes during heat treating.

Typical applications for various carbon compositions are for W-steels:

0.60–0.75% carbon: machine parts, chisels, setscrews; properties include medium hardness with good toughness and shock resistance.

0.76–0.90% carbon: forging dies, hammers, and sledges. 0.91–1.10% carbon: general purpose tooling applications that require a good balance of wear

resistance and toughness, such as rasps, drills, cutters, and shear blades. 1.11–1.30% carbon: files, small drills, lathe tools, razor blades, and other light-duty applications

where more wear resistance is required without great toughness. Steel of about 0.8% C gets as hard as steel with more carbon, but the free iron carbide particles in 1% or 1.25% carbon steel make it hold an edge better. However, the fine edge probably rusts off faster than it wears off, if it is used to cut acidic or salty materials.

Cold-work group

The cold-work tool steels are a group of steels used to cut or form materials that are at low temperatures. The group consists of three groups of steels: oil-hardening, air-hardening, and high carbon-chromium. This group possesses high hardenability and wear resistance, and average toughness and heat softening resistance. They are used in production of larger parts or parts that require minimal distortion during hardening. The use of oil quenching and air-hardening helps reduce distortion, avoiding the higher stresses caused by the quicker water quenching. More alloying elements are used in these steels, as compared to the water-hardening class. These alloys increase the steels' hardenability, and thus require a less severe quenching process and as a result are less likely to crack. They have high surface hardness and

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are often used to make knife blades. The machinability of the oil hardening grades is high and low for the high carbon-chromium types.

Oil-hardeningHere we have grade O1 of composition : 0.90% C, 1.0–1.4% Mn, 0.50% Cr, 0.50% Ni, 0.50% W. It is a very good cold work steel and also makes very good knives. It can be hardened to about 57-61 HRC.

Air-hardening

The first air-hardening grade tool steel was mushet steel, which was known as air-hardening steel at the time.

Modern air-hardening steels are characterized by low distortion during heat treatment because of their high-chromium content. They also harden in air due to their alloy than oil-hardening grades. Their machinability is good and they have a balance of wear resistance and toughness (i.e. between the D- and shock-resistant grades).

Grade Composition Notes

A21.0% C, 1.0% Mn, 5.0% Cr, 0.3% Ni, 1.0% Mo, 0.15–0.50% V

A common general purpose tool steel; it is the most commonly used variety of air-hardening steel. It is commonly used for blanking and forming punches, trimming dies, thread rolling dies, and injection molding dies.

A31.25% C, 0.5% Mn, 5.0% Cr, 0.3% Ni, 0.9–1.4% Mo, 0.8–1.4% V

A41.0% C, 2.0% Mn, 1.0% Cr, 0.3% Ni, 0.9–1.4% Mo

A60.7% C, 1.8–2.5% Mn, 0.9–1.2% Cr, 0.3% Ni, 0.9–1.4% Mo

This type of tool steel air-hardens at a relatively low temperature (approximately the same temperature as oil-hardening types) and is dimensionally stable. Therefore it is commonly used for dies, forming tools, and gauges that do not require extreme wear resistance but do need high stability.

A7

2.00–2.85% C, 0.8% Mn, 5.00–5.75% Cr, 0.3% Ni, 0.9–1.4% Mo, 3.9–5.15% V, 0.5–1.5 W

A80.5–0.6% C, 0.5% Mn, 4.75–5.50% Cr, 0.3% Ni, 1.15–1.65% Mo, 1.0–1.5 W

A9

0.5% C, 0.5% Mn, 0.95–1.15% Si, 4.75–5.00% Cr, 1.25–1.75% Ni, 1.3–1.8% Mo, 0.8–1.4% V

A10] 1.25–1.50% C, 1.6–2.1% This grade contains a uniform distribution of graphite particles to

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Mn, 1.0–1.5% Si, 1.55–2.05% Ni, 1.25–1.75% Mo

increase machinability and provide self-lubricating properties. It is commonly used for gauges, arbors, shears, and punches.

High carbon-chromium, D-type

The D-type, of the cold-work class of tool steels, contain between 10% and 13% chromium. These steels retain their hardness up to a temperature of 425 °C (797 °F). Common applications for these tool steels include forging dies, die-casting die blocks, and drawing dies. Due to their high chromium content, certain D-type tool steels are often considered stainless or semi-stainless, however their corrosion resistance is very limited due to the precipitation of the majority of their chromium and carbon constituents as carbides.

Grade Composition Notes

D2

1.5% C, 11.0–13.0% Cr; additionally 0.45% Mn, 0.030% P, 0.030% S, 1.0% V, 0.9% Mo, 0.30% Si

D2 is very wear resistant but not as tough as lower alloyed steels. The mechanical properties of D2 are very sensitive to heat treatment. It is widely used for the production of shear blades, planer blades and industrial cutting tools; sometimes used for knife blades.

1.2767 type

ISO 1.2767, also known as DIN X 45 NiCrMo 4, AISI 6F7, and BS EN 20 B, is an air-hardening tool steel with a primary alloying element of nickel. It possesses good toughness, stable grains, and is highly polishable. It is primarily used for dies in plastic injection molding application that involve high stresses. Other applications include blanking dies, forging dies, and industrial blades.

Shock-resisting group

The high shock resistance and good hardenability are provided by chromium-tungsten, silicon-molybdenum, silicon-manganese alloying. Shock-resisting group tool steels are designed to resist shock at both low and high temperatures. A low carbon content is required for the necessary toughness (approximately 0.5% carbon). Carbide-forming alloys provide the necessary abrasion resistance, hardenability, and hot-work characteristics. This family of steels displays very high impact toughness and relatively low abrasion resistance and can attain relatively high hardness (HRC 58/60). In the US, toughness usually derives from 1 to 2% silicon and 0.5-1% molybdenum content. In Europe, shock steels often contain 0.5-0.6 % carbon and around 3% nickel. 1.75% to 2.75% nickel is still used in some shock resisting and high strength low alloy steels (HSLA), such as L6, 4340, and Swedish saw steel, but it is relatively expensive. An example of its use is in the production of jackhammer bits.

High speed group

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T-type and M-type tool steels are used for cutting tools where strength and hardness must be retained at temperatures up to or exceeding 760 °C (1,400 °F). M-type tool steels were developed to reduce the amount of tungsten and chromium required.

T1 (also known as 18-4-1) is a common T-type alloy. Its composition is 0.7% carbon, 18% tungsten, 4% chromium, and 1% vanadium. M2 is a common M-type alloy.

Hot-working group

Hot-working steels are a group of steel used to cut or shape material at high temperatures. H-group tool steels were developed for strength and hardness during prolonged exposure to elevated temperatures. These tool steels are low carbon and moderate to high alloy that provide good hot hardness and toughness and fair wear resistance due to a substantial amount of carbide. H1 to H19 are based on a chromium content of 5%; H20 to H39 are based on a tungsten content of 9-18% and a chromium content of 3–4%; H40 to H59 are molybdenum based.

Special purpose group

P-type tool steel is short for plastic mold steels. They are designed to meet the requirements of zinc die casting and plastic injection molding dies.

L-type tool steel is short for low alloy special purpose tool steel. L6 is extremely tough. F-type tool steel is water hardened and substantially more wear resistant than W-type tool steel.

APPLICATIONS OF TOOL STEELS

Tool steels are used for cutting, pressing, extruding, coining, of metals and other materials. Their use for applications like injection molding due to their the resistance to abrasion is an important criterion for a mold that will be used to produce hundreds of thousands of parts is essential.

MICROSTRUCTURE OF TOOL STEELThe cutting performance of heat treated tool steel can be improved by obtaining finer grain size, a minimum amount of retained austenite, spheroid and finer carbide size and a unifom distribution of carbides. As mentioned above, the austenitizing temperature and quenching time should be appropriate, otherwise grain growth and an increased amount of retained austenite and segregation of carbides along grain boundaries will occur and can significantly reduce the life and cutting performance of tool steel.

HEAT TREATMENTHeat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming. 

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Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics. 

Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material. 

In heat treatment, the processing is most often entirely thermal and modifies only structure. Thermomechanical treatments, which modify component shape and structure, and thermochemical treatments which modify surface chemistry and structure, are also important processing approaches which fall into the domain of heat treatment. 

Tool Steel Analysis and CharacteristicsTool steels are vastly different from steel used in consumer goods. They are made on a much smaller scale with strict quality procedures and possess qualities necessary to perform a specific task, such as machining or perforating. Many different qualities in tool steels are sought based on a particular application need. These needs can be met by adding a particular alloy along with the appropriate amount of carbon. The alloy combines with carbon to enhance the steel’s wear, strength, or toughness characteristics. These alloys also contribute to the steel’s ability to resist thermal and mechanical stress.Heat-Treat ConsiderationsEach grade of tool steel has specific heat-treat guidelines to acquire optimum results for a given application. Tool steels are only as good as the heat treat they receive. The keys to achieving optimum results in heat treat include:1. Segregating by size and material type.2. Fixturing.3. Preheating.4. Soaking (austenitizing).5. Quenching (martensite transformation).6. Tempering.7. Freezing (cryogenics).Segregation by size is extremely important, because items of different sizes require adjustments in preheat, soak, and quench rates. Fixturing ensures even support and uniform exposure to heating and cooling during the heat-treat process.

Quenching is the sudden cooling of parts from the austenitizing temperature through the martensite transfer range. This transforms the steel from austenite to martensite, hardening the parts. Unfortunately,tool steels have a transformation range that is well below room temperature. This is one reason why cold-work steels benefit from freezing (cryogenics).Tempering is necessary to remove stress associated with the hardening process. Cold-work tool steels generally are tempered at 200 degrees C (400 degrees F) or less. Because of the low tempering temperature involved, one temper generally is adequate for cold-work tool steels.

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REASONS FOR HEAT TREATMENT OF TOOL STEEL ADVANTAGES OF HEAT TREATMENT OF TOOL STEEL DISADVANTAGES OF HEAT TREATMENT OF TOOL STEEL TABLE OF CONTENTS1.0 Introduction1.1 CATEGORIES OF TOOL STEEL

APPLICATIONS OF TOOL STEELS

MICROSTRUCTURE OF TOOL STEEL

HEAT TREATMENT OF STEEL HEAT TREATMENT OF tool STEEL REASONS FOR HEAT TREATMENT OF TOOL STEEL ADVANTAGES OF HEAT TREATMENT OF TOOL STEEL DISADVANTAGES OF HEAT TREATMENT OF TOOL STEEL

Purpose of heat treatment, heat treatment process variables. Heating and cooling of steels for heat treatment, homogeneity of austenite, austenitic grain size, determination and importance of austenitic grain size. TTT curves-formation and significance CC curves-significance. Heat treatment processes for steels: Annealing, normalizing, hardening, tempering, stress- relieving, spheroidizing, sub-zero treatment, austempering, martempering. Heat treatment defects. Hardenability – concept, determination and significance. Case hardening processes for steels. Thermomechanical treatments. Precipitation hardening. Heat treatment of some steels, namely, high speed steels, maraging steels, spring steels and ball bearing steels. Heat treatment of cast irons. Heat treatment of some important alloys of Al, Cu, Ti, Ni and Co.

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A TYPICAL HEAT TREATMENT OF A SCREW DRIVER BLADE AS A CASE STUDY In a brazing hearthAPPARATUS: A rotating table and fire brick.The fire bricks reflect the intense heat back on to the metal being heated. This is achieved by arranging the bricksin a semi-circle behind the metal being heated. Without the bricks, heat would escape and this would limit the temperature that could be reached.

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STAGE ONE:The screw driver blade is heated, slowly at first, warming up the whole blade. Then the heat is concentrated on the area at the end of the blade. This gradually becomes ‘red’ hot.

STAGE TWO:The screw driver blade is removed quickly from the brazing heart, with blacksmiths tongs and plunged into clean, cold water. Steam boils off from the water as the steel cools rapidly. At this stage the blade is very hard but brittle and will break easily.

STAGE THREE:The screw driver blade is cleaned with emery cloth and heated again on the brazing hearth. Heat is concentrated at the end of the steel blade. The steel must be watched very carefully as it changes colour quite quickly. A blue line of heat will appear near the end of the blade and it travels towards the tip as the temperature rises along the blade. When the line of blue reaches the tip the brazing torch is turned off. The blue indicates the correct temperature of

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‘tempering’.

STAGE FOUR:The screw driver blade is placed on a steel surface, such as an anvil face. This conducts the heat away and allows slow cooling of the screw driver blade. When cold, the blade should be tough and hard wearing and unlikely to break or snap. This is due to the tempering process.

USEFUL COLOUR INDICATORS OF TEMPERATUREWhen heating steel on the brazing hearth, colour changes take place. These can be used to indicate the temperature of the metal. The table below is a rough guide.

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The table below shows the temperatures and the associated colours required when tempering steel for particular uses. For instance, when making wood turning tools, they must be heated to a brown colour, whilst tempering.

CHIIn service, most tools are subjected to extremely high loads that are applied rapidly. They must withstand these loads a great number of times without breaking and without undergoing excessive wear or deformation. In many applications, tool steels must provide this capability under conditions that develop high temperatures in the tool. No single tool material combines maximum wear resistance, toughness, and resistance to softening at elevated temperatures. Consequently, selection of the proper tool material for a given application often requires a trade-off to achieve the optimum combination of properties. Table 1 gives the classifications and nominal compositions of various tool steels.

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23

24

25

26

27

28

29

30

31

32

33

34

35

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(a) These steels were not included in the March 1978 AISI Steel Products Manual, Tool Steels, in the main table of compositions nor in tables of heat-treating practice, because of their less common use.(b) Available with various carbon contents, in increments of 0.10% within this range.(c) Optional.(d) Intermediate high-speed steels M50 and M52, which are lower in alloy content than standard high-speed steels, have been employed successfully in applications requiring greater abrasion resistance than plain carbon steels, but less red hardness than high-speed steels. Typical uses include woodworking tools and hack saw blades. M50 and M52 steels meet the criteria promulgated by the American Society for Testing and Materials for intermediate high-speed steels but do not meet the more stringent criteria for standard high-speed steels.Most tool steels are wrought products, but precision castings can be used to advantage in some applications (additional information is available in the article "Wrought Tool Steels" in Volume 1 of ASM Handbook, formerly 10th Edition Metals Handbook). The powder metallurgy (P/M) process also is used in making tool steels; this process provides more uniform carbide size and distribution in large sections and special compositions that are difficult or impossible to produce by melting and casting and then mechanically working the cast product (additional information is available in the article "P/M Tool Steels" in Volume 1 of ASM Handbook, formerly 10th Edition Metals Handbook). For typical wrought tool steels, raw materials (including scrap) are carefully selected, not only for alloy content but also for qualities that ensure cleanliness and homogeneity in the finished product. Tool steels are generally melted in small-tonnage electric-arc furnaces to achieve composition tolerances economically, cleanliness and precise control of melting conditions. Special refining and secondary remelting processes such as argon oxygen decarburization (AOD), electroslag remelting (ESR), and vacuum arc remelting (VAR) (see the articles "Vacuum Melting and Remelting Processes" and "Degassing Processes (Converter Metallurgy)" in Volume 15 of ASM Handbook, formerly 9th Edition Metals Handbook), have been introduced to satisfy particularly difficult demands on

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tool steel quality and performance. The medium-to-high alloy contents of many tool steels require careful control of forging and rolling, which often results in a large amount of process scrap. Semifinished and finished bars are given rigorous in-process and final inspection. This inspection can be so extensive that both ends of each bar may be inspected for macrostructure (etch quality), cleanliness, hardness, grain size, annealed structure, and hardenability. The inspection may require that the entire bar be subjected to magnetic, particle, eddy current, and ultrasonic inspections for surface and internal discontinuities. It is important that finished tool steel bars have limited decarburization, which requires that annealing be done by special procedures under closely controlled conditions. Controlled atmosphere continuous annealing furnaces, vacuum furnaces, and protective coatings are often used to minimize decarburization during annealing. Such precise production practices and stringent quality controls contribute to the high cost of tool steels, as do the expensive alloying elements they contain. Insistence on quality in the manufacture of these specialty steels is justified, however, because tool steel bars generally are made into complicated cutting and forming tools worth many times the cost of the steel itself. Although some standard constructional alloy steels resemble tool steels in composition, they are seldom used for expensive tooling because, in general, they are not manufactured to the same rigorous quality standards as are tool steels.The performance of a tool in service depends on:*Proper tool design*Accuracy with which the tool is made*Selection of the proper tool steel*Application of the proper heat treatmentA tool can perform successfully in service only when all four of these requirements have been fulfilled.With few exceptions, all tool steels must be heat treated to develop specific combinations of wear resistance, resistance to deformation or breaking under high loads, and resistance to softening at elevated temperatures. A few simple shapes may be obtained directly from tool steel producers in correctly heat-treated condition. However, most tool steels first are formed or machined to produce the required shape and then heat treated as required. Figure 1 shows typical processing and heat-treatment sequences for tool steels as a function of time, temperature, and phase transformation. Improper finishing after heat treatment--principally grinding--can damage tool steels through the development of surface residual stresses and cracks. Some tools are heat treated (hardened) in a blank or semifinished state and subsequently ground, turned, or electrical discharge machined to create the final tool. Although these manufacturing techniques have progressed in recent years, metallurgical damage and surface stresses are still a major concern.

Fig. 1 Plots of temperature versus time showing sequence of operations required to produce tool steels. (a) Thermomechanical processing. (b) Hardening heat treatment. L, liquid; A, austenite; C, cementite; F, ferrite; Ms, temperature at which marten site starts to form on cooling; RT, room temperature. Source: Ref 1 Processing

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information and service characteristics of tool steels are presented in Tables 2, 3, 4. This information is essential in understanding the problems involved in selection, processing, and application of tool steels. Tool steel suppliers provide more specific information on the properties developed by specific heat treatments in the steels produced by their companies. They should be consulted as to the type of steel and heat treatment best suited to meet all service requirements at the least over-all cost.

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40

41

42

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(a) Time held at temperature varies from 15 min for small sections to 1 h for large sizes. Cooling is done in still air. Normalizing should not be confused with low-temperature annealing.(b) The upper limit of ranges should be used for large sections and the lower limit for smaller sections. Time held at temperature varies from 1 h for light sections to 4 h for heavy sections and large furnace charges of high-alloy steel.(c) For 0.25 Si type, 183 to 207 HB; for 1.00 Si type, 207 to 229 HB.(d) Temperature varies with carbon content: 0.60 to 0.75 C, 815 °C (1500 °F); 0.75 to 0.90 C, 790 °C (1450 °F); 0.90 to 1.10 C, 870 °C (1600 °F); 1.10 to 1.40 C, 870 to 925 °C (1600 to 1700 °F).(e) Temperature varies with carbon content: 0.60 to 0.90 C, 740 to 790 °C (1360 to 1450 °F); 0.90 to 1.40 C, 760 to 790 °C 1400 to 1450 °F).

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45

46

47

48

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(a) O, oil quench; A, air cool; S, salt bath quench; W, water quench; B, brine quench. (b) When the high-temperature heating is carried out in a salt bath, the range of temperatures should be about 15 °C (25 °F) lower than given in this line. (c) Double tempering recommended for not less than 1 h at temperature each time.(d) Triple tempering recommended for not less than 1 h at temperature each time.(e) Times apply to open-furnace heat treatment. For pack hardening, a common rule is to heat 1.2 min/mm (30 min/in.) of cross section of the pack. (f) Preferable for large tools to minimize decarburization.(g) Carburizing temperature. (h) After carburizing. (i) Carburized case hardness.(j) P21 is a precipitation-hardening steel having a thermal treatment that involves solution treating and aging rather than hardening and tempering. (k) Recommended for large tools and tools with intricate sections

50

51

52

53

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(a) A, air cool; B, brine quench; O, oil quench; S, salt bath quench; W, water quench.(b) After tempering in temperature range normally recommended for this steel.(c) Carburized case hardness.(d) After aging at 510 to 550 °C (950 to 1025 °F).(e) Toughness decreases with increasing carbon content and depth of hardening.

Physical properties--density, thermal expansion, and thermal conductivity--of selected tool steels are given in Tables 5 and 6.

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56

57

58

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(a) From 20 °C to 500 °C (70 °F to 930 °F).(b) From 20 °C to 600 °C (70 °F to 1110 °F).

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(c) From 20 °C to 260 °C (70 °F to 500 °F).(d) From 40 °C (100 °F)

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62

63

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NormalizingNormalizing requires slow and uniform heating above the transformation range to dissolve excess constituents, then cooling in still air (see the article "Normalizing of Steel" in this Volume). Normalizing breaks up nonuniform structures, relieves residual stresses, and produces greater uniformity in grain size--thus counteracting undesirable results of unequal reductions for different sections during forging, differences in temperature between varying thicknesses of sections, and the subsequent irregular cooling rates. Normalizing also conditions the steel for subsequent spheroidizing, annealing, or hardening.Applicability. Many tool steels harden even when cooled in still air; normalizing these steels for the purpose of refining a structure is not recommended. Tool steels that should not be normalized include all high-speed steels, all shock-resisting steels, all hot work steels, cold work steels of types A and D (except A10), and the mold steel P4.For other types of tool steel, normalizing is most commonly applied after forging and before annealing. Normalizing also may be used before full annealing for parts that are being hardened for a second time.Standard practice consists of heating to the normalizing temperature, soaking for a suitable time to allow the bar to reach a uniform temperature, and then cooling in still air. No special equipment is required, but the work should be protected against decarburization during heating.AnnealingTool steels usually are received from the supplier in the annealed condition. This condition allows the steel to be easily machined and heat treated. However, if they are subjected to hot or cold forming, often they must be fully annealed again before subsequent operations. If a tool is to be rehardened, it should first be thoroughly annealed. This procedure is important with the steels of higher alloy content; otherwise, irregular grain growth occurs and a mixed grain size (sometimes called fish scale or duplex grains) will result.

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Full annealing involves heating the steel slowly and uniformly to a temperature above the transformation range, holding it at the temperature for from 1 to 4 h (which is generally long enough for complete heat penetration), and cooling slowly at a controlled rate followed by air cooling.Atmosphere furnaces, salt baths, vacuum furnaces, or lead pots may be used for annealing. Requirements of the heating equipment include reasonably accurate temperature control and a means of preventing decarburization. In box or roller-hearth furnaces, surface protection often is accomplished through proprietary decarb-resistant coating or by packing the workpieces in pipes, in which they are then surrounded by nondecarburizing material, such as spent charcoal and mica, or cast iron chips. Furnaces with prepared atmospheres frequently are used for the annealing of tool steels.These packing materials may carburize, decarburize, or be neutral to steel, depending on the heating temperature, the carbon content of the steel, and the density of the packing (also the particle size of the packing material). The principal aim of such materials is to exclude the decarburizing gases from contact with the steel. If oxygen is to be excluded, the packing must be very tight and the sealing of the container perfect, for most gases can diffuse through ordinary seals at a surprisingly rapid rate at elevated temperatures. Materials that contain carbonaceous substances are, therefore, somewhat better where this is the case. In the annealing operation, where scale is present on the bars as charged in the furnace, decarburization may occur unless a carbonaceous material is present. Needless to say, the presence of moisture in the packing material is not tolerable as decarburization will take place very readily.Figure 2 shows the range of usefulness of each of the packing compounds. This should be used only as a guide, as the presence of scale, the age of the packing compound, the moisture content, and the carbon content of the steel will appreciably alter the range. Furthermore, the temperature at which one material will change from neutral to decarburizing is not necessarily as definite as that indicated. It is to be noted that the effect of carbon content is not included. The chart applies particularly to high-carbon steels.

Fig. 2 Approximate range of usefulness of selected packing compounds used in the annealing of tool steels.Temperatures of change from one behavior to another are actually not sharp. For cast iron chips, the temperature below which decarburization will take place depends on the carbon content of the chip. Source:Ref 2Cracking from thermal shock can be minimized by loading the furnace at a relatively low temperature (room temperature or a few hundred degrees Fahrenheit) to permit the furnace load to heat up slowly with the furnace. Following the soak at annealing temperature, the workpieces (and container, if used) should be cooled in the furnace at 8 to 22 °C/h (15 to 40 °F/h) to 540 °C (1000 °F) or lower. Below about 540 °C (1000 °F), the cooling rate for most tool steels is no longer critical, and the work may then be cooled in air. Typical annealed hardness values for the

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various tool steels are given in Table 1.Additional information is available in the articles "Annealing of Steel" and "Continuous Annealing of Steel" in this Volume.Isothermal annealing is an alternative method of cooling that consists of rapidly cooling the workload in the furnace from the annealing temperature to a temperature just below the transformation range and holding the load and furnace at this temperature for one or more hours. Following this period of soaking at just below the transformation range, the load may be safely air cooled. This process, known as isothermal annealing, is best suited for applications in which full advantage can be taken of the rapid cooling to the transformation temperature, and from this temperature to room temperature. Thus, for small parts that can be handled in salt or lead baths, or for light loads in batch furnaces, isothermal annealing makes possible large savings in time, as compared with the conventional slow furnace cooling. It can also be adapted conveniently to continuous annealing cycles where adequate equipment is available.Isothermal annealing offers no particular advantage for applications (such as the batch annealing of large furnace loads) in which the rate of cooling at the center of the load may be so slow as to preclude any rapid cooling to the transformation temperature. For such applications, conventional full annealing usually offers a better assurance of obtaining the desired properties.Stress Relieving

Stress relieving removes or reduces residual stress induced in tools by heavy machining or forming, and thereby decreases the probability of distortion or cracking during hardening of the tool (see the article "Stress-Relief Heat Treating of Steel" in this Volume).The ground surface of a hardened tool may be highly stressed after grinding but not cracked. The high stress may, however, cause cracks to develop immediately after grinding, before use, or during use. Ground tools with high residual stress can often be salvaged by stress relieving, immediately after grinding, at or just below the tempering temperature in order to maintain the specified tool hardness.Tools also develop high residual stress in use. It is sometimes advantageous to relieve this stress at each redressing of the tool by retempering at an appropriate temperature. This temperature should not exceed the tempering temperature; otherwise, undesirable softening will occur.Procedure. Stress relieving is most commonly performed in air furnaces or salt baths used for tempering. Neither the heating nor the cooling rate is critical, although cooling should be slow enough to prevent the introduction of new stress. Protection against scaling or decarburization is seldom required, unless the stress-relieving temperature is above 650 °C (1200 °F). Under some conditions, vacuum or inert atmosphere furnaces may be required to prevent scaling or discoloration.After stress relieving, it may be necessary to correct certain dimensions before hardening, because the relief of stress causes some dimensional change. Precision tools usually are stress relieved after machining and before hardening; it is often desirable to stress relieve after rough machining but before finish machining. Stress relieving after electrical discharge machining (EDM) work will reduce some of the residual stress but will not remove all of the effects of thismachining method.AustenitizingAustenitizing is the most critical of all heating operations performed on tool steels. Excessively high austenitizing temperatures or abnormally long holding times may result in excessive distortion, abnormal grain growth, loss of ductility, and low strength; this is especially true for high-speed steels, which are frequently austenitized at a temperature close to that at which melting begins. Underheating may result in low hardness and low wear resistance. At the time of quenching, if the center of a tool is cooler than the exterior, spalling or fracturing of the corners may result, particularly with water-hardening steels. Prior to heat treatment, all tool surfaces must be free of

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decarburization. Typically, steel is supplied decarb free of centerless ground bar or precision ground flats. If hot rolled material is purchased, sufficient stock must be removed. Austenitizing is the heat treatment where the final alloy element partitioning between the austenitic matrix (that will transform to martensite) and the retained carbides occurs. This partitioning fixes the chemistry, volume fraction, and dispersion of the retained carbides. The retained alloy carbides not only contribute to wear resistance, but also control austenitic grain size. The finer the carbides and the larger the volume fraction of carbides, the more effectively austenitic grain growth is controlled. Thus if austenitizing is performed at too high a temperature, undesirable grain growth may occur as the alloy carbides increasingly coarsen or dissolve into the austenite. The alloying elements not tied up in retained carbides are in solution in the austenite, and thus the carbides provide an important mechanism by which austenite composition is fixed. The austenite composition then sets the hardenability, Ms temperature, retained austenite content, and secondary hardening potential of a tool steel.Figure 3 shows the effect of increasing austenitizing temperature on the as-quenched, quenched and subzero cooled, and tempered hardness of an A2 tool steel. The highest as-quenched hardness is produced by austenitizing at 950 °C (1740°F), the recommended austenitizing temperature for A2. In this condition after quenching, the retained austenite content is finely dispersed and at a minimum and therefore subzero cooling has little effect on hardness. With increasing austenitizing temperature, more alloying elements go into solution, the Ms temperature drops, and more austenite is retained at room temperature. As a result, the as-quenched room temperature hardness decreases and subzero cooling has a greater effect as more of the large volume fraction of retained austenite transforms to martensite on subzero cooling.Figure 4 shows that eventually tempering, by a combination of retained austenite transformation and secondary hardening, will also raise the hardness of as-quenched structures with large amounts of retained austenite. Not shown is the deleterious increase in austenite grain size which develops as more and more carbides dissolve at the higher austenitizing temperatures.

Fig. 3 Plots of hardness versus tempering temperatures of as-quenched, quenched, and subzero cooled to -80°C (-110 °F), and quenched and subzero cooled to -180 °C (-290 °F) A2 tool steels to show effect of increasing austenitizing temperatures. (a) 950 °C (1740 °F). (b) 1000 °C (1830 °F). (c) 1050 °C (1920 °F). (d) 1100 °C (2010 °F). Source: Ref 3

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Equipment for austenitizing tool steels is chosen on the basis of steel composition, size and shape of workpieces, amount of stock removal after hardening, and production requirements. Vacuum furnaces, atmosphere furnaces, and salt baths have proved satisfactory for service over the entire austenitizing temperature range of 760 to 1300 °C (1400 to 2375°F). Lead pots are suitable for the temperature range of about 760 to 925 °C (1400 to 1700 °F).Workpieces must be supported during austenitizing. Lead and salt provide some of the support, but in atmosphere furnaces, special attention must be given to prevent workpieces from sagging or making contact with the furnace brickwork.During austenitizing, continuous control of the furnace internal environment must be maintained to prevent workpieces from becoming carburized or decarburized. Salt baths must be rectified; atmospheres must be controlled for proportion of gases and dew point; lead baths must be kept free of contamination. Vacuum furnaces must be maintained at low leak rates and partial pressure control at austenitizing temperatures, above 1095 °C (2000 °F).Preheating for Austenitizing. Preheating tool steels before austenitizing is sound practice, but it is not always required. For small pieces of simple shape, preheating may be eliminated. Preheating normally is employed as a safeguard against the cracking and extreme distortion resulting from the thermal shock undergone by a cold workpiece when it is exposed to the high temperature of the austenitizing furnace.Preheating is especially beneficial for the highly alloyed hot work and high-speed steels, because it gives them a greater length of time to reach thermal equilibrium and eliminates most of the risk of prolonged exposure to austenitizing temperatures such as decarburization and the tendency for surfaces of large sections to experience longer exposure times.Tools that are to be austenitized in salt are usually preheated in salt, but they may be preheated in an atmosphere furnace if it is more convenient. Tools that are to be austenitized in an atmosphere furnace are preheated in a gaseous

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atmosphere.Procedure. Preheating is usually done in a furnace adjacent to the austenitizing furnace, although it is possible to preheat and austenitize in the same furnace. In the latter procedure, once the workpiece is heated through to the preheat temperature, the furnace temperature is raised to the austenitizing temperature and the workpiece is thus brought to the austenitizing temperature without leaving the furnace. The practicality of this one-furnace procedure depends on the difference between preheating and austenitizing temperatures for the type of steel being treated and on production requirements. Some experts do not recommend this procedure for high-speed steels, especially where high-inertia furnaces are used, because the difference between preheating and austenitizing temperatures for these steels may be as much as 485 °C (875 °F). In high-volume operations, where preheating is frequently performed solely to shorten production time, separate furnaces can be used for preheating and austenitizing.

QuenchingQuenching from the austenitizing temperature may be done in water, brine, oil, salt, inert gas, or air, depending on composition and section thickness. The quenching medium must cool the workpiece rapidly enough to obtain full hardness; it is poor practice, however, to use a quenching medium with a cooling capacity that exceeds requirements, because of the possibility that cracking or excessive distortion may occur. Tool steels that will harden during air cooling are frequently hot quenched to the range 540 to 650 °C (1000 to 1200 °F) after austenitizing. Quenching time is long enough for decomposition of austenite to begin. After hot quenching, the steels are air cooled or oil quenched to ambient temperature. Hot quenching minimizes distortion without adversely affecting hardness and spaIls away or prevents the hard scale from forming on most air-hardening steels during air cooling. Additional information is available in the article "Quenching of Steel" in this Volume.Martempering is often utilized to minimize distortion without sacrifice of hardness in oil-hardening tool steels or in extremely thin sections of water-hardening tool steels (see the article "Martempering of Steel" in this Volume).Workpieces are quenched from the austenitizing temperature in an agitated bath of oil or salt. Bath temperature should be near the temperature at which martensite starts to form on cooling (Ms), usually about 31 °C (57 °F) above it. Time in the bath should be just sufficient for temperature to equalize throughout the workpieces, which are then air cooled to room temperature prior to tempering.TemperingTempering modifies the properties of quench-hardened tool steels to produce a more desirable combination of strength, hardness, and toughness than obtained in the quenched steel (see the article "Tempering of Steel" in this Volume). The as-quenched structure of tool steel is a heterogeneous mixture of retained austenite, untempered martensite, and carbides.More than one tempering cycle may be necessary to produce an optimum structure. It is normally desirable to transform all retained austenite to ensure complete hardness, improve toughness, and minimize distortion during service. This can be more nearly accomplished by two or more shorter tempering cycles than by a single and longer cycle (see Fig. 4).In the higher-alloy tool steels, a small amount of untempered martensite is formed from retained austenite during the cool-down from the first tempering cycle. It is good practice to double temper to ensure more nearly complete transformation of retained austenite and to temper freshly formed martensite. For some highly alloyed grades of tool steel, triple or quadruple tempering is recommended. The changes that take place in the microstructure during tempering of hardened tool steels are time-temperature dependent. Time at tempering temperature should not be less than 1 h for any given cycle. Most manufacturers of high-speed steels recommend multiple tempers of 2 h or more each to attain the desired microstructure and properties. Maintaining recommended tempering times, temperatures, and number of tempers (a minimum of two) ensure attainment of consistent tempered martensitic structures and

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overcomes uncertainties caused by variations in the amount of retained austenite in the as-quenched condition. These variances are functions of differences in heat chemistry, prior thermal history, hardening temperatures, and quenching conditions. Other factors that influence the tempering requirements of high-speed steels are:

Increasing the free (matrix) carbon content increases the amount of retained austenite in the as-quenched condition

The amount of retained austenite significantly affects the rate of transformation, particularly for short tempering cycles. Multiple tempering is more important to attain an acceptable structure if short tempering times are used

Cobalt in alloys such as M42 reduces the amount of retained austenite in the as-quenched condition and accelerates the transformation of the retained austenite during tempering Enough time should be allowed during tempering for the temperature to be distributed uniformly throughout the tools before time at temperature is counted. This is especially true for low tempering temperatures and for tools that have large sections. Table 7 indicates the time needed for various section sizes to reach uniform temperature in different kinds of furnaces. If not enough time is allowed for the tool to reach the tempering temperature, the result will be nonuniform tempering and possible damage to the tool. Color of the oxide film should not be used as a guide in tempering, because these temper colors indicate only the surface temperature of the tool, not the internal temperature. Grinding cracks in hardened tools may be caused by inadequate tempering.

(a) Data are given in minutes per millimeter, and in minutes per inch, of diameter or thickness, with furnace maintained at the temperature indicated in column 1. Data may be used as a guide for charges of irregular shapes and

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quantities by estimating total size of charge and applying the above allowance to the number of inches from outside to center of charge.(b) Times indicated are for tools with dark or scaled surfaces. If surfaces are finish ground, or otherwise brightened, twice as much time should be allowed in a still hot air oven. No extra allowance need be made for bright surfaces in a circulating oven or in an oil bath.(c) Oil baths are usually not used above 205 °C (400 °F).Proper tempering depends on the accurate determination of the temperature of the load and on proper spacing of workpieces in the load to ensure that it is uniformly heated. The most common medium used for tempering tools is recirculating-atmosphere furnaces, where the atmosphere may be flue gas, nitrogen, argon, or even a partial vacuum.Regardless of the medium used for tempering, accurate means of temperature control are mandatory for reproducible results.Procedure. Before a tool steel part is tempered, it should be cooled in the quenched medium or in air until it can be held in the hand without discomfort (near 50 °C, or 120 °F, for most steels). For particularly large or intricate parts, it is essential to temper as soon as possible after the quench, to prevent cracking.Heating to the tempering temperature should be slow, to obtain uniform distribution of temperature within the tool and to prevent the nonuniform relief of hardening stress that could cause cracking or warping. Satisfactory results may be obtained by charging the tools into a freely circulating medium at the desired tempering temperature and then permitting them to reach this temperature. If tempered in a liquid medium, the tools should be placed in a basket and not permitted to come in contact with the hot walls or bottom of the pot or tank. Heat transfer is most rapid for molten lead baths, less rapid for salt and oil baths, and slowest in still air. Cooling after tempering should be relatively slow in order to prevent development of residual stress in the steel. Still air cools at a satisfactory rate.Equipment. Recirculating-air furnaces have several advantages over most other types of equipment used for tempering.For example, such a furnace can be cooled rapidly between batches of tempering temperatures so that successive work loads may enter the furnace safely at a low temperature. Another advantage of the recirculating-air furnace is its relatively low heat transfer rate, which permits the load to be brought to temperature more slowly. As-quenched tools heated too rapidly may develop cracks. Recirculating-air furnaces also usually afford a wider range of useful tempering temperatures than other tempering mediums, with no hazards of fire or burns from splashing of hot liquids.Surface Treatments and Cold Treating Carburizing of tool steels is usually restricted to special applications. Mold steels, however, are commonly carburizing and then case hardened. A marked increase in surface carbon renders most tools too brittle for their intended uses. However, tools made of shock-resisting steel, hot work steel, and especially the lower-carbon types of high-speed steel are sometimes carburized to advantage for use in certain die applications involving severe wear. Carburizing is also useful for applications such as cold work dies that require extreme wear resistance and that are not subjected to impact or highly concentrated loading. All the common methods of carburizing (gas, pack, and liquid) have been employed for these special applications (see the articles "Gas Carburizing," "Pack Carburizing," and "Liquid Carburizing and Cyaniding" in this Volume). Case depths are shallower, about 0.05 to 0.25 mm (0.002 to 0.010 in.), rather than the 0.75 to 1.5 mm (0.030 to 0.060 in.) that is common on conventional carburizing steels.Carburizing temperatures, typically 1040 to 1065 °C (1900 to 1950 °F) are held for 10- to 60-min durations. The carburizing treatment also serves as an austenitizing treatment for the whole tool. A carburized case on high-speed steels has a hardness of 65 to 70 HRC but does not have the high resistance to softening at elevated temperatures exhibited by normally hardened high-speed steel.Nitriding successfully increases the life of all types of high-speed steel cutting tools. For nitrided high-speed steel

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taps, drills, and reamers used in machining annealed steel, five-fold increases in life have been reported, with average increases of 100 to 200%. Obviously, if this nitrided case is removed when the tool is reground, the tool must then be retreated, which reduces the cost advantage of the process.Gas nitriding, however, produces a case that is too brittle for most applications. Gas nitriding of tool steels is limited to applications such as hot work tool steels (H grades) and tooling for aluminum extrusion dies. Additional information is available in the article "Gas Nitriding" in this Volume.Liquid nitriding of finished high-speed steel tools in cyanide-base salt baths at 510 to 565 °C (950 to 1050 °F) is a common method of increasing tool life because it provides a light case, increasing both surface hardness and resistance to adhesion. Nitriding time ranges from 15 min to 2 h, resulting in case depths up to about 0.05 mm (0.002 in.). Due to environmental considerations, sources for liquid nitriding are becoming rare. Additional information is available in the article "Liquid Nitriding" in this Volume.Vacuum Nitriding. Vacuum furnaces, especially vacuum tempering furnaces, can be used for nitriding of tools. The tempering furnace is ideally suited for this purpose because it is designed to operate in the nitriding temperature range.For high-speed steels and other tools tempered at 455 to 595 °C (850 to 1100 °F), the nitriding cycle can be incorporated into the final tempering cycle, by introducing a partial pressure of ammonia during the final temper. In this way, the vacuum furnace operates similar to a normal nitriding furnace, except that nitrogen or the vacuum itself acts as the dilutant for nitriding, replacing dissociated ammonia. Cycle times would be the same as for gas nitriding.Plasma Nitriding. Another process used for nitriding tool steels is plasma nitriding, also known as ion nitriding. It is used in the nitriding of high-speed (M and T series), cold work (A and D series), and hot work (H series) tool steels (Ref 5). This process is also a vacuum process, but employs the principle of glow discharge to provide energy for heating and nitriding (see Fig. 5). Because it relies on electrical energy to dissociate gases, activate surfaces, and to provide energy for reaction, ordinary nitrogen at pressures in the range 0.1 to 1 kPa (1 to 10 mm Hg) are all that is required for nitriding. In addition, by adjusting the amount of nitrogen, the surface white layer can be closely controlled or eliminated. As in the other methods, the cycles are very short. In addition, the temperature range is greatly expanded, to as low as 350 °C (660°F), because the glow discharge reaction is not dependent on ammonia breakdown as in gas nitriding. This permits greater flexibility in choice of nitriding temperature so that surface hardness, case depth, and core hardness can be optimized for a given steel.

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Titanium Nitride Coatings. Several techniques are being used for the application of nitride coatings. Most processes are proprietary and carried out on a commercial basis. The most frequently applied material is titanium nitride. Very thin coatings in the 0.025 mm (0.001 in.) thickness range or less produce 69 HRC high hardness, cutting edges on drills, reamers, and other cutting tools. See the article "High-Speed Tool Steels" in Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook.Sulfide Treatment. A low-temperature (190 °C, or 375 °F) electrolytic process using sodium and potassium thiocyanate provides a seizing-resistant iron sulfide layer. This process can be used as a final treatment for all types of hardened tool steels without much danger of overtempering.Oxide coatings, provided by treatment of the finish-ground tool in an alkali-nitrate bath or by steam oxidation, prevent or reduce adhesion of the tool to the workpiece. Oxide coatings have doubled tool life--particularly in machining of gummy materials such as soft copper and nonfree-cutting low-carbon steels.Chromium plating of finished high-speed steel tools with 0.0025 to 0.0125 mm (0.1 to 0.5 mil) of chromium also prolongs tool life by reducing adhesion of the tool to the workpiece. Chromium plating is relatively expensive, and precautions must be taken to prevent tool failure in service due to hydrogen embrittlement.Electroless nickel plating has been used successfully as a replacement for chromium plating, both in routine production and for salvage plating operations on tool steel parts. Because plating by this method is accomplished by means of chemical reduction, it does not depend on any galvanic coupling between dissimilar metals, and there is no electrolysis involved. Therefore, there is no danger of hydrogen embrittlement. Plated hardness is in the high Rockwell50's range, with good, uniform plated thickness on all surfaces, and the plated surfaces have a low coefficient of friction.

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Cold Treating. The main purpose of cold treating tool steels (to -75 °C, or -100 °F, or lower) is to transform retained austenite in the unfinished tool and thus to provide dimensional stability in subsequent finishing operations. The use of cold treatment on properly heat-treated cutting tools does not affect tool performance. When used, cold treatment should be performed between the first and second tempering operations. Although cold treating an as-quenched tool is more effective in transforming retained austenite than after the first temper, it does increase the chance of cracking.Additional information is available in the article "Cold Treating and Cryogenic Treatment of Steel" in this Volume.THE HEAT TREATMENT FURNACESHeat treatment of tool steels is as important to their success as the selection of the grade itself. Machine tools or production dies made from tool steels should never have their rigid metallurgical requirements compromised or out-weighed by cost considerations. Attempting to reduce production costs by bypassing steps in the heat treat processing of tool steels will yield an end product that fails to meet tool life expectations and does not justify its high initial expense.These metallurgical requirements involve the control of the surface condition and chemistry, accurate control of the temperatures often up to 1315 °C (2400 °F), the time at a given temperature, and the control of the heating and cooling rates. Special attention must be paid to these requirements in the design, construction, and operation of the furnaces used to heat treat tool steels, especially those used for hardening, where the metallurgical factors involved become all-important.Tool steels are typically heat treated in ceramic-lined salt bath furnaces, in vacuum furnaces, in controlled atmosphere furnaces, and in fluidized-bed furnaces.Salt Bath FurnacesMolten salts of various compositions are well adapted to all operations in the heat treatment of tool steels. For tools that cannot be ground after hardening or for tools that require an excellent surface condition and the maintenance of sharp edges, salt bath heating provides excellent results. Table 1 lists various salt bath compositions and processing temperatures for the heat treating of tool steels. The salt bath method of hardening is predominant with high-speed steel tools. With correct operating conditions, tools can be heat treated without carburization, decarburization, and scaling. The surface will be fully hard with a minimum of distortion. Three types of salt baths are generally used:

Preheating baths High-temperature baths Quenching baths

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Preheating serves to minimize thermal shock, equalize temperature, and minimize the amount of time required at the high-temperature stage. The high-temperature salt bath serves as the austenitizing step. The function of the quenching bath is to equalize the temperature as well as to ensure a clean surface after heat treatment.Most tools heat treated in salt baths are fully hard from surface to core regardless of the section thickness. Because salt baths provide temperature uniformity in preheating, in high-temperature heating, and in quenching, distortion and residual stress are minimized. Tools that are heat treated in molten salt baths are heated by conduction with the molten salt providing a ready source of heat as required. Although steels come in contact with heat through the tool surfaces, the core of a tool rises in temperature at approximately the same rate as its surface. Heat is quickly drawn to the core from the surface. Salt baths provide heat at a rate equal to the heat absorption rate of the total tool. Convection or radiation heating methods are unable to maintain the rate of heating necessary to reach equilibrium with the rate of heat absorption. The ability of a molten salt bath to supply heat at a rapid rate enhances the uniformity of properties and resultant high quality of tools heat treated in salt baths. Heat-treating times are also shortened; for example, a 25 mm (1 in.) diam bar can be heated to temperature equilibrium in 4 min in a salt bath, while 20 to 30 min would be required to obtain the same properties in convection or radiation furnaces.Salt baths are an efficient method of heat treating tool steels; about 93 to 97% of the electric power consumed in a salt bath operation goes directly into heating. Tool steels that are heat treated in molten salts typically are processed in ceramic-lined furnaces with submerged or immersed electrodes containing chloride-base salts.

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Immersed-Electrode Salt Bath FurnacesCeramic-lined furnaces with immersed (over-the-side) electrodes have greatly extended the useful range and capacity of molten salt equipment when compared with externally heated pot furnaces (see Fig. 1). Detailed information is available in the article "Salt Bath Equipment" in this Volume.

Submerged-Electrode Salt Bath FurnacesSubmerged-electrode furnaces have the electrodes placed beneath the working depth for bottom heating. Figure 2 is a cutaway showing typical construction of a submerged-electrode furnace. Detailed information is available in the article "Salt Bath Equipment" in this Volume.

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Automatic Heat Treating of Tool SteelsFigure 3 illustrates three different heat treating arrangements for the production heat treatment of tool steels. Table 2 gives relative process times and temperatures for heat treating, and Table 3 gives process times for twist drills. The systems are equipped for cycles ranging from less than 1 min to 10 min. The parts are suspended on tong-type fixtures and are carried through the process by a chain conveyor on carrier bars. To facilitate rapid transfer of the tool steels, rotary transfer arms are placed between the preheat and the high heat units and between the high heat and the quench units. Transfer-arm placement is chiefly governed by the production rate; however, transfer arms are always required between the high heat and the quench units to satisfy metallurgical conditions. The lines also have areas above the furnaces to accommodate air cooling of the tools. In special cases, lines will be made with a station for an isothermal nitrate quench after the neutral salt quench. This additional stage allows rapid reduction of the temperature of the tools and reduces the air cooling time from 24 times to 6 times the time at the high-heat temperature. Caution: If as little as 600 ppm of nitrate salts are allowed to enter the high heat furnace, extreme surface damage can be done to the tool being heat treated.

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(a) See Table 3 for drill sizes and times in the high heat indicated by an "X" in this table.

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80

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Fig. 3 Process designs for automated salt bath furnaces for heat treating high-speed tool steels. Installations can be custom designed to meet specific customer requests. (a) Does not include wash and rinse. (b) Similar to (a), but includes wash and rinse operation necessitating relocation of load and unload operations. (c) Similar to (b), but includes second quench and a variation in wash cycles specified by customerRectification of Salt BathsNeutral salts used for austenitizing steel become contaminated with soluble oxides and dissolved metals during use, resulting from a reaction between the oxide layers present on fixtures and workpieces and the chloride salts. Because the buildup of resulting oxides and dissolved metals renders the bath oxidizing and decarburizing toward steel, the bath must be rectified periodically.Baths of salts such as salt mixtures No. 1 and 2 in Table 1 can be rectified with silica, methyl chloride, or ammonium chloride. The higher the temperature of operation, the more frequent the need for rectification. Baths in which the electrodes protrude above the surface require daily rectification with either ferrosilicon or silicon carbide. Baths operated above 1080 °C (1975 °F) require rectification a minimum of at least once a day, with more frequent rectification certainly recommended. During rectification of a bath, the silica combines with the dissolved metallic oxides to form silicates.Although these silicates settle out as a viscous sludge that can be removed, sufficient soluble silicates can remain to cause the bath to become decarburizing. If the bath is not rectified, it becomes more viscous than water. Any sludge or salts obtained as a by-product of the heat treatment must be disposed of in accordance with federal, state, and local

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regulations.Methyl chloride bubbled through the bath or the submerging of ammonium chloride pellets in a perforated cage in the bath are more effective methods of rectifying salt baths. The ammonium chloride pellets react with the oxides to regenerate the original neutral salt without sludge formation or bath thickening. To remove dissolved metals from high-temperature baths, graphite rods are introduced at operating temperature. The graphite reduces any metallic oxides to metals that adhere to the rod. The metal can be scraped off and the rod reused.To control the decarburizing tendency of high-temperature baths, test specimens frequently should be hardened by quenching in oil or brine. A file-soft surface indicates a need for more rectification. This test may be supplemented by analysis of the bath. High-heat baths containing in excess of 0.5% BaO are likely to be decarburizing to steel.The following method can be used to rectify austenitizing baths such as salt mixtures No. 2 and 3 of Table 1:

Add 57 g (2 oz) of boric acid for each 45 kg (100 lb) of salt, after every 4 h of operation Insert a 75 mm (3 in.) graphite rod into the bath for 1 h for every 4 h of operation

Controlled Atmosphere FurnacesIn selecting an atmosphere that will protect the surface of tool steel against the addition or the depletion of carbon during heat treatment, it is desirable to choose one that requires no adjustment of composition to suit various steels. An ammonia-based atmosphere (American Gas Association, or AGA, class 601) meets this requirement and has the advantage of being sufficiently reducing to prevent oxidation of high-chromium steels. In the range of dew points generally found in this gas, -40 to -50 °C (-40 to -60 °F), there is no serious depletion of carbon, because the decarburizing action is slow and any loss of carbon at the surface is partially replaced by diffusion from the interior. For applications in which high superficial hardness is important, a carburized surface can be obtained by the addition of about 1% methane (CH4) to the atmosphere. Although ammonia-based atmosphere costs more than an endothermic gas atmosphere, this seldom becomes important because tool treating furnaces generally are comparatively small and therefore require a correspondingly small quantity of gas.Endothermic-based atmospheres are often used for the protection of tool steel during heat treatment. Suggested ranges of dew point for an AGA class 302 endothermic atmosphere when used for hardening some common tool steels are listed in Table 4. Relatively short heating times for hardening small tools allow treatment to be carried out with the theoretical carbon balance of the atmosphere varying over a rather wide range. However, for the hardening of large die sections, the particular composition of the die steel being treated requires careful control of the atmosphere if carburization or decarburization is to be avoided during the relatively long heat-treating cycle.Table 4 Ranges of endothermic-atmosphere dew point for hardening tool steelsData compiled for short times at temperature; furnace dew point; AGA class 302 atmosphere

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Vacuum FurnacesOne of the most important considerations that must be met when heat treating tool steels is that the treatment must be accomplished with minimal change of the surface of the workpiece. Minimizing the exposure to air during heat treatment by minimizing or reducing the quantity of air in a vessel as with creating a partial vacuum is an excellent method for retaining workpiece surface integrity. Vacuum furnaces with pressures of 26 Pa to 1.3 mP (200 to 0.01 m Hg) are possible with the sophisticated pumping equipment integral to vacuum furnaces.Vacuum furnaces have historically been popular with heat-treating processes such as brazing, sintering, and outgassing. More recently, vacuum furnaces have become predominant for hardening of selected tool steels. One reason for the widespread use of vacuum furnaces is the freedom from environmental problems they afford the user. In contrast to salt bath heat treating, disposal problems are eliminated with the use of vacuum furnace heat treating. Another reason for the widespread use of vacuum furnaces is their flexibility. Vacuum furnaces can be designed for operating temperatures in excess of 2760 °C (5000 °F) and can be programmed to run an almost limitless variety of stress relieving, preheating, hardening, and quenching cycles. Design of computer hardware and software will allow these steps to be programmed individually or sequentially to enhance productivity.Hot Wall Furnaces. Until recently, vacuum furnaces were inhibited by technical considerations in their use for hardening of tool steels. Two factors limited their use of vacuum furnaces in early hot wall designs. First, the retort in which the vacuum was developed lost considerable strength when it was heated and would tend to collapse. Secondly, a retort was limited in the type of cooling or quenching techniques which are required by tool steels.Cold Wall Furnaces. Vacuum furnaces now incorporate a heating unit inside a vacuum chamber that is of double-wall construction. Between the two walls, water or coolant is circulated for effective cooling of the vacuum chamber, therefore enabling high-temperature operation. These cold wall vacuum furnaces have been designed by various manufacturers and offer countless variations in size, pumping capacity, heating capacities, quenching methods, speed, computerization, and so on. In cold wall furnaces, the electric heating elements are located inside the retort. The heating elements can be made of a refractory metal (molybdenum) or from graphite rods or cloth. The heating

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elements are surrounded by refractory metal baffles to provide insulation and direct radiant reflection. Centered or positioned within the furnace is a refractory (metal) hearth on which a fixtured or basketed work load can be positioned.Single-Chamber Vacuum Furnaces. A simple vacuum furnace (Fig. 4) consists of one chamber in which the workpiece is both heated and cooled. Cooling or quenching is accomplished by back filling or blowing inert gas across the workpieces. In order to quench rapidly enough to obtain the desired microstructure of tool steel, it is necessary to increase the pressure of the quench gas (usually nitrogen). This is accomplished by high-velocity, high-pressure blowers which have reported cooling gas pressures of up to 60 kPa (6 bar).

The cooling rate required will vary depending on the type of steel used and the size and shape of the workpiece. One must also consider flow patterns and furnace load when evaluating vacuum heat treatment. A variety of vacuum furnace designs have been developed that produce a wide range of cooling rates by varying gas pressures, gas velocities, and gas flow patterns. In some cases, gas quenching may not be adequate to achieve the necessary cooling rate for a component, and other quenching methods may need to be considered (that is, salt bath, fluidized bed, or oil quenching) (see Table 1).Multiple-chamber vacuum furnaces or integrated quench furnaces have been designed to improve throughput or enhance quench rate. Vacuum furnaces typically have thermocouples available at several locations in the furnace as well as on the surface of the load or within the confines of the load itself.Multiple-chamber furnaces (see Fig. 5) allow nearly continuous hardening of components. In such systems three chambers of modules exist:

A purge (loading chamber) A multiple-zone heating chamber A quench chamber

A loaded tray automatically moves into the purge chamber where decompression begins. Once the vacuum level is similar to the level in the heating chamber, the tray or basket is moved through an insulated door for heating. Heating is accomplished by transfer through multiple preheating zones and one final high-heat zone. Meanwhile, another tray has moved into the purge chamber. Once the preprogrammed time interval has elapsed in the high-heat zone, the basket is transferred to the final quench chamber for immersion. Ultimately, the basket is transported from the quench chamber through a door to an unloading tray.

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Additional information is available in the article "Heat Treating in Vacuum Furnaces and Auxiliary Equipment" in this Volume.Furnace Kinetics. The suitability of a vacuum furnace to harden a particular component is governed by many factors, not the least of which is the quenching capability. With gas quenching, the effects of gas variables such as pressure, velocity, and flow patterns are significant. Fundamentally, in the cooling of any steel, the process is limited by:

Gas parameters which control the rate of heat from the surface of the component (surface thermal resistance) Component parameters which control the rate of heat transfer within the component from the center to the

surface (component thermal resistance effect) of the workpieceIn general, the gas parameters predominate in determining the cooling rate in large diameter components (greater than 250 mm, or 10 in., diameter). Both types of parameters must be taken into consideration.Gas Parameters. The gas parameter constituent of heat removal is described by the following equation: Q = h A · .T (Eq 1) where Q is the heat removal rate, h is the heat transfer coefficient, A is the surface area of component, and T is the temperature difference between the component and the gas.During the initial cooling period, the gas temperature has only a minor effect on the workpiece. However, after this initial cooling period, the component cooling rate becomes increasingly sensitive to changes in gas temperature with the cooling rate decreasing as the gas temperature increases.Two important features of furnace design that affect gas temperature are:

Heat exchanger type, location, and size, because these factors control the bulk gas temperature into the furnace

Flow distribution, which controls the local gas temperature around the workpieceHigh gas temperatures usually occur only in the initial cooling period of a workpiece, when the effect of gas temperature, as discussed previously, is minimal.The effect of heat transfer coefficient on the cooling rate of a 25 mm (1 in.) diameter slug is shown in Fig. 6.

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Two practical considerations to be taken into account when increasing either gas velocity or pressure are:High-pressure vacuum furnaces are required to be designed and built to stringent safety regulationsIncreases in both gas velocity and pressure affect the design of the blower and the power required to recirculate the gases (doubling the gas velocity increases the blower power by a factor of eight, while doubling the gas pressure only increases the blower power by a factor of two)The heat transfer coefficient, h, is also a function of the gas properties. The effect of four gases on the cooling of 25 mm (1 in.) diameter slugs is demonstrated in Fig. 9. Nitrogen is usually the gas of choice because:Hydrogen is explosive and must be used with extreme careHelium is expensiveArgon gives poor cooling rates

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Thus, it is evident that the cooling rates of steel components are not only determined by gas parameters such as gas temperature, gas velocity, and gas pressure but also depend on the physical properties (that is, conductivity, density, and viscosity) of the gas itself.In practice, it is the gas velocity and the gas pressure that are the most significant factors in controlling component cooling rates.Component Parameters. Component size, shape, and material properties control the rate of heat transfer within components from the center of the material core to the surface of the material. Material properties (that is, density, specific heat, and thermal conductivity) vary only marginally from one steel to another and hence have been considered constants for the purpose of this discussion. Component size and shape can vary greatly.The effect of diameter on cooling is shown in Fig. 10. At the surface of the component, the cooling rate is inversely proportional to the component diameter; thus, increasing the diameter by a factor of two decreases the cooling rate by a factor of two. The temperature at the center of the component lags behind the temperature at the surface of the component. This effect is more clearly shown in Fig. 11, where the ratio of surface temperature to center temperature of the component is plotted over a wide range of diameters and heat transfer coefficients.

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At low heat transfer coefficient values, gas parameters predominate over the cooling rate and negligible differences exist between the temperature at the surface and the temperature at the center of the component. As the heat transfer coefficient is increased, the component parameters begin to restrict the cooling rate of the component and large differences begin to develop between the temperature at the surface and the temperature at the center of the component. These temperature differences can cause distortion and cracking in large diameter components.It is usually the cooling rate at the center of the component that is of most interest. The variation in center cooling rate in M2 tool steel over the temperature range 1200 to 600 °C (2190 to 1110 °F), is plotted for a range of heat transfer coefficients in Fig. 12.

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For the large 250 mm (10 in.) diameter component, the center cooling rate increases only marginally with large increases in the heat transfer coefficient when compared to increases in the center cooling rate gained in a small component. For such large diameter components, even fast oil or salt quenching (h is approximately 1000 to 5000 W/m 2 · K, or 200 to 900Btu/ft 2 · h · °F) may not provide the center cooling rate required to develop the desired steel hardness properties.Two important conclusions drawn from this discussion of how component parameters affect cooling rates are:

High heat transfer coefficients can cause large variations in temperature between the center and surface of components (particularly as the diameter increases) that may result in cracking and/or distortion

Even high heat transfer coefficients may not be able to cool the center of large diameter components fast enough to harden them adequately

Fluidized-Bed FurnacesFluidized-bed furnaces offer another method of heat treating tool steels (see also the article "Fluidized-Bed Equipment" in this Volume). This method uses a solid rather than a liquid or gas for the heat transfer medium. In general, the furnace is composed of a layer of small mobile particles of an inert refractory (for example, aluminum oxide or silica sand) in a container which is heated and fluidized by a flowing stream of gas. Objects to be heat treated are immersed directly into the bed of particles.A fluid bed results when a gas is passed upward through a bed of small solid particles at a rate fast enough to lift these particles and to create turbulence. This motion of particles, similar to that of a fluid, is shown in Fig. 13. When gas is forced upward through small holes in a supporting plate, two forces meet to raise the particles: the buoyancy of the gas and the retarding force known as aerodynamic drag.

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Fig. 13 Schematic showing principle of the fluidized-bed furnace.(a) Initially, the gas flows upward through the permeable base to agitate the particles as the pressure is gradually increased. (b) Eventually, the gas flow is sufficient to lift the small particles of refractory materials and to transform the particle movement into a violent turbulent motion. Although the particles are actually solid, the fluidized bed simulates the motion of a liquid. Source: Ref 2Most fluidized-bed furnaces are used at temperatures below 1095 °C (2000 °F), although some manufacturers have furnaces capable of treating components to temperatures through 1205 °C (2200 °F). This temperature limitation is related to the exposure damage or wear and tear on the retort materials. Fluidized beds have been designed to perform a wide variety of heat-treating tasks including stress relieving, preheating, hardening, quenching, annealing, and tempering as well as a variety of surface treatments such as carburizing, nitriding, and steam tempering. This discussion will deal primarily with aspects of neutral hardening of tool steels.Heat transfer with fluidized-bed furnaces is particularly good and offers characteristics approaching that of molten salt bath furnaces. Heating properties of the fluid bed can be adjusted through a wide range because there are many parameters that can be varied. Some of the major variable parameters are:

Particle properties (size, shape, bulk density, and absolute density) Properties of the gas used to fluidize the bed (density, viscosity, heat capacity, and thermal conductivity) System properties (flow of gas through the bed, total weight of the particles in a given bed, cross section and

shape of the retort or bed container, and type of permeable plate used to support the particles) One of the major attributes of the fluidized bed is the high rate at which heat can be transferred from the bed of particles to an immersed object. Coefficients of heat transfer on the order of 400 to 740 W/m 2 ·K (70 to 130 Btu/ft 2 · h · °F) are possible. This heat flow rate is two to ten times higher than that provided by normal convection or radiation. In addition, the rate of heat transfer in the full bed is relatively independent of the emissivity of the object which is immersed and the temperature level. The turbulence of the fluidized bed is important in mixing and can effectively minimize thermal gradients within the bed.

Figure 14 illustrates the nature of heat transfer in a fluidized bed. Under curve 1, the bed is nonfluidized in a static state with low heat transfer rates that increase only slightly with velocity. After the minimum fluidization velocity (Vmf) is reached, the heat transfer coefficient, h, increases rapidly over a comparatively narrow velocity range (curve 2). At a certain optimum velocity (Vopt), the heat transfer coefficient reaches a maximum (hmax) and then tends to diminish as the fluidized bed attains more gas-like properties (curve 3). The actual heat transfer rate experienced in the fluidized bed depends on the fluidizing gas velocity and its thermal conductivity, the size and density of the bed particles, their thermophysical properties, and on the geometry and structural design features of the furnace.

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Generally, the principal parameter, other than velocity, that affects the heat transfer coefficient is the particle size with the coefficient rising as the particle diameter is decreased. These parameters result in heat transfer coefficients as high as 570 W/m 2 · K (100 Btu/ft 2 ·h · °F), which is up to five times that which can be obtained in a conventional open-fired furnace and is similar to that obtained in liquid baths. The comparison of the heating rate in a fluidized-bed furnace with other typical modes of heating is shown in Fig. 15.

Fluidized-bed heat-treating furnaces are manufactured by several suppliers and are available in three fundamental configurations. Two of the configurations are fluidized by the products of combustion and are known as internally fired and externally fired types. For the third configuration, known as the indirectly heated type, the fluidization and the heating are accomplished independent of one another. The indirectly heated type is most often used for neutral hardening and therefore is more applicable to tool steel heat treating.Because the heating and fluidization modes of an indirectly heated fluidized-bed furnace are independent of one another, this type of furnace is used where special atmospheres are required by the product. Literally, any gas may be used for fluidization and this type of furnace can accommodate a wide range of processes such as carburizing, carbonitriding, steam treating, and bright annealing. An example of an indirectly heated fluidized-bed furnace is shown in Fig. 16.Although the furnace shown is heated electrically, it should be emphasized that a fluidized-bed furnace may also be fuel fired (simply by replacing the electric elements on the outside with a suitable burner system) or both fuel fired

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and electrically heated. In special configurations, furnaces may also be cooled to operate at subambient temperature conditions.

The fluidized-bed particles offer some similarities to salt baths and can provide a supporting neutral environment. The fluidized particles do not collect on the work surface and therefore there is no dragout or subsequent cleaning required.The aluminum oxide or silica oxide particles can become contaminated but are typically not considered an environmental hazard as are lead and salt compounds used in the other heat treat methods. The workpiece, upon removal from the high-temperature bed, can, however, be exposed to surface contamination such as decarburization during transfer to a quenching media. Because multiple fluidized-bed furnaces or a combination of furnaces are typically used in conjunction with each other during tool steel heat treatment, such factors must be considered in the overall layout of a heat treat department.Heat Treating of Specific Classes of Tool SteelsIntroductionHEAT-TREATING PROCEDURES vary significantly among classes of tool steels and with intended application. The preferred heat-treating and hardening procedures, as well as mechanical properties and applications are discussed in this article with respect to: water- and air-hardening tool steels, oil-hardening and high-carbon, high-chromium cold-work steels, low-alloy and special-purpose high-speed tool steels, and shock-resisting tool steels. Specific examples of heat-treating procedures for specific applications for hot-work tools are given.Water-Hardening Tool SteelsWater-hardening tool steels containing 0.90 to 1.00% C are the most widely used. Carbon content affects heat-treating temperatures as indicated in Table 1, which outlines recommended heat-treating practices for these steels.

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(a) Holding times vary from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large furnace charges.(b) For large tools and tools with intricate sections, preheating at 565 to 650 °C (1050 to 1200°F) is recommended.As a class, water-hardening tool steels are relatively low in hardenability, although they are arbitrarily classified and available as shallow-hardening, medium-hardening, and deep-hardening types. Their low hardenability is frequently an advantage, because it allows tough core properties in combination with high surface hardness. Low cost and adaptability to simple heat treatment are additional advantages offered by these steels.

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Water-hardening tool steels are so termed because they are most commonly quenched in an aqueous medium. There are exceptions, however; for example, thin sections may be satisfactorily quenched in oil with less distortion and danger of cracking than if quenched in water or brine.Example 1: Analysis of Maximum Diameter in Oil-Quenched Water-Hardening Steel Punch Yielding Minimum 60 HRC Hardness at Selected Austenitizing Temperatures.In one plant, it was desirable to harden small-diameter punches in oil to reduce breakage and consequent downtime of the presses. A study was made to determine the maximum diameters of water-hardening tool steels that could be fully hardened to a minimum of 60 HRC by oil quenching. Results of the study, indicating the relationship between austenitizing temperature, type of steel, and punch diameter, are shown in Fig. 1.

Experimentation proved that a greater degree of uniformity was obtained if the punches were normalized prior to hardening. Normalizing temperatures applied were: 870 °C (1600 °F) for punches up to 6.4 mm ( 14 in.) in diameter; 900°C (1650 °F) for those over 6.4 mm ( 14 in.) in diameter. As indicated in Fig. 1, austenitizing temperature varied from 790 to 900 °C (1450 to 1650 °F), depending on punch diameter. The punches were austenitized by being heated vertically in a neutral salt bath. They were also quenched vertically, in a compounded oil containing additives. The quenching oil was maintained at 50 to 60 °C (120 to 140 °F) and circulated up and around workpieces at 190 L/main (50 gal/min).Normalizing. Except in special instances where experience has proved it beneficial (as in the preceding example), normalizing is not recommended for water-hardening tool steels as received from the supplier. Normalizing is recommended for these steels after forging or before reheat treatment, for refining the grain and producing a more uniform structure. Recommended normalizing temperatures are given in Table 1; as indicated, optimum temperature varies with carbon content.Decarburization during air cooling will be minimized if parts are heated in a protective atmosphere or a neutral salt bath.Parts heated in salt are additionally protected during the cooling period by the film of salt that adheres to their surfaces when they are removed from the salt bath. After parts have cooled, the film of salt can be easily removed (except from recesses such as tapped holes) by a water rinse. Additional information is available in the article "Normalizing of Steel" in this Volume.Annealing. Tool steels of the W types are received from the supplier in the annealed condition. Thus, annealing by the user is usually unnecessary. Annealing is applied to forged or cold-worked carbon tool steel to soften it for easier

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machining, to relieve residual stress, and to produce a structure suitable for hardening. Annealing may be done in an atmosphere furnace (provided the furnace is of a type that can be cooled slowly to below 540 °C, or 1000 °F), in a vacuum, or in an ordinary air furnace after the piece has been protected against surface decarburization by being packed in a suitable container with an inert material. Protection against decarburization (but not against oxidation) may be obtained also by copper plating the surface or by applying a surface-protecting paint. (Not all of these paints are equally effective, and some are difficult to remove; the prospective user should investigate such a paint by trying it under his conditions of operation and then inspecting the treated part for decarburization.) The workpiece should be heated to the annealing temperature (Table 1) and held at temperature for from 1 h, for thin sections, to about 4 h, for heavy sections.When the steel has been placed in a pack to prevent surface reactions, a general rule of thumb is to allow the assembly to soak at temperature for 1 h per inch of pack cross section. Work should then be cooled in the furnace at a rate not exceeding 22 °C/h (40 °F/h), to 510 °C (950 °F). Below this temperature, cooling rate is not critical. Hardness after annealing should be in the range of 156 to 201 HB.Stress relieving prior to hardening is sometimes employed to minimize distortion and cracking. The procedure consists of heating the work to 650 to 720 °C (1200 to 1325 °F) and cooling in air. Usually, stress relieving of water-hardening tool steel is limited to complex or severely cold-worked parts.Example 2: Elimination of Cracking in a W2 Piston by Stress Relieving Prior to Hardening.A piston of W2 steel for a pneumatic clay digger varied in section thickness by as much as 6 to 1. Cracking occurred in the cupped end section when the pistons were hardened by conventional practice. Stress relieving or preheating at 675 °C (1250 °F) prior to hardening eliminated this difficulty.In most instances, stress relieving after hardening and grinding is not employed. Periodic stress relieving of tools thathave been in service will reduce the stresses imposed by such service, and is believed to be beneficial in extending servicelife. Temperatures used for this purpose should never exceed those used for tempering the steel after hardening.Austenitizing temperatures for water-hardening tool steels normally vary from 760 to 845 °C (1400 to 1550 °F), asindicated in Table 1. Higher temperatures are sometimes used for special purposes (Fig. 1). Hardenability increases as austenitizing temperature increases. The optimum time at austenitizing temperature is from 10 to 30 min. Preheating is unusual except for very large tools or those with intricate cross sections (such as the W2 piston cited in Example 2). If surfaces are to be protected against scaling or decarburization, an atmosphere furnace, lead bath, or salt bath is required. It is particularly important to protect shallow-hardening steels against scaling and decarburization. Severe scaling can interfere with heat transfer during quenching and slow the required high rate of cooling. Decarburization will produce a soft surface on any tool steel, but in a deep-hardening steel it can be ground off until the underlying hard high-carbon area is reached. Grinding a shallow-hardening steel will frequently expose the soft core.Atmospheres. Excellent results are obtained by austenitizing water-hardening tool steels in a slightly oxidizingatmosphere, as the data in Fig. 2, obtained in tests on type W2, indicate. Oxidizing atmospheres are inexpensive, and are usually produced by controlled direct-firing burners. The light scale that is produced is removed by the vigorous water or brine quench.

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Endothermic atmospheres also are used, but close control is necessary to match the carbon potential of the atmosphere to the carbon content at the surface (Fig. 2). Also, endothermic installations are more expensive than the controlled-burner technique mentioned above.Salt baths are widely used and frequently preferred over other heating mediums for hardening type W tool steels (see the article "Salt Bath Equipment" in this Volume).Example 3: Advantages and Limitations of W1 and W2 Dies Heat Treated in Salt Baths.In one plant, salt baths were found to be superior to atmosphere furnaces for heat treating die sections of W1 and W2(0.90 to 1.05% C) because die sections could be hardened in limited areas by being suspended and only partly immersed in the salt bath, and because long sections, such as die wiper plates measuring 25 by 100 by 760 mm (1 by 4 by 30 in.), could be hardened in a salt bath with less distortion.Salt baths are usually lower in initial cost than endothermic-atmosphere installations. Neutral salts such as No. 3 in Table 1 of the article entitled "Processes and Furnace Equipment for Heat Treating of Tool Steels" in this Volume, are commonly used. A salt of this type will operate satisfactorily in either steel-lined or ceramiclined pot furnaces, but maintenance cost will be less with ceramic linings. Immersed-electrode heating of these furnaces is recommended. High-temperature salt baths will cause severe decarburization (see Fig. 2) if not closely controlled. A recommended method of rectification for control of these baths is indicated in the article, "Processes and Furnace Equipment for Heat Treating of Tool Steels" in this Volume.Other disadvantages of salt baths are that salt dragout necessitates frequent replenishment of the bath, particularly when many small parts are being treated and that salt is sometimes difficult to remove from parts having complex shapes or tapped holes.Lead baths also are used for austenitizing water-hardening steels and have advantages and limitations paralleling those of the salt bath, specifically with complex shapes and tapped holes, as described above. Both the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) have stringent regulations to avoid lead poisoning.Fluidized-bed furnaces (see the article "Fluidized-Bed Equipment" in this Volume) represent a special class ofatmosphere furnaces that exploit the excellent heat transfer properties of a ceramic medium that is liquefied via the application of a gas flow. Fluidized-bed furnaces are extremely versatile furnaces because they have the capability tocontrol blend gases that generate the desired carbon potential. In addition, fluidized-bed equipment can also be applied to processes such as ammonia gas nitriding and steam oxide surface coatings.

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Quenching. To produce maximum depth of hardness in water-hardening tool steels, it is essential that they be quenched as rapidly as possible. In most instances, water or a brine solution consisting of 10% NaCl (by weight) in water is used. Occasionally, for an even faster quench, an iced brine solution is employed. Cooling rate is a function of size of workpiece as well as of quenching medium; for this reason, small pieces can be quenched in oil (Fig. 1). This is particularly useful when heat treating thin-section tools in an atmosphere furnace containing an integral oil quench.Tempering. Water-hardening tool steels should be tempered immediately after hardening, preferably before they reach room temperature; about 50 °C (120 °F) is optimum. Salt baths, oil baths, and air furnaces are all satisfactory for tempering. However, working temperatures for both oil and salt are limited; the minimum for salt is about 165 °C (325°F), and the maximum for oil is usually about 205 °C (400 °F).All parts made of these steels should be tempered at temperatures not lower than 175 °C (350 °F). One hour attemperature is usually adequate; additional soaking time will further lower hardness. Figure 3 shows the effect oftempering temperature on hardness of water-hardening tool steels austenitized at 790, 815, and 845 °C (1450, 1500, and 1550 °F) and quenched in brine.

Fig. 3 Effect of tempering temperature on surface hardness of water-hardening tool steels austenitized at threedifferent temperatures and quenched in brine. Specimens held for 1 h at the tempering temperature in a recirculating-air furnace. Cooled in air to room temperature. Data represent 20 25 mm (1 in.) diam specimens for each steel. Compositions of steels: shallow hardening, 0.90 to 1.00 C, 0.18 to 0.22 Mn, 0.20 to 0.22 Si, 0.18 to 0.22 V; medium hardening, 0.90 to 1.00 C, 0.25 Mn, 0.25 Si, no alloying elements; deep hardening, 0.90 to 1.00 C, 0.30 to 0.35 Mn, 0.20 to 0.25 Si, 0.23 to 0.27 Cr Tools should be placed in a warm 95 to 120 °C (200 to 250 °F) furnace immediately after quenching and then brought to the tempering temperature with the furnace. This is particularly necessary when quenched tools are being accumulated for tempering in a single batch. Allowing quenched tools to stand at room temperature or placing them in a cold furnace will lead to cracking. Except for large pieces, the work will heat at about the same rate as the furnace. The low temperatures used in tempering eliminate the need for atmosphere control. A double temper is frequently used to temper any martensite that may have formed from retained austenite during quenching, and in the first tempering cycle.The resistance to fracture by impact initially increases with tempering temperature to about 180 °C (360 °F) but falls off rapidly to a minimum at about 260 °C (500 °F). This is known as 500 °F embrittlement or tempered martensiteembrittlement (see the article "Embrittlement of Steels" in Volume 1 of ASM Handbook, formerly 10th Edition Metals Handbook). For tools subjected to impact loading, tempering temperature should be selected to give an optimum combination of hardness and impact resistance.Shock-Resisting Tool SteelsRecommended heat-treating practices for shock-resisting tool steels are outlined in Table 2. These steels may be obtained with several variations in composition, for specific applications (for example, S1 steel is available with 0.30 or 0.50% Mo or with up to 0.90% Si). The user of these nonstandard compositions should: (a) obtain from the manufacturer information as to the modifications required in heat treatment, or (b) select a heat treatment recommended for the shock-resisting tool steel of standard composition that most closely resembles the modified

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steel. The latter procedure should be followed only after the treatment has been tried on test samples.

Not rec, not recommended.(a) Lower limit of range should be used for small sections, upper limit for large sections. Holding time varies from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large charges; for pack annealing, hold for 1 h per inch of pack cross section.(b) Maximum. Rate is not critical after work is cooled to about 510 °C (950 °F).(c) Preferable for large tools to minimize decarburization.(d) For open furnace heat treatment. For pack hardening, hold for 12 h per inch of pack cross section.Normalizing is not recommended for the shock-resisting tool steels.Annealing. The high-silicon types (S2, S4, S5, and S6) are susceptible to graphitization and decarburization. Annealing these types at temperatures higher than those indicated in Table 2 may produce a softer structure, but it will also increase the danger of graphitization. The silicon types should not be soaked at temperature. Surfaces should be protected against decarburization by heating in a protective atmosphere or a vacuum furnace, by the use of pack annealing, or by the application of proprietary paints.Pack annealing consists of surrounding parts with inert material inside a closed container, heating the container to the recommended temperature, and slow cooling. The selection of a packing medium for use with shock-resisting tool steels is difficult; the same general practice has produced different results in different plants. Dry silica sand is

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usually satisfactory for type S1, and a combination of new and used carburizing compound is usually satisfactory for S2, S4, and S5. Burned-off cast iron chips, spent pitch coke, lime, and mica are sometimes used, also. Cast iron chips decrease in carbon content and should not be used indefinitely; lime and mica should be used carefully, because they are insulators.Excessive thicknesses of inert material should not be packed around parts, because this complicates handling andlengthens heating time. Wrapping parts tightly in brown paper before surrounding them with packing material helps to keep the surfaces clean. Additional information is available in the article "Introduction to Heat Treating of Tool Steels" in this Volume.Proprietary paints are available that are intended to protect steel surfaces from decarburization during annealing. The use of such paints is simpler than the use of a pack anneal, but not all of these paints are effective. Moreover, considerable difficulty may be experienced in removing such paints after heat treatment. The prospective user should test any such paint on a sample of steel prior to adopting it in practice.Stress relieving before hardening is seldom required for shock-resisting tool steel, except for extremely intricate parts of widely varying section thickness (to minimize distortion and cracking) and parts subjected to excessive stock removal (to relieve stresses induced by machining). Treatment of such parts, which involves no microstructural transformation, consists of heating them to 650 °C (1200 °F), (soaking should be avoided), furnace cooling to about 510 °C (950 °F), and then removing them from the furnace to cool in air. Stress relieving of tools after tempering is seldom done. In some instances, however, increased tool life has been obtained by removing tools from service and stress relieving them (at a temperature no higher than the original tempering temperature) before returning them to service.Example 4: Doubling the Tool Life of Shock-Resistant Tool Steels Used in Swaging Stainless Steels with Stress Relief.In one plant, shock-resisting steel tools used for swaging stainless steel would sink a definite amount after a time inservice. If kept in service, these tools would crack after swaging about 40,000 parts. However, by withdrawing the tools after sinking had ceased and stress relieving them at 230 °C (450 °F) for 1 h per inch of cross section, tool life was more than doubled.Austenitizing temperatures for shock-resisting tool steels vary from 815 to 955 °C (1500 to 1750 °F). Preheating is not mandatory, but it is sometimes desirable for large tools, to minimize distortion, shorten time at the austenitizingtemperature, and speed up production.These steels may be austenitized in electric or fuel-fired furnaces or in salt or lead baths. Generally, for austenitizingtemperatures below 870 °C (1600 °F), a slightly oxidizing environment is best, whereas above 870 °C (1600 °F) areducing atmosphere is required. If a semimuffle fuel-fired furnace is used, the desired atmosphere can be obtained at low cost by adjustment of burners. However, if electrically heated or full-muffle fuel-fired furnaces are used, a prepared atmosphere from an external source is required.Neutral salt baths are a practical means of heating the type S steels. A salt mixture such as No. 3 in Table 1 of the article, "Processes and Furnace Equipment for Heat Treating of Tool Steels" in this Volume, is satisfactory for types S2, S4, and S5, whereas a mixture such as No. 2 in that table will be more suitable for heating S1. A recommended method of controlling these salts to prevent decarburization of the work is indicated in the discussion of rectification in the article, "Processes and Furnace Equipment for Heat Treating of Tool Steels.".If atmosphere furnaces or neutral salt baths are not available, the shock-resisting steels can be heated in a pack of neutral material such as burned pitch coke or cast iron chips. Packing mediums must be free of oil or other contaminants. Before being placed in the pack, tools should be wrapped with heavy brown paper, to prevent packing material from adhering to them as they are removed for quenching. Types S2, S4, and S5 should be quenched soon after they reach the austenitizing temperature; types S1 and S7 are soaked at temperature for 15 to 45 min before being quenched (Table 2). Types S1

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and S7 have the highest hardenability of these steels. The other types, although lower in hardenability than S1 and S7, are higher in hardenability than the W steels.Tempering. Both the tungsten and the silicon types of shock-resisting tool steel resist softening from tempering to agreater degree than carbon tool steels. Secondary hardening does not occur in these steels, except to a minimal degree in some compositions of the tungsten type.The effect of tempering temperature on the hardness of various types and compositions of the S steels is shown in Fig. 4.

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Tools made of shock-resisting steel should be tempered immediately after quenching, or cracking is likely to result,especially if they are quenched in water or brine.Example 5: Study Conducted to Determine Maximum Elapsed Time Required Between Quenching and Tempering Treatments to Prevent Cracking.One plant made an extensive study on how much time could be safely permitted between quenching and tempering

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of tools made of shock-resisting steels. Results of this study are given in Table 3.

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In this same plant, tool records indicate that double tempering is beneficial for tools made from the S steels. The firsttempering operation is done at a temperature 30 to 55 °C (50 to 100 °F) lower than that of the second and final tempering operation.Surface treatments such as carburizing and carbonitriding are often applied to S1 steel. Types S4 and S5 do not takean effective carburized case.Oil-Hardening Cold-Work Tool SteelsRecommended heat-treating practices for oil-hardening cold-work tool steels is summarized in Table 4.

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(a) Holding time, after uniform through heating, varies from about 15 min, for small sections, to about 1 h, for large sections. Work is cooled from temperature in still air.(b) Lower limit of range should be used for small sections, upper limit for large sections. Holding time varies from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large charges; for pack annealing, hold for 1 h per inch of pack cross section.(c) Maximum. Rate is not critical after cooling to below 540 °C (1000 °F).(d) Sections larger than 38 mm (1 12 in.) will be softer.Normalizing is desirable and sometimes necessary for parts that have been forged or heated previously to temperatures much higher than the proper austenitizing temperature, because it produces a more uniformly refined grain structure. Recommended normalizing temperatures are given in Table 4. Work should be held at temperature for 15 min to 1 h, depending on section size; prolonged soaking is not desirable. When tools are to be hardened after normalizing, precautions must be taken in order to avoid decarburization during normalizing. If tools are to be subsequently machined, annealing is recommended in preference to normalizing.Annealing. Finished or semifinished tools made from oil-hardening cold-work steels should be protected fromdecarburization or carburization during annealing. This can be accomplished by the use of dry exothermic furnaceatmospheres. More often, however, it is accomplished by pack annealing, wherein work to be annealed is packed in a box and surrounded with inert protective material, such as clean cast iron chips or 6-to-8 mesh spent pitch coke. Packannealing permits the use of an open furnace; also, slow heating and cooling occur naturally in the packed box. However, it is important that the work be soaked long enough to permit it to reach the annealing temperature. Recommended annealing temperatures, cooling rates, and expected hardness values are given in Table 4.Type O1 steel may also be cycle annealed (Table 5). Cycle annealing offers little advantage for large loads, but withindividual tools that can be conveniently handled in liquid baths or other conventional furnaces, it enables substantialsavings in time.

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Stress Relieving. In most instances, stress relieving of finished tools prior to final hardening does not noticeably lessen distortion during hardening. If extreme dimensional accuracy after hardening is required, tools should be stress relieved after rough machining but before final light machining. A recommended stress-relieving treatment consists of heating to 650 to 675 °C (1200 to 1250 °F), holding at temperature for 1 h per inch of thickness, and then air cooling.Preheating of the O steels will minimize distortion during subsequent hardening. It is almost always required for tools that are to be austenitized in liquid baths. Recommended preheating temperatures are listed in Table 4. Open furnaces can be used for preheating, but if scale-free and oxide-free hardening is required, preheating must be done with atmosphere control.Austenitizing. Recommended austenitizing temperatures for oil-hardening coldwork tool steels are given in Table 4.Work that has been preheated may either be transferred to an austenitizing furnace or be heated to the austenitizingtemperature in the same furnace in which it was preheated.Decarburization and scaling can be effectively minimized in liquid salt or lead baths, and in furnaces with controlledatmospheres (such as endothermic gas, dissociated ammonia, and argon or other inert gases). However, in all of thesethere is some danger of decarburization if conditions are not controlled. Oxides in the molten baths or excess water vapor in the various gases will cause decarburization. The atmospheres of gas-fired or oil-fired semimuffle furnaces can be adjusted to contain from 2 to 4% O2, a condition that will eliminate decarburization but not oxidation. Types O1 and O2 can be satisfactorily austenitized in such an atmosphere, but it is not recommended for types O6 and O7. All type O steels may be austenitized in semimuffle furnaces if packed in inert materials such as spent pitch coke and clean cast iron chips.Adequate time must be allowed to ensure that packed work reaches prescribed temperature. If salt baths are used, a salt mixture such as No. 3 in Table 1 of the article, "Processes and Furnace Equipment for Heat Treating of Tool Steels" in this Volume, is recommended. For a suitable method of controlling this bath, see the discussion of rectification in that article.Quenching. The optimum temperature range for quenching baths consisting of conventional oils is 40 to 60 °C (100 to 140 °F); agitation is recommended. Quenching oils that contain additives (fast oils) increase the cooling rate of the steel and permit more latitude in the operating temperature of the bath. Tools may be quenched in these oils at 80 °C (180 °F) without loss of hardness.Martempering. If control of distortion is particularly important, martempering is sometimes advantageous. Inmartempering, the work is quenched in a bath of oil or molten salt that is usually held about 15 to 30 °C (25 to 50 °F)above the temperature (Ms) at which martensite starts to form from austenite on cooling, and is held in the bath long enough to allow it to attain substantially equalized temperature throughout. The work is then removed from the bath

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and air cooled. The slow cooling through the martensitic transformation range permits the transformation of austenite to martensite to take place uniformly throughout the piece, thus minimizing distortion. Figure 5 presents a comparison of the dimensional changes in tools made of O1 steel that were oil quenched with those in tools of the same steel martempered at 230 °C (450 °F) for 10 min; the martempered tools exhibited markedly less distortion.

Fig. 5 Dimensional changes in O1 tools. Tools sketched, made of O1 steel, were used for comparison ofdimensional changes resulting from martempering at 230 °C (450 °F) for 10 min, and from oil quenching. Fivetools of each design, processed by each method, were measured on 5 different days. (a) Maximum change inflatness along the 180 mm (7 in.) dimension was 0.25 mm (0.020 in.) after oil quenching and 0.005 mm (0.0002 in.) after martempering. (b) Maximum change of the 19 mm ( 34 in.) slot width was 0.1 mm (0.0039 in.) after oil quenching and 0.33 mm (0.0012 in.) after martempering.Tempering. The O steels should be tempered immediately after quenching (preferably before they reach roomtemperature). These steels usually are not tempered below 120 °C (250 °F) or above 540 °C (1000 °F); the mostcommonly used temperature range is from 175 to 205 °C (350 to 400 °F). Tempering times vary with section size. Often, a time at temperature of 1 h per inch of thickness (minimum dimension of heaviest section) or per inch of diameter, with a minimum of 1 h, is used. Typical hardness values obtained with various tempering temperatures for oil-hardening tool steels are given in Fig. 6. The upper curve in each graph represents results from austenitizing at the higher side of the range of temperatures indicated, and the lower curve represents results from austenitizing at the lower side.

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Fig. 6 Hardness as a function of tempering temperature, for oil-hardening cold-work tool steels. Steels O1, O2,and O6 were austenitized at the temperatures indicated, and then oil quenched. For O7 steel, large uniform sections were austenitized at 800 to 830 °C (1475 to 1525 °F) and water quenched, and other sections were austenitized at 830 to 870 °C (1525 to 1600 °F) and oil quenched. Duration of tempering was 1 h. Conventional tools made from the O steels are seldom subjected to multiple tempering or subzero treatment. However, for some special tools, such as gages, where dimensional stability is critical, multiple tempering is desirable. In such instances the workpieces should be cooled to below 65 °C (150 °F) prior to each retempering. Subzero cooling to -75 °C(-100 °F) or lower is also helpful in achieving dimensional stability.Medium-Alloy Air-Hardening, and High-Carbon High-Chromium, Cold-Work Tool SteelsRecommended heat-treating practices for medium-alloy air-hardening cold-work tool steels (group A) and high-carbon high-chromium cold-work tool steels (group D) are summarized in Table 6.

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Not rec, not recommended.

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(a) Holding time, after uniform through heating, varies from about 15 min, for small sections, to about 1 h, for large sections. Work is cooled from temperature in still air.(b) Lower limit of range should be used for small sections, upper limit for large sections. Holding time varies from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large charges; for pack annealing, hold for 1 h per inch of pack cross section.(c) Maximum rate, to 540 °C (1000 °F) unless footnoted to indicate otherwise.(d) To 705 °C (1300 °F).(e) For open furnace heat treatment. For pack hardening, hold for 12 h per inch of pack cross section.(f) Hardness varies with austenitizing temperature.(g) To 650 °C (1200 °F).(h) One manufacturer recommends cooling from 760 to 540 °C (1400 to 1000 °F), then reheating to 730 °C (1350°F) and cooling.Normalizing. Except for type A10 (see Table 6), normalizing is not recommended for any of the steels in groups A and D.Annealing. These steels are usually supplied in the annealed condition by the manufacturer. However, they should be annealed after forging and prior to rehardening. Annealing is required also for previously hardened or welded tools that are to be reworked.Recommended annealing temperatures for the various types are given in Table 6. Tools should be heated slowly anduniformly to the annealing temperature. Slow heating is particularly important if a hardened tool is being annealed.Cycle, or isothermal, treatments may be employed for annealing some A and D steels (Table 5).Stress Relieving. Tools made of A and D steels that cannot be ground after hardening are sometimes stress relievedafter rough machining. This is particularly advisable for delicate tools and tools that vary markedly in cross section. Stress relieving is used also on tools that are machined to final shape, if these tools can be straightened after stress relieving and before final heat treatment. There is little advantage in stress relieving completed tools if they cannot be straightened prior to hardening, because a good preheat will relieve stresses, and the distortion which occurs in either case will remain uncorrected. Recommended temperatures for stress relieving are:

Usually, tools can be stress relieved at these temperatures without surface protection. Tools are commonly held attemperature for 1 h/in. of cross section (minimum of 1 h) and then air cooled.Preheating. Steels of the A and D groups are usually preheated before being austenitized for hardening. Preheatingreduces subsequent distortion in the hardened parts by minimizing nonuniform dimensional changes during austenitizing. Preheating simpler tools made of grades A4, A5, A6, and A10 can often be eliminated if they are

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austenitized in a furnace instead of a liquid bath, because these steels are austenitized at lower temperatures.Recommended preheating temperatures are listed in Table 6. Holding time at temperature is usually 1 h per inch ofmaximum cross section. Preheating temperatures of 790 to 815 °C (1450 to 1500 °F) are used for tools made from A2, A3, A7, A8, or A9, or from any of the D steels. For these higher temperatures, a liquid bath or a protective furnace atmosphere is required in order to prevent scaling and decarburization.Austenitizing. Steels of groups A and D can be austenitized in molten salt baths or in various types of furnaces usinggaseous atmospheres. Because of their lower austenitizing temperatures, types A4, A5, A6, and A10 may also beaustenitized in molten lead, or in open furnaces with oxidizing atmospheres. However, the latter methods are notsatisfactory for the other A steels or for the D steels, because of their higher austenitizing temperatures.If salt baths are used, salt mixtures such as No. 2 or No. 3 in Table 1 of the article, "Processes and Furnace Equipment for Heat Treating of Tool Steels," are recommended; the choice between the two depends on required working temperature range. These mixtures may be rectified (for the prevention of decarburization) by the method indicated in that article. Procedures for austenitizing two different parts made of D2 steel by salt bath and by endothermic atmosphere furnace processes are shown in Table 7.

(a) After austenitizing, inserts 200 by 305 by 38 mm (8 by 12 by 1 12 in.) were double tempered (2 h at 510 °C or 950 °F, air cool; 2 h at 510 °C or 950 °F, air cool) and then nitrided for 48 h at 510 °C (950 °F).(b) Salt bath furnace was immersed-electrode type, 380 by 760 by 915 mm (15 by 30 by 36 in.) deep.(c) Manual loading requires 1 12 min per piece.(d) Furnace was radiant-tube type, 610 by 915 by 455 mm (24 by 36 by 18 in.) high.(e) After austenitizing, die inserts were double tempered (4 h at 190 °C or 375 °F, air cool; 4 h at 190 °C or 375 °F, air cool).(f) Second preheat was necessary because of faster heating rate of salt bath.In some instances, austenitizing will cost more with atmosphere furnaces than with salt baths, and in other instances the reverse will be true. Atmospheres that have proved suitable for austenitizing the A and D steels are endothermic, dry dissociated ammonia, and dry hydrogen.

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Endothermic gas produced by catalytic combination of air and fuel gas is the most widely used atmosphere. Thisrelatively inexpensive gas can be adjusted for desired carbon potential and controlled by dew point.Dry dissociated ammonia (dew point, -50 °C, or -60 °F) and dry hydrogen (dew point, -75 °C, or -100 °F) are used inapplications in which complete freedom from discoloration is required. Vacuum, which excludes all atmosphere, can also be used to austenitize the A and D steels; it is particularly suitable for these steels because their air-hardeningcharacteristics permit slow cooling rates. Like the O steels, the A and D steels may be packed and then austenitized in semi-muffle furnaces. The packing materials and heat-treating procedures employed are similar to those described in the previous section on austenitizing of the O grades.Steels of groups A and D must be held at their austenitizing temperatures long enough to obtain required carbide solution if they are to attain maximum hardness. However, hardening from excessively high austenitizing temperatures will increase the retained austenite. Although retained austenite can be decreased by repeated tempering or subzero cooling (or both), it should be avoided.Quenching. Steels of groups A and D, except D3, will attain maximum hardness by cooling in still air, unless sections are extremely large. However, the hardenability of these steels varies with different types, as indicated in Table 8.

Depending on section size, hardenability, and complexity of shape, the following methods are used to obtain increasingly accelerated cooling of nominally air-hardening steels:

Cool in still air--that is, atmospheric air undisturbed by artificial circulation Cool in fan air--that is, the current of air discharged from a fan Cool in air blast--that is, the discharge from a high-pressure line

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Oil quench to black--that is, quench in oil until the steel is below the temperature at which it glows dullred, then cool to room temperature in air

Oil quench by conventional practiceTempering practices for A and D steels parallel those described for O steels in the preceding section. Tempering isusually begun when the work reaches a temperature of about 50 to 65 °C (120 to 150 °F). However, these steels retain some austenite at this temperature range. To maximize transformation of austenite to martensite, cooling to room temperature, or to subzero temperature (see the article "Cold Treating and Cryogenic Treatment of Steel" in this Volume), is sometimes applied.Opinions vary greatly as to the merits of subzero cooling, because it increases the probability of cracking during thecooling cycle. The more usual practice is to begin tempering when parts reach about 50 to 65 °C (120 to 150 °F) and then double or triple temper. Multiple tempering is effective in decreasing the amount of austenite retained in A and D steels and is a common practice in heat treating them. The general precautions and tempering practices outlined for O steels in the preceding section are followed for the A and D steels. However, because most of the steels in groups A and D (except A4, A5, and A6) soften less rapidly than the group O steels with an increase in tempering temperature (Fig. 7 and 8), higher tempering temperatures can be used for the A and D steels. A minimum tempering temperature of 205 °C (400 °F) is a common requirement for A2, A7, and D steels. Tempering temperatures as high as 550 °C (1025 °F) are frequently used, and even higher temperatures are used for special requirements.

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It will be noted in Fig. 7 and 8 that certain steels (notably A2 and D2) exhibit higher hardness after being tempered atabout 540 °C (1000 °F) than after being tempered at temperatures 55 to 110 °C (100 to 200 °F) lower. This reversal in the usual relationship is known as secondary hardening, and is caused by transformation of retained austenite during tempering at the higher temperatures, near 540 °C (1000 °F). When a steel can be tempered to the same hardness at more than one temperature (for instance, D2 to 58 to 59 HRC), it is advisable to select the highest tempering temperature that will produce the desired hardness. This will yield added toughness and may prevent tool breakage in service.Nitriding. The A steels (particularly A2 and A7) and the D steels are often nitrided after being hardened and

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tempered.Nitriding may be done either in a salt bath or in an atmosphere of dissociated ammonia. High tempering temperatures of 510 to 540 °C (950 to 1000 °F) are used on steels that are to be nitrided. Excessively high nitriding temperatures, with a recommended range of 510 to 540 °C (950 to 1000 °F), will reduce hardness of the base metal and should not be used.Austenitizing at a higher temperature when hardening prior to nitriding will minimize loss of hardness during nitriding of some D steels (note graph for D2 in Fig. 8). For details, see the article "Gas Nitriding" in this Volume.Hot-Work Tool SteelsNominal compositions of chromium, tungsten, and molybdenum types of hotwork tool steels are given in Table 1 of the article entitled "Introduction to Heat Treating of Tool Steels " in this Volume. The steels in the group denoted in Table 1 as "Other Alloy Tool Steels" are included here in the discussion of hot-work tool steels, because they are also used extensively for hot-work applications. Table 9 summarizes the heat-treating practices commonly employed for this composite group of tools steels.

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A, air; O, oil; S, salt; Not rec, not recommended; Not req, not required.(a) Holding time, after uniform through heating, varies from about 15 min, for small sections, to about 1 h, for large sections. Work is cooled from temperature in still air.(b) Lower limit of range should be used for small sections, upper limit for large sections. Holding time varies from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large charges; for pack annealing, hold for 1 h per inch of pack cross section.(c) Maximum rate, to 425 °C (800 °F) unless footnoted to indicate otherwise.(d) For open furnace heat treatment. For pack hardening, hold for 12 h per inch of pack cross section.(e) Temper to precipitation harden.(f) To 540 °C ( 1000 °F).(g) To 480 °C (900 °F).(h) To 370 °C (700 °F).(i) To 205 to 175 °C (400 to 350 °F), then air cool.(j) Temper immediately.(k) For isothermal annealing, furnace cool to 650 °C (1200 °F), hold for 4 h, furnace cool to 425 °C (800 °F), then air cool.

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(l) For isothermal annealing, furnace cool to 670 °C (1240 °F), hold for 4 h, furnace cool to 425 °C (800 °F), then air cool.(m) (p) Cool with forced-air blast to 205 to 175 °C (400 to 350 °F), then cool in still air.(n) Air cool from annealing temperature.(o) Furnace cool, at 22 °C (40 °F), (max) per h, to 425 °C (800 °F), reheat to 595 ± 14 °C (1100 ± 25 °F), furnace cool to 425 °C (800 °F), then air cool.(p) Furnace cool at 17 °C (30 °F) per h to 540 °C (1000 °F), reheat to 790 °C (1450 °F), furnace cool at 11 °C (20 °F) per h to 540 °C ( 1000 °F), then air cool.(q) Heat in pack or in controlled atmosphere.(r) To 50 °C ( 125 °F).(s) Pack heating, 59 to 60 HRC; atmosphere heating, 54 to 55 HRC.(t) For isothermal annealing. hold at 845 °C (1550 °F) for 2 h, furnace cool to 745 °C (1375 °F), hold for 4 to 6 h, then air cool.Normalizing. Because these steels as a group are either partially or completely air-hardening, normalizing is notrecommended except for the high-nickel steel 6F7. After forging or before reheat treating, 6F7 may be normalized byheating to 845 to 870 °C (1550 to 1600 °F), preferably in a controlled atmosphere, and cooling in still air.Annealing. Recommended annealing temperatures, cooling practices, and expected hardness values are given in Table 9. Heating for annealing should be slow and uniform to prevent cracking, especially when annealing hardened tools. Heat losses from the furnace usually determine the rate of cooling; large furnace loads will cool at a slower rate than light loads. For most of these steels, furnace cooling to 425 °C (800 °F), at 22 °C max (40 °F max) per h, and then air cooling, will suffice.For types 6F2, 6F3, and 6H1, an isothermal anneal (Table 9) may be employed to advantage for small tools that can be handled in salt or lead baths or for small loads in batch-type furnaces; however, isothermal annealing has no advantage over conventional annealing for large die blocks or large furnace loads of these steels.To minimize scaling and decarburization, small parts are usually pack annealed, while large and heavy die blocks aremore commonly annealed in controlled-atmosphere furnaces.Packing material should preferably be spent cast iron chips or spent pitch coke-petroleum coke heated to 1205 °C (2200°F) in a semiclosed container to drive off gas and moisture. Lime, sand, or mica is sometimes used, but under such material if mixed with a small amount of charcoal or other carburizing material, the steel may be decarburized. Packing material should be dry and free of all oxidizing materials, should separate all metal surfaces, and should fill the container.Containers should be sealed after packing. Holding time at the annealing temperature is 1 h per inch of containerthickness. The H steels must have a neutral packing material, because they are extremely susceptible to both carburization and decarburization.In controlled-atmosphere furnaces, the work should be supported so that it does not touch the bottom of the furnace. This will ensure uniform heating and permit free circulation of the atmosphere around the work. Workpieces should be supported so that they will not sag or distort under their own weight.Grades 6F4 and 6F7 may be annealed without packing or controlled atmosphere if light scaling is not objectionable,because they are annealed at lower temperatures (Table 9).Stress Relieving. It is sometimes advantageous to stress relieve tools made of hot-work steel after rough machining but prior to final machining, by heating them to 650 to 730 °C (1200 to 1350 °F). This treatment minimizes distortion during hardening, particularly for dies or tools that have major changes in configuration or deep cavities. However, closer dimensional control can be obtained by hardening and tempering after rough machining and prior to final machining, provided that the final hardness obtained by this method is within the machinable range.

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Preheating prior to austenitizing is nearly always recommended for all hot-work steels except 6G, 6F2, 6F3, and 6F5. These four steels may or may not require preheating, depending on size and configuration of the workpieces.Recommended preheating temperatures for all the other types are given in Table 9. Die blocks or other tools for open furnace treatment should be placed in a furnace that is not over 260 °C (500 °F). Work that is packed in containers may be safely placed in furnaces at 370 to 540 °C (700 to 1000 °F). Once the workpieces (or container) have attained furnace temperature, they are heated slowly and uniformly, at 65 to 110 °C (150 to 200 °F) per h, to the preheating temperature (Table 9) and held for 1 h per inch of thickness (or per inch of container thickness, if packed). Thermocouples should be placed adjacent to the pieces in containers. Controlled atmospheres or other protectivemeans must be used above 650 °C (1200 °F) to minimize scaling and decarburization. A slightly reducing atmosphere is especially recommended for preheating of H41.For certain parts--for example, intricate die-casting dies--preheating is omitted. Distortion of such parts is sometimeslessened by packing them and heating them slowly and uniformly throughout the entire range to the quenchingtemperature.Austenitizing temperatures recommended for the hardening of hot-work tool steels are given in Table 9. Rapid heating from the preheating temperature to the austenitizing temperature is preferred for types H16 through H43 and for type 6F4.Except for steels H10 through H14 (see Table 9), time at the austenitizing temperature should only be sufficient to heat the work completely through; prolonged soaking is not recommended. Time cycles for several specific conditions are indicated in the next section of this article entitled "Examples of Heat-Treating Procedure for Hot-Work Tools." The equipment and method employed for austenitizing are frequently determined by the size of the workpiece. For tools weighing less than about 230 kg (500 lb), any of the methods would be suitable. However, larger tools or dies would be difficult to handle in either a salt bath or a pack.Tools or dies made of hot-work steel must be protected against carburization and decarburization when being heated for austenitizing. Carburized surfaces are highly susceptible to heat checking. Decarburization causes decreased strength, which may result in fatigue failures; and on die-casting dies, the molten casting metal will weld on to decarburized surfaces and may cause washout because of poor wear resistance of the decarburized surface. However, the principal detrimental effect of decarburization is to mislead the heat treater as to the actual hardness of the die. To obtain specified hardness of the decarburized surface, the die is tempered at too low a temperature. The die then goes into operation at excessive internal hardness and breaks at the first application of load.An endothermic atmosphere produced by a gas generator is probably the most widely used protective medium. The dew point is normally held from 2 to 7 °C (35 to 45 °F) in the furnace, depending on carbon content of the steel and operating temperature. A dew point of 3 to 4 °C (38 to 40 °F) is ideal for most steels of type H11 or H13 when austenitized at 1010°C (1850 °F).The packing of work in spent pitch coke before heating it for austenitizing has been used extensively in small shopswhere it has not been feasible to invest in special equipment. This procedure is generally used for small dies. New pitch coke is generally heated to 1040 to 1205 °C (1900 to 2200 °F) to burn off any combustibles that may be present as well as to remove any excessive moisture. The spent pitch coke is then sifted to remove the fines (the coke should also be sifted before re-use). Normal procedure for this method is to wrap the workpiece in plain brown wrapping paper and place it in a heat-resistant metal box in the bottom of which is about 50 mm (2 in.) of spent pitch coke. The workpiece should be covered and surrounded with approximately 50 to 100 mm (2 to 4 in.) of spent pitch coke. The cover is then placed on the box and sealed with a refractory paste. The box is then ready to be placed in a furnace, which need not be provided with controlled atmosphere.Quenching. Hot-work steels range from high to extremely high in hardenability. Most of them will achieve fullhardness by cooling in still air; however, even with those types having the highest hardenability, sections of die

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blocks may be so large that insufficient hardening results. In such instances, an air blast or an oil quench is required to achieve full hardness. Hot-work steels are never water quenched. Recommended quenching media are listed in Table 9. If blast cooling is used, air should be blasted uniformly on the surface to be hardened. All air must be dry. When being air quenched, dies or other tools should not be placed on concrete floors or in locations where water vapor may strike them.Some of the hot-work steels (especially the tungsten and molybdenum types) will scale considerably during cooling to room temperature in air. An interrupted quench reduces this scaling by eliminating the long period of contact with air at elevated temperature, but it also increases distortion. The procedure is best carried out by quenching from theaustenitizing temperature in a salt bath held at 595 to 650 °C (1100 to 1200 °F), holding in the quench until the workpiece reaches the temperature of the bath, and then withdrawing the piece and allowing it to cool in air. An alternative, but less precise, procedure is to quench in oil at room temperature or slightly above and judge by color (faint red) when the workpiece has reached 595 to 650 °C (1100 to 1200 °F); the piece is then quickly withdrawn and permitted to cool to room temperature in air. While cooling, the pieces should be placed in a suitable rack, or be supported by wires, in such a manner that air is permitted to come in contact with all surfaces.Steel H23 requires a different type of interrupted quench, because ferrite precipitates rapidly in this steel at 595 °C (1100°F) and Ms is below room temperature. Type H23 should be quenched in molten salt at 165 to 190 °C (325 to 375 °F) and then air cooled to room temperature. This steel will not harden in quenching but will do so by secondary hardening during the tempering cycle.Parts quenched in oil should be completely immersed in the oil bath, held until they have reached bath temperature, and then transferred immediately to the tempering furnace. Oil bath temperatures may range from 55 to 150 °C (130 to 300°F), but should always be below the flash point of the oil. Oil baths should be circulated and kept free of water.Tempering. Hot-work tool steels should be tempered immediately after quenching, even though sensitivity to cracking in this stage varies considerably among the various types (for example, air-quenched 6F4 may be safely kept at room temperature for several hours before tempering, whereas 6G, 6F2, and 6F3 are susceptible to cracking if they are cooled substantially below 175 °C, or 350 °F, before tempering).Hot-work steels are usually tempered in air furnaces of the forced-convection type. Salt baths are used successfully for smaller parts, but for large complex parts salt bath tempering may induce too severe a thermal shock and cause cracking.The effect of tempering temperature on hardness of hot-work tool steels is shown in Fig. 9(a), 9(b), and 9(c).

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Multiple tempering ensures that any retained austenite that transforms to martensite during the first tempering cycle is tempered before a tool is placed in service. Multiple tempering also minimizes cracks due to stress originating from the hardening operation.

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Multiple tempering has proved particularly advantageous for large or sharp-cornered die blocks that are not permitted to reach room temperature before the first tempering operation.Example 6: Heat Treatment of Selected Hot-Work Steels.In one plant where many die blocks are heat treated, standard practice is as follows. When the dies have air cooled to 52 °C (125 °F), they are placed in a tempering furnace maintained at 565 °C (1050 °F). After the dies have reached furnace temperature, they are soaked for 1 h per inch of thickness. The dies are then air cooled to room temperature. Second and third tempering operations are carried out in the same manner, except that temperature may be increased as required in order to obtain desired hardness.Most of the hot-work steels have secondary hardening characteristics; H23 is the most pronounced in this respect (Fig.9(a), 9(b), and 9(c)). As with A2 and D2 (discussed previously), these secondary-hardening hot-work steels should be tempered at the highest temperature at which the desired hardness can be produced.Surface Hardening. Although tools and dies made of the hot-work steels usually have sufficient hardness to performthe tasks for which they were designed, they are occasionally surface hardened to acquire improved resistance to wear or heat for special applications. The two principal processes that have been used for this purpose are carburizing and nitriding.Carburizing is usually limited to hot-work steels having a carbon content of 0.35% or lower. Type H12 has beenreported to achieve a carburized surface hardness of 60 to 62 HRC. The carburized case should be shallow--for example, 0.4 mm (0.015 in.) maximum--or severe embrittlement will occur. The greater the thermal shock (or gradient) present in service--as in die casting--the shallower the case must be.Nitriding. Gas or liquid nitriding is sometimes applied to the hot-work steels to increase resistance to heat or wear, orboth. For instance, dies for hot extrusion are sometimes nitrided to increase service life. One disadvantage of nitriding, however, is the difficulty it imposes on the reworking of tools or dies. Another disadvantage is that it may accentuate heat checking. Hot-work steels should be hardened and tempered before being nitrided, but should be neither decarburized nor carburized.The quality and depth of the nitrided case are influenced by the chemical composition of the steel and by the time and temperature of nitriding. The presence of nitride-forming elements such as chromium and vanadium is helpful to the attainment of a satisfactory case. The fact that most of the hot-work steels reach a secondary hardening peak when tempered in the vicinity of 540 °C (1000 °F) is beneficial, because nitriding is usually accomplished in a range of 510 to 540 °C (950 to 1000 °F) over a period of 15 to 24 h. The nitrided case, in addition to being very hard, may be brittle.Brittleness increases with depth of case; hence, shallow, 0.08 to 0.2 mm (0.003 to 0.008 in.), nitrided cases are usually applied.Examples of Heat-Treating Procedure for Hot-Work ToolsTools and dies made of hot-work steel extend over an extremely wide range of sizes and weights (sometimes up to several tons, as in the largest die blocks). Therefore, details of heat-treating techniques may vary considerably. The following examples give details of procedures that have proved successful in practice:Example 7: Heat Treating of an H21 Hot Extrusion Die.A typical method for heat treating a 75 mm (3 in.) thick, 200 mm (8 in.) OD, 75 mm (3 in.) hole, hot extrusion die made of H21 steel comprises the following:Preheat at 815 to 845 °C (1500 to 1550 °F), either in a slightly oxidizing atmosphere or in neutral salt

Transfer to furnace (6 to 12% reducing atmosphere or neutral salt bath) operating at 1175 °C (2150 °F).Hold in furnace for approximately 20 min after the die has reached 1175 °C (2150 °F)

Col in still air to about 65 °C (150 °F)Temper at 565 °C (1050 °F) for 4 h

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Cool to near room temperature Retemper at 650 °C (1200 °F) for 4 h Air cool

Example 8: Heat Treating of an H11 Mandrel.Mandrels made of H11, used in conjunction with the H21 die in Example 7, above, are heat treated as follows:

Preheat at 760 °C (1400 °F) in a slightly oxidizing atmosphere Transfer to atmosphere furnace (1 to 3% excess O2) operating at 1010 °C (1850 °F) and hold for 20 min

plus 5 min for each inch of thickness Air cool to near room temperature (oil quenching can also be used)

Temper (or, preferably, double temper) for desired hardnessExample 9: Heat Treating of an H13 Die Block.One plant employs the following procedure for heat treating die blocks made of H13 that weigh less than 23 kg (50 lb):1. Insert eyebolt to facilitate handling2. Wrap die block in waxed paper and place in a heat-resistant container on a bed of spent pitch coke 75 to100 mm (3 to 4 in.) deep3. Seal cover on container with asbestos paste4. Place container in furnace (not atmosphere-controlled) operating at 760 °C (1400 °F); bring to furnacetemperature and hold for 4 h5. Raise furnace temperature at 30 °C (50 °F) per h to 1010 °C (1850 °F) and hold charge at thistemperature for 6 h6. Remove die block from container by use of eyebolt7. Cool in still air to 345 °C (650 °F) (temperature-indicating crayons may be used), then place in furnace operating at 345 °C (650 °F) and cool in furnace at 30 °C (50 °F) per h to 95 °C (200 °F). (If the die block has no sharp corners or major changes in configuration, the interrupted cooling may be omitted)8. Remove from furnace and cool in air to 40 °C (100 °F)9. Place in tempering furnace operating at 565 °C (1050 °F), bring to furnace temperature and hold for 8 h, air cool to room temperature and check hardness10. Repeat step 9, except that it may be necessary to increase tempering temperature so that final hardnesswill be 46 to 49 HRCExample 10: Heat Treating of an H13 Die Block.The following procedure has proved successful for heat treating large die blocks, 1590 kg (3500 lb), made of H13 steel:1. Load die block into electrically heated bell-type furnace. The sequence of operations begins whenfurnace temperature reaches 95 °C (200 °F)2. Raise furnace temperature at 30 °C (50 °F) per h to 370 °C (700 °F)3. Introduce nitrogen atmosphere to furnace and increase furnace temperature at 55 °C (100 °F) per h to790 °C (1450 °F); hold for 1 h, then shut off nitrogen, introduce endothermic atmosphere--dew point, 3to 4 °C (38 to 40 °F)--and hold for an additional 5 h4. Increase furnace temperature at 55 °C (100 °F) per h to 1040 °C (1900 °F) and hold for 6 h5. Remove die block and air cool to 65 °C (150 °F)6. Place die block in tempering furnace operating at 205 °C (400 °F); bring to furnace temperature andhold for 7 h7. Increase furnace temperature at 40 °C (100 °F) per h to 565 °C (1050 °F) and hold for 16 h

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8. Air cool to room temperature. Hardness is about 46 to 48 HRC9. Temper die block a second time, repeating steps 6, 7, and 8 but increasing final temperature to 580 °C(1075 °F) because a finished hardness of 42 to 43 HRC is desired10. Temper a third time, repeating steps 6, 7, and 8 without modificationExample 11: Heat Treatment of a Premium Grade H13 Die(Ref 1). Most aluminum die casting dies fail by thermal fatigue of the die surface. Repeated cyclic stressing caused by alternate heating and cooling of the die surface leads to formation of a pattern of check-like cracks that are indicated as protrusions of the die casting surface and eventually expand and merge to cause further failure of the die.Die life varies widely (20,000 to 2,000,000 shots are typical) depending on the aggressiveness of the process conditions, whether the die is preheated prior to being put into service, the metallurgy of the steel used, and the heat treat microstructure.Hot-work die steels containing 5% Cr are normally used (H13 is the most common in the United States) because theyhave:*A good combination of toughness and strength for safety and service*Good hardenability in thick sections*Good hot hardness and strength to resist cyclic stressing at operating temperatures up to 400 °C (750 °F)*Good resistance to tempering during service which gradually lowers the surface hardness and the fatigueStrength Carbide precipitation, other than that produced during tempering, is known to be deleterious to the fatigue life of H13 tool steel. Fast quenching rates required to suppress intermediate precipitation of carbides (such as proeutectoid grain-boundary carbide, pearlite, or bainite) also produce large thermal gradients within the tool which can cause distortion or even cracking. The larger the tool, the greater the risk of cracking.Austenitizing temperature was approximately 1040 °C (1900 °F) for the continuous cooling test. Test results are shown in Fig. 10.

Fig. 10 Effect of intermediate cooling rates on the microstructure and toughness of H13 tool steel austenitizedat 1075 °C (1970 °F). (a) Continuous cooling transformation (CCT diagram) showing variation in microstructurewith varying cooling rates. (b) Variation in toughness as a function of carbides ejected from austenite. Hardnessmaintained at constant value. Source: Ref 1After tempering to 46 HRC, the H13 bars were finish machined to produce specimens for impact energy testing.As shown in Fig. 11, the results of the 12 treatments processed by continuous cooling indicate that three regimes oftoughness occur. The slowest cooling rates of 2 to 4 °C/min (4 to 7 °F/min) produce structures with some pearlite andheavy grain-boundary carbides. These structures result in low toughness values of 3 to 8 J (2 to 6 ft · lbf) at room

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temperature for the Charpy V-notch test. Increasing the cooling rate to 10 to 31 °C/min (17 to 56 °F/min) eliminates the pearlite and provides martensitic/bainitic matrices but considerable quantities of grain-boundary carbide remain. These faster cooling rates produce room-temperature Charpy V-notch toughness values of 16 to 20 J (12 to 15 ft · lbf). The fastest cooling rates result in greatly reduced grain-boundary carbide precipitation and virtually 100% martensitic matrices. These structures have room-temperature Charpy V-notch toughness values of 24 to 26 J (18 to 19 ft · lbf).

Fig. 11 Plot of impact energy versus cooling rate showing three distinct regions of toughness obtained for 12treatments of premium H13 tool steel. Note that all test values lie inside these three regions.This study clearly shows that increasing the quenching rate increased the impact strength but the effect is nonlinear.Structures containing pearlite rates should be avoided by quenching at rates faster than 9.5 °C/min (17 °F/min) untilbelow the nose of the pearlite curve. This results in bainite/martensite structures with some grain-boundary carbides and an anticipated notch impact strength of 15 to 22 J (11 to 16 ft · lbf).Faster quenching at rates of over 58 °C/ min (105 °F/min) will give a martensite structure with less grain-boundarycarbide. The small increase in impact strength to 23 to 26 J (17 to 19 ft · lbf) may not be considered worth the increased risk of cracking and distortion.Example 12: Heat Treating of a 6F3 Forging Die.

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1. Preheat at 260 °C (500 °F)2. When dies have attained furnace temperature, raise temperature at 55 to 85 °C (100 to 150 °F) per h to915 °C (1675 °F); use controlled atmosphere above 760 °C (1400 °F)3. Hold at 915 °C (1675 °F) for 6 h4. Air-blast cool to 175 °C (350 °F) (temperature-indicating crayons used)5. Place in tempering furnace operating at 175 to 205 °C (350 to 400 °F). When dies have attained furnacetemperature, raise temperature at 85 °C (150 °F) per h to 595 °C (1100 °F) and hold for 9 h6. Air cool to room temperature; check hardness7. Retemper, repeating steps 5 and 6 except for final temperature, which will depend on hardness obtainedfrom first tempering6F2, 6F4, and H12 Components Used in Forging of Pinions. The following three examples indicate theprocedures employed in one plant for heat treating 6F2, 6F4, and H12 components used for hot upset forging of pinions.Example 13: Heat Treating of a 6F2 Heading Tool and Gripper Die.Final hardness of 40 to 42 HRC is obtained on this tool by preheating, austenitizing, quenching, tempering, andretempering as follows:1. Preheat at 260 °C (500 °F)2. When dies have attained furnace temperature, raise temperature at 55 to 85 °C (100 to 150 °F) per h to855 °C (1575 °F)3. Hold at 855 °C (1575 °F) for 1 h per inch of thickness4. Quench in oil at 55 °C (130 °F), to 175 °C (350 °F) (temperature-indicating crayons used); transfer asquickly as possible to tempering furnace5. Place in tempering furnace operating at 175 to 205 °C (350 to 400 °F). When dies have attained furnacetemperature, raise temperature at 55 to 85 °C (100 to 150 °F) per h to 595 °C (1100 °F) and hold for 1 h per inch of thickness6. Cool in still air to room temperature; check hardness7. Retemper, repeating steps 5 and 6 except for final temperature, which depends on hardness obtained from first temperingExample 14: Heat Treating of a 6F4 Slab Insert.These inserts, requiring final hardness of 39 to 41 HRC, are heat treated as follows:1. Preheat at 260 °C (500 °F)2. When inserts have attained furnace temperature, raise temperature at 55 to 85 °C (100 to 150 °F) per h to 815 °C (1500 °F) (use controlled atmosphere above 760 °C, or 1400 °F)3. Solution treat at 1020 °C (1870 °F) for 1 h per inch of thickness4. Cool in still air to room temperature. Note: When these inserts were quenched to only 95 °C (200 °F), threads broke out of the die during tapping5. Precipitation harden by heating at 260 °C (500 °F) until temperature of insert equals furnace temperature, raising furnace temperature at 55 to 85 °C (100 to 150 °F) per h to 450 °C (840 °F), holding at 450 °C (840 °F) for 3 h plus 1 h per inch of thickness (minimum time, 4 h), and air cooling to room temperatureExample 15: Heat Treating of an H12 Punch Insert.

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Heat treating to a desired final hardness of 40 to 42 HRC comprises the following procedure:1. Preheat at 260 °C (500 °F)2. When inserts have attained furnace temperature, raise temperature at 55 to 85 °C (100 to 150 °F) per h to 870 °C (1600 °F) (use controlled atmosphere above 760 °C, or 1400 °F)3. Raise furnace temperature to 1010 °C (1850 °F); hold inserts at 1010 °C (1850 °F) for 1 h per inch of thickness4. Quench inserts in still air until they are cool enough to be hand-held. Transfer immediately to the tempering furnace5. Place in tempering furnace operating at 260 °C (500 °F). When inserts have attained furnace temperature, raise temperature at 55 to 85 °C (100 to 150 °F) per h to 595 °C (1100 °F). Hold at 595 °C (1100 °F) for 1 h per inch of thickness6. Cool in still air and check hardness7. Retemper, repeating steps 5 and 6 except for final temperature, which will depend on the hardness that was obtained from the first tempering cycle8. Temper for a third time, if time permitsHigh-Speed Tool SteelsHigh-speed tool steels are used primarily for cutting tools, such as broaches, chasers, cutters, drills, hobs, reamers, and taps. Nominal compositions of these steels are given in Table 1 of the article, "Introduction to Heat Treating of Tool Steels," in this Volume. Recommended heat-treating practices are summarized for two standard groups of high-speed steels and one intermediate group in Table 10 of this article; note that normalizing of high-speed tool steels is not recommended.

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O, oil; A, air; S, salt; Not rec, not recommended.(a) Pack annealing is recommended, for minimum decarburization. Steels should be held at temperature for 1 h per inch of thickness of the container.(b) Maximum. Rate is not critical after work (in pack, if employed) has been furnace cooled to 650 °C (1200 °F).(c) If steels are austenitized in a salt bath, austenitizing temperatures should be 14 °C (25 °F) lower than those in the ranges given.The market for tungsten high-speed steels is basically limited to Europe. Molybdenum high-speed steels are the materials of choice in the United States for machine tool applications.

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Recommended Heat-Treating Procedures Based on Steel Group and TypeSteels in the intermediate group, M50 and M52, are less expensive than standard high-speed steels and may occasionally be used in place of standard high-speed steels.The intermediate high-speed steels do not resist tempering to the same extent as M2, and therefore, they cannot be expected to perform as well as high-speed steels in applications where red hardness is important. For example, in such applications as lathe tools and drills, where the tool is in continuous contact with the workpiece and high surface temperatures are the rule, M50 and M52 steels would not be expected to perform as well as standard high-speed steels.When contact with the workpiece is intermittent or surface temperatures are low, in such applications as hack and band saw blades, blanking dies, and some special woodworking tools, M50 and M52 steels may perform adequately. M50 steel is also used in ball and roller bearing races used at elevated temperatures. Other applications include woodworking tools, hydraulic pump assemblies, pump pistons, and pump vanes. If greater abrasion resistance is required, but not as much as afforded by standard high-speed steels, then M52 may be a logical choice.Annealing. High-speed steel must be fully annealed after forging or when rehardening is required. To minimize decarburization, pack annealing in tightly closed containers is recommended. The packing material can be dry sand or lime to which a small amount of charcoal has been added; burned cast iron chips also are satisfactory. Because the packing material acts to insulate the container and thereby slow down heating, the container should be filled in such a way with the steel to be annealed that a minimum amount of packing material is required.After the steel has reached the annealing temperature range (Table 10), it should be held at temperature for 1 h per inch of thickness of the container and should then be slowly cooled in the furnace (at a rate not exceeding 22 °C, or 40 °F per h) until it reaches a temperature of 650 °C (1200 °F), when a faster rate of cooling is permissible.Preheating. Austenite begins to form at about 760 °C (1400 °F), and preheating for hardening to slightly above this temperature will minimize stresses that might be set up because of the transformation. If the prevention of partial decarburization is important, a preheating temperature of 705 to 790 °C (1300 to 1450 °F) generally will be used. When this is not a problem, preheating at 815 to 900 °C (1500 to 1650 °F) is satisfactory.Double preheating--in one furnace at 540 to 650 °C (1000 to 1200 °F) and in another at 845 to 870 °C (1550 to 1600 °F)-- is often recommended to minimize thermal shock and to increase the productivity of the equipment. If a single preheat is used, the T types of high-speed steels are preferably preheated at 815 to 870 °C (1500 to 1600 °F), and the remaining M types at 730 to 845 °C (1350 to 1550 °F). It is common practice to preheat for twice the length of time required at the austenitizing temperature. Accordingly, to ensure a uniform flow of work, the capacity of the preheating installation is generally twice that of the austenitizing installation. Although preheating is recommended for all high-speed steels, small tools and those that do not incorporate sharp notches or abrupt changes in section, such as small tool bits and solid drill rod blanks, may be placed directly into the austenitizing furnace with reasonable safety. If consumable carbonaceous muffles are used, the preheating temperature must not exceed about 650 °C (1200 °F), because the type of atmosphere they provide is ineffective in preventing decarburization at higher temperatures. Decarburization is detrimental to a heat-treated tool requiring finished edges andsurfaces.Austenitizing. High-speed steels depend on the solution of various complex alloy carbides during austenitizing to develop their heat-resisting qualities and cutting ability. These carbides do not dissolve to

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an appreciable extent unless the steel is heated to temperatures near the melting point. Therefore, exceedingly accurate temperature control is required in austenitizing high-speed steel. Steels containing about 3% or more vanadium may be held at the austenitizing temperature approximately 50% longer than the lower-vanadium types. The relatively pure vanadium carbide phase inherent in the microstructure of the high-vanadium steels is virtually insoluble at temperatures below the melting point and acts to restrict grain growth, thus permitting longer soaking times without detriment. It should be noted that the tungsten,molybdenum chromium, and cobalt contents rather than the vanadium content are the key factors in determining the soaking time required in austenitizing. However, the recommended austenitizing temperatures for these steels should not be exceeded.Single-point tools intended for heavy-duty cutting often can be effectively austenitized at 8 to 17 °C (15 to 30 °F) above the nominal austenitizing temperature. The higher temperature increases alloy solution, temper resistance, and hot hardness, but it also results in some sacrifice in toughness. To prevent rapid wear-out of fine-edged tools such as taps and chasers, austenitizing temperatures of 1040 to 1080 °C (1905 to 1975 °F) are recommended. Punches and dies that do not require maximum hardness may be austenitized for maximum toughness at temperatures 55 to 110 °C (100 to 200 °F) below the nominal temperature. Other adjustments in austenitizing temperature depend on the type of heating equipment employed. Full-muffle furnaces employing a controlled atmosphere rich in carbon monoxide are usually operated at the higher temperature of the recommended range. Salt baths usually are operated 15 to 30 °C (30 to 50 °F) below the top of the range.The effect of austenitizing temperature on the as-quenched hardness of M2 steel is shown in Fig. 12. Below 1175 °C (2150 °F), M2 cannot develop full hardness on quenching, because of insufficient carbide solution. At temperatures above approximately 1230 °C (2250 °F), the as-quenched hardness of M2 decreases because of too much carbon and alloy solution and an excess of retained austenite in the as-quenched steel.

Figure 13 illustrates the improved toughness of M2, as measured by the Izod unnotched impact test, that results from the use of lower-than-normal austenitizing temperatures. Numerous investigators have shown

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that the optimum means for attaining maximum toughness in high-speed steel is through reduced austenitizing temperatures rather than by full austenitizing and over-tempering to an equivalent hardness level.

Quenching. High-speed steels can be quenched in air, oil, or molten salt. However, except for thin tools, which are air quenched between plates to keep them straight, it is customary to quench in oil from muffle or semimuffle furnaces and in molten salt from a high-temperature salt bath. After its temperature has

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been equalized in the salt quench, the tool is air cooled. For large cutters heated in a furnace, an interrupted oil quench is often used to minimize quenching strains and prevent cracking. This consists of cooling the cutters in the oil only until they lose color (about 540 °C, or 1000 °F) and then cooling in air. Cooling rates ranging from 335 to 555 °C/min (600 to 1000 °F/min) are required to develop desirablemicrostructures (no carbide precipitation on grain boundaries) when cooling from austenitizing temperatures down to 760 °C (1400 °F).After quenching, high-speed steel tools usually possess high residual stress, and to prevent cracking, it is good practice to transfer them from the quenchant to a tempering furnace before they have cooled to below 65 °C (150 °F). This is particularly important for large or intricate tools, for which a delay between quenching and tempering or permitting the work to cool to too low a temperature will usually induce cracking. If the work cannot be transferred to a tempering furnace at once, it should be put in a holding furnace maintained at 120 to 205 °C (250 to 400 °F) until a tempering furnace is available. Vacuum furnaces equipped or modified to enhance quenching capabilities have found increasing use in high-speed steel hardening applications. Ongoing vacuum furnace technology developed since 1985 has been incorporated into the production of section sizes up to 75 mm (3 in.) with increases in the mechanical properties of high-speed steels over conventional vacuum furnace equipment produced components. Vacuum hardening has advantages over salt hardening in terms of environmental safety, and energy costs. With proper fixturing (see the article "Heat-Resistant Materials for Furnace Parts, Trays, and Fixtures" in this Volume), vacuum hardening can also minimize distortion.Bainitic hardening has been used in a few applications. To produce a primary bainitic structure, this treatment is performed by arresting the quench from the austenitizing temperature at approximately 260 °C (500 °F), holding for 4 h, then cooling to room temperature. This produces a structure with about 55% bainite and the remainder retained austenite.Subsequent tempering at normal tempering temperature transforms the retained austenite and tempers the bainite to a Rockwell C hardness 1 to 3 points lower than normal for the selected tempering temperature.Partial Hardening to Improve Machinability. Annealed high-speed steel may be partially hardened toapproximately 270 to 300 HB to improve machinability. At these hardnesses, high-speed steels, including the sulfurized types, are less likely to tear in shaving or back-off operations. Typical heat treating to achieve this result consists of heating to 855 to 870 °C (1575 to 1600 °F), holding for at least 1 h, quenching in oil, and tempering at 635 to 665 °C (1175 to 1225 °F) to obtain the desired hardness. If the austenitizing temperature does not exceed 870 °C (1600 °F), this treatment will not cause grain coarsening in the final hardening operation.Certain machining operations, such as drilling and rough milling, should be performed in the annealed condition to obtain maximum tool life.Tempering. As shown in Fig. 15 for an M2 steel austenitized at 1220 °C (2225 °F), the hardness of high-speed steel is directly affected by tempering temperature and time. From the slope of the curves in Fig. 15, it can be seen that M2 undergoes secondary hardening at temperatures above approximately 370 °C (700 °F) and that secondary hardening proceeds at higher temperatures up to about 595 °C (1100 °F), depending on time at temperature. These temperatures approximate the practical limits for most tempering operations; lower temperatures do not evoke the secondary hardening response, and higher temperatures produce hardnesses considerably lower than are usually desired.

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The effect of austenitizing temperature on the tempering characteristics of several high-speed steels tempered from 480 to 675 °C (900 to 1250 °F) is shown graphically in Fig. 17. For all of the steels for which data are plotted, the highest austenitizing temperature results in maximum solution of alloy carbides--which, during subsequent tempering, produces the maximum response to secondary hardening.

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High-speed steels normally are subjected to a minimum of two separate tempering treatments within the range of 540 to 595 °C (1000 to 1100 °F). The actual tempering temperature depends on tool type (for example, drills typically require temperatures of 540 to 555 °C, or 1000 to 1030 °F, and taps typically require temperatures of 560 to 580 °C, or 1040 to 1080 °F). The duration of each treatment is usually 2 h or more at temperature. This process ensures attaining consistent martensitic structures, because the amount of retained austenite in the as-quenched condition will vary significantly because of variations in heat chemistry, prior thermal history, hardening temperature, and quenching conditions.It is essential that the time-temperature combination of the first tempering operation be adequate to condition the retained austenite. Consequently, the first tempering treatment is sometimes longer and at a slightly higher temperature than the second, because the latter is used to temper the freshly formed martensite that develops on cooling from the first temper.Moreover, multiple tempering gains in importance in attaining an acceptable structure if short tempering times are used. The hardness of single and double tempered M2 steel austenitized at various temperatures, as affected by tempering temperature, is shown in Fig. 18.

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Tempering at too low a temperature or for too short a time, or both, may not adequately condition the 20 to 30% retained austenite present after initial quenching, and the steel will still retain abnormally large quantities of austenite after cooling from the initial temper. This austenite will not transform until the steel is cooled from the second temper, and a third temper is then required to temper the martensite so formed. It should be noted that the second temper provides a negligible increase in hardness. In order to carry these reactions as near to completion as possible, high-speed steel should always be cooled to room temperature between tempers. The beneficial effect of multiple tempering on mechanical properties of T1 high-speed steel is shown in Table 11.

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Forced-air furnaces are generally conceded to be the most desirable for tempering high-speed steel, because the heat is transmitted from the heating elements to the work by convection; consequently, the transfer of heat is gradual, and there is little danger of the work cracking as the result of thermal shock. It is advisable to place the work in a tempering chamber maintained in the temperature range of 205 to 260 °C (400 to 500 °F) and to bring the work up to the tempering temperature slowly with the furnace. This is particularly important for large or intricate tools, because too rapid a heating rate may lead to cracking.The very rapid heating rates of molten lead or salt baths, and the attendant thermal shock, usually militate against their successful use for tempering high-speed steel tools of other than simple shape and design, unless they are preheated to about 315 °C (600 °F) before being introduced into the bath.Refrigeration treatment may be employed to transform retained austenite. The application of a refrigeration treatment is recommended for high-alloy high-speed steels such as M42, M3 (class 2), and

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CPM Rex 60. Best results are obtained when the refrigeration treatment is performed after the quenching operation. The hardened or hardened and tempered tool is cooled to at least -85 °C (-120 °F) and then tempered or retempered at normal tempering temperatures. Carburized surfaces will respond satisfactorily to the -85 °C (-120 °F) treatment, even when they have been tempered prior to refrigeration.Nitriding. Liquid nitriding is preferred to gas nitriding for high-speed steel cutting tools because it is capable of producing a more ductile case with a lower nitrogen content.Although any of the liquid nitriding baths or processes may be used to nitride high-speed steel, the commercial bath consisting of 60 to 70% sodium salts and 30 to 40% potassium salts is most commonly employed. The nitriding cycle for high-speed steel is of relatively short duration, seldom exceeding 1h; in all other respects, however, the procedures and equipment are similar to those used for low-alloy steels.The cyanide baths employed in liquid nitriding introduce both carbon and nitrogen into the surface layers of the nitrided case. Normally, the highest percentages of both elements are found in the first 0.025 mm (0.001 in.) surface layer. For carbon and nitrogen gradients, see the section on liquid nitriding.The effect of time in a liquid nitriding bath at 565 °C (1050 °F) on the nitrogen content of the first 0.025 mm (0.001 in.) surface layer of a T1 high-speed steel is shown in Table 12. A nitrogen content of 0.06% was obtained in the first 3 min at temperature, and it gradually increased to 1.09% at the end of a 6-h cycle at this temperature.

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As shown in Table 13, carbon also was absorbed by the steel, at nitriding temperatures as low as 455 °C (850 °F). In a 30-min nitriding cycle, the carbon content of the first 0.025 mm (0.001 in.) surface layer increased with an increase in the nitriding temperature. However, it was reported that only a portion of the carbon was absorbed by the steel, most of the carbon being mechanically attached to the surface, filling microscopic pits. (This pitting is not dangerous under normal conditions, because the pits are shallower than ordinary grinding or machining marks.)

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High-speed steel tools that are nitrided in fresh baths or for short times show steep nitrogen and hardness gradients. To avoid these steep gradients, which are believed responsible for the brittleness of the case after such treatments, the use of longer immersion time, higher temperature, or a thoroughly aged bath is recommended. To avoid brittleness of case when relatively short immersion times are used, the cyanate content of the bath should exceed 6%. These conditions often will lower the surface hardness as well as the hardness gradient.Figure 19 compares the hardness gradients obtained on specimens of T1 high-speed steel nitrided at 565 °C (1050 °F) for 90 min in a new bath and for various lengths of time in an aged bath.

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Nitriding of decarburized high-speed steel tools should be avoided, because it results in a brittle surface condition. For those surfaces that have been softened from grinding, nitriding is frequently employed as an offsetting corrective measure.Liquid nitriding provides high-speed steel tools with high hardness and wear resistance and a low coefficient of friction. These properties enhance tool life in two somewhat related ways. The high hardness and wear resistance lower the abrading action of chips and work on the tool, and the low frictional characteristics serve to create less heat at and behind the tool point, in addition to assisting in the prevention of chip pickup (see the article "Wrought Tool Steels" in Volume 1 of ASM Handbook, formerly 10th Edition Metals Handbook).Plasma nitriding (also known as ion nitriding, glow-discharge nitriding, and the glow-discharge deposition process) is a heat treatment that uses a large electrical potential to ionize (break down) a treatment gas into ions which are attracted to the surface of the workpiece. When the reaction is properly controlled, the hardened case obtained is similar to a liquid nitride case.Detailed information is available in the article "Plasma (Ion) Nitriding" in this Volume.Steam treating produces a nonuniform, soft layer of iron oxide on the surface of finished high-speed steel tools. This layer, approximately 0.005 mm (0.0002 in.) thick, has lubricant-retaining and antigalling properties, and in some applications will improve tool life by reducing tool-edge buildup. The oxide layer is removed from the tool after a short interval of operation; during this interval, the cutting surfaces of the tool develop a burnished surface that adds further to antigalling characteristics.

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Steam treatment requires a special furnace with a sealed retort from which all air can be displaced by steam, which is admitted at controlled rates. The presence of excessive levels of moisture in the furnace prior to the admission of the steam will cause rusting and an unsatisfactory surface finish.A typical processing cycle involves placing the work in the special furnace, heating to approximately 370 °C (700 °F), and equalizing. After a suitable equalizing time, which depends on the load, the steam is admitted at controlled rates for approximately h. The furnace is then partly sealed to develop positive steam pressure, and the temperature is raised to 525 °C (975 °F). The steam can then be shut off and the work removed from the furnace and cooled normally.The treatment produces a blue-black film whose appearance is improved by subsequent dipping in oil. This treatment may sometimes be combined with normal tempering treatments, because the type of film produced is relatively insensitive to temperature up to approximately 580 °C (1075 °F). Steam treating offers an additional advantage for tools hardened in salt baths, because it effectively reduces the pitting that can result from adhering salt.Carburizing is not recommended for high-speed steel cutting tools because of the extreme brittleness of the case so produced. However, it is suitable for applications requiring extreme wear resistance in the absence of impact or highly concentrated loading, such as are encountered with certain types of cold-work dies made from high-speed steel. At the same level of hardness, the carburized layer does not have the heat resistance of normal high-speed steel because carbides in the microstructure are predominantly Fe3C, rather than the complex alloy carbides characteristic of high-speed steel.Carburizing cycles for high-speed steel consist of packing in a carburizing medium, heating to approximately 1040 to 1065 °C (1900 to 1950 °F) long enough to develop the depth of case desired, and air cooling. The usual holding time at carburizing temperature is from 10 to 60 min, to produce a case 0.05 to 0.25 mm (0.002 to 0.010 in.) deep. Deeper cases should be avoided because of the extreme brittleness which develops. This treatment carburizes the surface and serves as the austenitizing treatment for hardening the entire piece. The carburized layer will harden to 65 to 70 HRC at the surface.Hardening of Specific Machine ToolsHigh-speed tool steels are used extensively as materials for broaches, chasers, milling cutters, drills, taps, reamers, form tools, hobs, thread rolling dies, threading dies, tool bits, and bearing components.Broaches require maximum edge hardness because of the continuous cutting action and light chip load to which they are subjected. This indicates a minimum hardness of 65 HRC for the standard grades and 66 HRC for the premium grades of high-speed steel.Broaches should be suspended vertically in the hardening furnace to avoid undue distortion, and should be quenched under controlled and uniform cooling conditions. Broaches should be straightened while still warm from the hardening operation, and should be cooled to at least 65 °C (150 °F) before tempering. These precautions are particularly important for large diameters.Chasers, because they usually are quite small, present no particular problem in hardening with regard to straightness or residual stress. Hardness recommendations for chasers depend largely on the type of application and the pitch of the thread. Recommended hardnesses for chasers used to cut threads in steel are listed in Table 14.

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For cutting cast iron or plastics, chasers should be heat treated to the maximum attainable hardness, because these materials are cut without any significant cutting force but require maximum abrasion resistance. For Acme threads, however, it is sometimes advisable to underharden.Milling Cutters. Fine-tooth cutters and those with fragile forms should be hardened to 63 to 64 HRC. Heavy-duty milling cutters and cutters for use on soft, abrasive materials should be hardened to the maximum hardness obtainable for the particular type of steel.Drills. Hardening techniques for drills vary, depending on the diameter of the drill. Straightness of these tools is extremely important. Various jigging methods are employed, but it is usually advisable to heat treat drills vertically suspended by their shanks in order to reduce distortion in the hardening operation. Straightening is best accomplished in the as-hardened condition before tempering. In tempering, the tempering furnace must not be overloaded, and all drills must receive the correct tempering temperature and time at temperature.Specific recommendations for the hardness of drills for cutting steel are as follows:

Most drills 5 mm ( 316 in.) in diameter and smaller are usually hardened to 63 to 65 HRC. (Drills of this size used for plastics, aluminum, or magnesium may have hardness as high as 65 HRC)

Drills over 5 mm ( 316 in.) in diameter, to 63 to 65 HRC Heavy-duty drills normally use grades of high-speed steel providing hardnesses equal to or higher

than those noted above. (These drills generally are designed for maximum rigidity and require maximum abrasion resistance)

Taps, like drills, are slender in section and require hardening techniques that minimize distortion; this generally means hardening in the vertical position suspended in suitable jigs. Taps should be straightened in the as-hardened condition before tempering. Tempering of these tools must be carefully controlled to allow adequate heating time. Specific hardness recommendations for taps that are to be used to cut steel are listed in Table 14.Reamers encounter a minimum chip load but require maximum wear resistance. For this reason, they are always hardened to the maximum hardness attainable for each grade of steel.Form tools of all types also should have maximum hardness. In general, a minimum of 65 HRC is necessary, and for the premium grades hardnesses ranging from 68 to 70 HRC are frequently desirable.Hobs. Because of their shaving action, hobs require maximum edge hardness. They may become oval in shape if they are not placed in the hardening furnace in the vertical position. Such placement may require

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special fixtures. Techniques and temperatures in both hardening and tempering must be accurately controlled if tools of this type are to be produced successfully and economically.The hardness of fragile tooth forms may have to be reduced to 62 to 64 HRC to avoid breakage, although the lower hardness results in a shorter production life.Thread rolling dies are usually made of A2 or D2 steel, although dies made of high-speed steel frequently afford superior results, particularly in rolling the harder materials. For fragile thread forms, thread rolls should be hardened to 60 to 62 HRC. For heavier thread forms and those used to roll high-strength materials, hardnesses of 63 to 65 HRC are recommended; however, at these higher hardnesses, dies are more susceptible to breakage.Threading Dies. Most threading dies are made of carbon steel; however, button and acorn dies justify the use of high-speed steel. The relation between hardness and thread form for threading dies is the same as that recommended for taps and chasers.Tool Bits. Standard tool bits, as well as cheeking tools, offset-head bits, and other special types, all require maximum hardness. Standard-duty tool bits should be hardened to 65 to 66 HRC, whereas tool bits made from the higher-alloy high-speed steels should be hardened to 67 to 69 HRC when possible.Bearing Components. The heat treatment of M50 high-speed steel bearing components for aerospace applications must be capable of producing a part with high hardness, uniformly fine grain size, and dimensional stability over a wide temperature range.M50 steel has a nominal composition of 0.83C-4.0Cr-4.0Mo-1.0V with a Ms temperature of approximately 163 to 166 °C (325 to 330 °F). The time-temperature transformation (TTT) diagram for M50 is illustrated in Fig. 20.

Virtually any cooling rate capable of cooling the austenitized part to 205 °C (400 °F) or below in 15 min will produce high hardness. To minimize distortion, residual stress and crack susceptibility, a cooling similar to the idealized rate shown in Fig. 20 is desirable.

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The following practices and procedures are recommended for heat treating M50 bearing components to provide optimum bearing properties:

M50 can be satisfactorily heat treated in vacuum or protective atmosphere furnace. However, most bearing manufacturers prefer to heat treat these bearing components in a neutral molten salt bath or baths

Parts should be preheated prior to the austenitizing cycle to minimize the required soak time at the high austenitizing temperature. If a single preheat is employed, a bath temperature of 815 to 870 °C (1500 to 1600 °F) with a cycle of 5 to 15 min is recommended. If multiple preheat baths are available, recommended bath temperatures and cycles are listed in Table 15.

The high-temperature bath cycle is the most critical operation in heat treating M50 steel. Following preheating, parts should be austenitized at 1105 to 1120 °C (2025 to 2050 °F) for 3 to 10 min,depending on cross section and gross load weight. Optimum cycles in the austenitizing bath may beestablished empirically by varying the soak cycle in the high-temperature bath in 12 -min increments andevaluating resultant grain size and hardness. Grain size is more easily measured on as-quenched samples; however, hardness should be checked on parts subsequent to final tempering operations. Ideally, the cycle will be as short as possible to minimize grain growth while producing desired hardness

Following austenitizing, parts should be quenched in 540 to 595 °C (1000 to 1100 °F) molten salt for 5 to 10 min. The quench minimizes internal stresses and the core-to-surface thermal differential prior to subsequent air cooling and martempering operations

Parts should be subjected to a 175 to 190 °C (350 to 375 °F) martemper bath for 5 to 15 min following quench or quench/air cool operations. The martemper bath, which operates between 15 and 30 °C (25 and 50 °F) above the Ms temperature for M50, equalizes core-to-surface thermal differentials and facilitates subsequent transformation of austenite into martensite with minimal residual stress, distortion, or cracking potential. To avoid undesirable intermediate transformation products, the interval between austenitizing and martempering should not exceed 15 min

Following martempering, parts should be air cooled to room temperature prior to washing, tempering, or subzero treatment. The air-cooling equipment and conditions should provide uniform cooling of parts from the 175 to 190 °C (350 to 375 °F) martempering bath to room temperature within 30 to 60 min.

Shorter cooling rates may result in increased residual stress, distortion, or susceptibility to stresscracking

M50 steel requires multiple tempers to provide maximum toughness and dimensional stability. Parts should be subjected to a minimum of three tempers of 540 to 550 °C (1000 to 1025 °F) for 2 to 4 h, with cooling to room temperature between each temper. Failure to cool to below 40 °C (100 °F) between tempers may result in retained austenite. Tempering may be performed either in neutral molten salts or in atmosphere or air furnaces

Subjection to subzero temperatures prior to and/or after initial tempering enhances transformation of retained austenite to martensite. Common deep-freeze cycles for M50 are -70 to -85 °C (-90 to -120 °F) for 2 to 4 h. Use of lower temperatures provides little if any added benefit. The deep-freeze cycle provides maximum benefit when employed before tempering; however, it is not recommended for parts not subjected to martempering or parts susceptible to cracking. When parts are subzero treated before tempering, caution should be exercised to ensure that the total elapsed time between martempering and tempering does not exceed 5 h. Use of prior stress-relief cycles reduces effectiveness of deep-freeze operation. When equipment, time constraints, or

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part design are unfavorable for performing deep freezing prior to tempering, the parts should be subjected to deep freeze between the first and second tempering operations

Parts requiring re-treating should be annealed prior to rehardening to minimize susceptibility todeveloping duplex/nonuniform grain

(a) Time predicated on relative load size/bath capacity

Low-Alloy Special-Purpose Tool SteelsNominal compositions of the low-alloy special-purpose tool steels are given in Table 1 of the article entitled "Introduction to Heat Treating of Tool Steels" in this Volume. These steels are similar in composition to the water-hardening tool steels, except that the addition of chromium and other elements provides the L steels with greater wear resistance and hardenability. Types L1, L3, L4, and L7 are similar to the production steel 52100 and are used for similar applications.Because of their relatively low austenitizing temperatures, the L steels are easily heat treated. Recommended heat-treating practices are summarized in Table 16.

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(a) Holding time, after uniform through heating, varies from about 15 min, for small sections, to about 1 h, for large sections. Work is cooled from temperature in still air.(b) Lower limit of range should be used for small sections, upper limit for large sections. Holding time varies from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large charges; for pack annealing, hold for 1 h per inch of pack cross section.(c) Maximum. Rate is not critical after cooling to below 540 °C (1000 °F).(d) These steels are seldom preheated.(e) Typical average values; subject to variations depending on austenitizing temperature and quenching mediumNormalizing should follow forging or any other operation in which the steel has been exposed to temperatures substantially above the transformation range. For the L steels, normalizing consists of through heating to 870 to 900 °C (1600 to 1650 °F) and cooling in still air. The use of a protective atmosphere is recommended.Annealing must follow normalizing and precede any rehardening operation. Recommended annealing temperatures and cooling rates, as well as expected as-annealed hardness values, are given in Table 16.

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Stress relieving prior to hardening may be advantageous for complex tools to minimize distortion during hardening. A common practice for complex tools is to rough machine, heat to 620 to 650 °C (1150 to 1200 °F) for 1 h per inch of cross section, cool in air, and then finish machine prior to hardening.Austenitizing temperatures recommended for hardening the L steels are listed in Table 16; preheating is seldom employed for steels in this group.Salt or lead baths and atmosphere furnaces are all satisfactory for austenitizing these steels. A neutral salt, such as No. 3 in Table 1 of the article entitled "Processes and Furnace Equipment for Heat Treating of Tool Steels," is recommended.This salt may be deoxidized, for control of decarburization, by the method indicated in the section on rectification of salt baths in the article "Processes and Furnace Equipment for Heat Treating of Tool Steels" in this Volume.Quenching. Oil is the quenching medium most commonly used for the L steels. Water or brine may be used for simple shapes, or for large sections that do not attain full hardness by oil quenching. Rolling-mill rolls made of L7 are an example of parts for which water or brine quenching is used. These steels respond well to martempering.Tempering. Tools made of the L steels should be quenched only to a temperature at which they can be handled with bare hands, about 50 °C (125 °F), and should be tempered immediately thereafter; otherwise, cracking is likely to occur.The tempering characteristics of these steels are plotted in Fig. 21. For most applications, the S steels are used at near-maximum hardness. It is recommended that tools made of any of these low-alloy steels be tempered at a minimum of 120°C (250 °F), even though maximum hardness is desired. Double tempering also is recommended.

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Fig. 21 Hardness of low-alloy special-purpose tool steels after tempering for 2 h

Carbon-Tungsten Special-Purpose Tool SteelsNominal compositions of carbon-tungsten special-purpose tool steels are given in Table 1 of the article entitled "Introduction to Heat Treating of Tool Steels" in this Volume. Recommended heat-treating practices for these steels are summarized in Table 17.

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W, water: B, brine; O, oil.

(a) Holding time, after uniform through heating, varies from about 15 min, for small sections, to about 1 h, for large sections. Work is cooled from temperature in still air.(b) Lower limit of range should be used for small sections, upper limit for large sections. Holding time varies from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large charges; for pack annealing, hold for 1 h per inch of pack cross section.(c) Maximum cooling rate. Rate is not critical after steel has been cooled to below 540 °C (1000 °F).(d) Typical average hardness values; subject to variations depending on austenitizing temperature and quenching medium employed As a group, these steels are shallow hardening and usually are quenched in water or brine. Steel F3, because of the chromium addition, is the highest in hardenability.Normalizing and Annealing. These steels should be normalized after they have been forged or otherwise subjected to temperatures above their hardening temperatures. Normalizing and annealing practices are essentially the same as those recommended in the preceding section (see "Low-Alloy Special-Purpose Steels" ) of this article. Recommendations for normalizing and annealing the F steels are given in Table 17.Stress relieving as outlined previously for the low-alloy special-purpose steels may be advantageously applied also to the F steels. The same procedure as that described for the L steels would be used.Austenitizing. Preheating and austenitizing temperatures recommended for the carbon-tungsten special-purpose tool steels are given in Table 17. Equipment and practices are generally the same as those previously described for the low-alloy special-purpose steels.Quenching. Water or brine quenching causes high distortion in parts made of the F steels. This is often used to advantage in the rehardening of worn dies that have been used for cold drawing of bars and tubes.

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Such dies are flush quenched--that is, a spout of water is directed into the bore, thus causing shrinkage and allowing further use of dies for the same product size.Tempering. Because tools made of the F steels (cold drawing dies, for example) are used mainly for applications requiring wear resistance, they are usually placed in service at or near their maximum hardness. Therefore, tempering temperatures higher than 205 °C (400 °F) are seldom used. The effect of tempering temperature on hardness for the F steels is shown in Fig. 22.

Mold SteelsThe principal use of these type P steels is for plastic molds. However, some steels, such as P4, P20, and P21, are used also for die-casting dies. The several types vary widely in composition (see Table 1 of the article "Introduction to Heat Treating of Tool Steels"), from the unalloyed hubbing iron P1, to P4, P6, and P21, which contain over 5% total alloying elements.The wide variations in composition, method of forming the mold cavity, molding method, and material to be molded are major influences on choice of mold material as well as method of heat treating. The two most common methods of heat treating the mold steels are (1) preharden the steel (or partially machined mold or die) to about 30 to 36 HRC, finish machine, and use at this hardness level and (2) case harden by carburizing. Nitrided molds have proved successful in some instances, but nitriding is not used extensively. When molds are carburized or nitrided, the same procedures are used as for production steels.Heat-treating practices for the mold steels are summarized in Table 18. P21 is a special type of mold steel that is heat treated by the manufacturer and delivered ready for the user to machine and place in operation without further treatment.As noted in Table 18, this steel is hardened by solution treating and aging.

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W, water; B, brine; O, oil; A, air; Not rec, not recommended; Not req, not required;

(a) Holding time, after uniform through heating, varies from about 15 min, for small sections, to about 1 h, for large sections. Work is cooled from temperature in still air.(b) Lower limit of range should be used for small sections, upper limit for large sections. Holding time varies from about 1 h, for light sections and small furnace charges, to about 4 h, for heavy sections and large charges; for pack annealing, hold for 1 h per inch of pack cross section.(c) Maximum. Rate is not critical after cooling to below 540 °C (1000 °F).(d) When applicable.(e) Solution treatment: Hold at 705 to 730 °C (1300 to 1350 °F) for 1 to 3 h, quench in air or oil; approximate solution treated hardness, 24 to 28 HRC. Aging treatment: Reheat to 510 to 550 °C (950 to 1025°F); approximate aged hardness, 40 to 30 HRCAnnealing temperatures and expected resulting hardness values are indicated in Table 18. For some types, such as P1, the annealing temperature is not critical. A more important factor is surface protection, especially if the mold cavities will be formed by hubbing. If surfaces are allowed to carburize, even slightly, during annealing, subsequent rubbing will be impaired.Usually, parts are packed in an inert material such as spent pitch coke and are held at annealing temperature only long enough to become heated through; they are then cooled in the pack to below 540°C (1000 °F), after which they may be removed from the pack. If rubbing is to follow, it is usually preferable to use the lower side of the annealing temperature range to minimize the danger of carburizing, even though annealing at the higher side of the range will result in slightly lower hardness. Atmosphere-controlled furnaces that can be programmed for slow cooling can also be used for annealing.For hubbing deep cavities, two or more in-process anneals are sometimes required.

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When cavities will be formed entirely by machining (sometimes a combination of hubbing and machining is used), annealing usually is neither necessary nor desirable, because slightly harder structures can be machined more easily.Steels as received from the manufacturer are usually suitable for machining. If hardened molds require reworking, they can be annealed as recommended in Table 18.Additional Heat Treatments. Variations in heat treatment, as necessitated by differences in composition, properties, and intended use, are discussed in the following sections for steels P1 to P20.P1 steel, although shown in Table 1 of the article entitled "Introduction to Heat Treating of Tool Steels" as containing no alloying elements, may contain about 0.10% V, which promotes a finer grain after carburizing, with no apparent sacrifice in hubbability. This steel usually is used only for hubbed molds for injection molding of general-purpose plastics.P1 steel can be carburized by any of the regular practices. Whether the steel is reheated to the austenitizing temperature or quenched from a programmed furnace depends on equipment used. Full hardness (Table 18) can be achieved only by water or brine quenching. Practice varies as to working hardness range.A minimum tempering temperature of 175 °C (350 °F) is recommended. This will retain a finished surface hardness of 60 HRC or slightly higher. However, a more commonly desired hardness range is 54 to 58 HRC, which is obtained by tempering at 260 to 315 °C (500 to 600 °F). If the distortion encountered from water quenching cannot be tolerated for a particular mold design, a type of mold steel that can be hardened by oil quenching must be used instead of P1.P2 steel also is a hubbing steel, although it is less easily hubbed than P1. Carburizing and hardening practice and the working hardness range are the same as for P1, except that the alloy content of P2 increases hardenability so that full hardness can usually be obtained by oil quenching, thus minimizing distortion.P3 steel is also hubbed, but it is less easily hubbed than P1 or P2. Except that P3 is usually oil quenched, the carburizing and hardening practice for it is essentially the same as that outlined above for P1. The operating hardness range may vary from 54 to 64 HRC, but common practice is to temper at about 315 °C (600 °F) to achieve a final hardness of 54 to 58 HRC.P4 steel is sometimes used hubbed, but because of its resistance to cold deformation it is more often used for machined molds or dies. Of all the steels in this group, P4 is the most resistant to wear and to softening by tempering. Because of these properties, it is commonly used for injection molding of plastics that require high curing temperatures and for dies used for die casting low-melting alloys. For the latter application, a common practice is to carburize P4 in cast iron chips to obtain a slight increase in carbon content at the surface. The effect of carburizing practice, as well as case and core hardness values after tempering, is shown in Fig. 23.

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Fig. 23 Tempering characteristics of carburized mold steels.(a) Upper curve represents steel carburized in hardwood charcoal 915 to 925 °C (1675 to 1700 °F) for 8 h, air cooled in pack, reheated at 940 to 955 °C (1725 to 1750 °F), cooled in air and tempered. Middle curve represents steel carburized in cast iron chips at 940 to 955 °C (1725 to 1750 °F), removed from pack, cooled in air and tempered. (b) Surface hardness after heating at temperature for 2 h in carburizing compound, oil quenching, and tempering Because of its high alloy content, P4 steel can be hardened by air cooling. However, it is sometimes quenched in oil to minimize scaling during cooling. For use in plastic molds, the most common working range is 56 to 60 HRC, which may be obtained by tempering the carburized and hardened molds at 205 to 315 °C (400 to 600 °F) (see Fig. 23).P5 steel, in which chromium is the major alloying element, approaches P1 in ease of hubbing and has a core strength equivalent to that of P3. After carburizing, a surface hardness of 65 HRC can be achieved by water quenching, or slightly lower values by oil quenching. Choice of quenching medium depends on mold configuration, allowable distortion, and required hardness. A common working range is 54 to 58 HRC; this can be obtained by tempering at about 260 °C (500°F).P6 steel, because it can seldom be annealed to a hardness of less than 183 HB (Table 18), is difficult to hub, and hence it is usually used for machine-cut cavities. It can be carburized by conventional practice. Because of its hardenability, heavy sections of P6 can be oil quenched to full hardness from 790 to 815°C (1450 to 1500 °F). The as-quenched surface hardness is not quite so high as for some other types, because the high nickel content of P6 promotes retention of austenite. Some of this retained austenite is transformed in tempering, with the result that after tempering up to about 260°C (500 °F) the hardness will be little or no lower than that obtained after quenching. By tempering at 315 °C (600 °F), the most common working hardness range (54 to 58 HRC) is obtained. In some plants, a working hardness range of 58 to 61 HRC, obtained by tempering at 260 °C (500 °F), is considered preferable.P20 steel is a popular mold material for either injection or compression molding, and also for die casting low-melting alloys.For injection molding of the general-purpose plastics or die casting of low-melting alloys, P20 is usually used in the prehardened condition. It is available at hardness levels of about 300 HB or slightly higher. In this condition, cavities are machined and the dies or molds placed in service without further heat treatment. Annealed molds or dies can be austenitized at 845 to 870 °C (1550 to 1600 °F), oil quenched, and tempered at 540 °C (1000 °F), to obtain a hardness of about 300 HB.

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Type P20 is often carburized for molds used in compression molding, particularly for molding the more abrasive plastics.Carburizing temperatures no higher than 900 °C (1650 °F) are recommended for this steel, because higher temperatures may impair polishability; otherwise, conventional carburizing practice is used, and molds may be quenched in oil directly from the carburizing temperature. A common working range is 54 to 58 HRC.Tempering characteristics for P20 carburized at two different temperatures are given in Fig. 23.This steel is sometimes nitrided for special applications. Conventional nitriding practice is employed. Before being nitrided, P20 should first be quenched and tempered to about 300 HB as outlined above, and cavities should be machined; following this sequence will ensure freedom from carburization or decarburization.

REFERENCE

G.A. Roberts and R.A. Cary, Tool Steels, 4th ed., American Society for Metals, 1980

K.-E. Thelning, Steel and Its Heat Treatment, 2nd ed., Butterworths, 1984

S.G. Fletcher and C.R. Wendell, ASM Met. Eng. Q., Vol 1, Feb 1966, p 146

J.R.T. Branco and G. Krauss, Heat Treatment and Microstructure of Tool Steels for Molds and Dies, in Tool Materials for Molds and Dies, G. Krauss and H. Nordberg, Ed., Colorado School of Mines Press, 1987, p 94-117

E.J. Radcliffe, Gas Quenching in Vacuum Furnaces: A Review of Fundamentals, Ind. Heat., Nov 1987, p 34-39

J.D. Stauffer and C.O. Pederson, Principles of the Fluid Bed, Met. Prog., April 1961, p 78-82

A. Fennell, Continuous Heat Treating with Fluidized Beds, Ind. Heat., Sept 1981, p 36-38

J.E. Japka, Fluidized-Bed Furnace Heat Treating Applications for the Die Casting Industry, Die Cast. Eng., May-June 1983, p 22-26

ASM Metals HandBook Volume 4 - Heat Treating 1991.