4. metal alloys: structure and strengthening by heat treatment 2005 pearson education south asia...

4. Metal Alloys: Structure and Strengthening by Heat Treatment

2005 Pearson Education South Asia Pte Ltd

4.8 Hardenability of Ferrous Alloys

• The capability of an alloy to be hardened by heat treatment is called its hardenability.

• It is a measure of the depth of hardness that can be obtained by heating and subsequent quenching.



4.8.1 The end-quenching hardenability test

• In this commonly used Jominy test a round test bar 100-mm (4 in.) long, made from a particular alloy, is austenitized.

• It is then quenched directly at one end with a stream of water at 24°C.• Each composition of an alloy has its particular hardenability band.




• Fig 4.20 (a) shows (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels, as obtained from the end-quench test.

• Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment of metals for better control of properties.



4.8.2 Quenching Media

• The fluid used for quenching the heated alloy also has an effect on hardenability.• Agitation is also a significant factor in the rate of cooling. The more vigorous the agitation, the higher is

the rate of cooling.• Water is a common medium for rapid cooling. However, the heated metal may form a vapor blanket

along its surfaces due to the water-vapor bubbles that form when water boils at the metal–water interface.



4.8.2 Quenching Media

• Polymer quenchants can be used for ferrous as well as for nonferrous alloy quenching, and new quenchants are being developed regularly.



4.9 Heat Treatment of Nonferrous Alloys and Stainless Steels

• Heat-treatable aluminum alloys, copper alloys, martensitic stainless steels, and some other stainless steels are hardened and strengthened by a process called precipitation hardening.

• This heat treatment is a technique in which small particles (of a different phase and called precipitates) are dispersed uniformly in the matrix of the original phase.

• Fig 4.21 (a) shows the phase diagram for the aluminum-copper alloy system. (b) Various microstructures obtained during the age-hardening process.



4.9.1 Solution treatment

• In solution treatment, the alloy is heated to within the solid-solution kappa phase and then cooled rapidly—for instance, by quenching it in water.

• This alloy has moderate strength and considerable ductility.



4.9.2 Precipitation hardening

• The structure obtained in A in Fig. 4.21b can be made stronger by precipitation hardening.• This structure is stronger than that in A, although it is less ductile.

Aging• Because the precipitation process is one of time and temperature, it is also called aging, and the property improvement is known

as age hardening. • If carried out above room temperature, the process is called artificial aging.




Aging• Several aluminum alloys harden and become stronger over a period of time at room temperature; this process is called natural

aging.• Natural aging can be slowed down by refrigerating the quenched alloy (cryogenic treatment).• In the precipitation process, if the reheated alloy is held at the elevated temperature for an extended period of time, the

precipitates begin to coalesce and grow.• This process is called overaging, and the resulting alloy is softer and weaker.




Aging• Fig 4.22 shows the effect of aging time and temperature on the yield stress of 2014-

T4 aluminum alloy. • Note that, for each temperature, there is an optimal aging time for maximum strength.




Maraging• This is a precipitation-hardening treatment for a special group of high-

strength iron-based alloys. • The word maraging is derived from martensite age hardening.



4.10 Case Hardening• The heat-treatment processes described thus far involve microstructural alterations and property changes in the bulk of the material or component by means of through hardening.• Several surface-hardening processes are available (Table 4.1):

a. Carburizing (gas, liquid, and pack carburizing)

b. Carbonitriding

c. Cyaniding

d. Nitriding

e. Boronizing

f. Flame hardening

g. Induction hardening

h. Laser hardening



4.10 Case Hardening



4.10 Case Hardening

• Laser beams and electron beams also are used effectively to harden both small and large surfaces, such as gears, valves, punches, and locomotive cylinders.

• These methods also are used for thorough hardening of relatively small parts.• Decarburization is the phenomenon in which alloys containing carbon lose carbon from their surfaces

as a result of heat treatment or of hot working in a medium (usually oxygen) that reacts with the carbon.



4.11 Annealing

• Annealing is a general term used to describe the restoration of a cold-worked or heat-treated alloy to its original properties.

• The annealing process consists of the following steps:

1. Heating the workpiece to a specific range of temperature in a furnace.

2. Holding it at that temperature for a period of time (soaking).

3. Cooling in air or in a furnace.



4.11 Annealing

• Full annealing is a term applied to the annealing of ferrous alloys.• Fig 4.23 shows the heat-treating temperature ranges for plain-

carbon steels, as indicated on the iron-iron carbide phase diagram.



4.11 Annealing

• To avoid excessive softness from the annealing of steels, the cooling cycle may be done completely in still air. This process is called normalizing.

• Fig 4.24 shows the hardness of steels in the quenched and normalized conditions as a function of carbon content.



4.11 Annealing

Process annealing• During process annealing (also called intermediate annealing, subcritical

annealing, or in-process annealing), the workpiece is annealed to restore its ductility, part or all of which may have been exhausted by work hardening during cold working.



4.11 Annealing

Stress-relief annealing• To reduce or eliminate residual stresses, a workpiece generally is subjected to stress-relief annealing (or simply stress

relieving).

Tempering• If steels are hardened by heat treatment, then tempering or drawing is used in order to reduce brittleness, increase ductility

and toughness, and reduce residual stresses.



4.11 Annealing

Tempering• Fig 4.25 shows the mechanical properties of oil-quenched

4340 steel as a function of tempering temperature.



4.11 Annealing

Austempering• In austempering, the heated steel is quenched rapidly from the austenitizing temperature to avoid

formation of ferrite or pearlite.• In modified austempering, a mixed structure of pearlite and bainite is obtained. • The best example of this practice is patenting, which provides high ductility and moderately high strength.



4.11 Annealing

Martempering (Marquenching)• In martempering, the steel or cast iron is first quenched from the austenitizing

temperature in a hot-fluid medium, such as hot oil or molten salt.• In modified martempering, the quenching temperature is lower, and thus, the cooling

rate is higher. The process is suitable for steels with lower hardenability.



4.11 Annealing

Ausforming• In ausforming (also called thermomechanical processing) the steel

is formed into desired shapes within controlled ranges of temperature and time to avoid formation of nonmartensitic transformation products.



Example 4.1 Heat Treatment of an Extrusion Die

The heat treatment of parts to obtain a certain hardness involves several considerations regarding the material and its desired properties. The specific heat-treating process has to be planned carefully, and it often requires considerable experience, as is noted in this example.

A hot-extrusion die 200-mm around, 75-mm long, and having a round hole of 75 mm, is made of H21 hot-worked. A typical method for heat treating such a die suitable for hot extrusion of metals is outlined as follows:



Example 4.1 Heat Treatment of an Extrusion Die

1. Preheat the die at 815° to 845°C, either in a slightly oxidizing atmosphere or in neutral salt.

2. Transfer it to a furnace operating at 1175°C in a 6 to 12% reducing atmosphere or neutral salt bath; hold it in the furnace for about 20 minutes after the die has reached 1175°C.

3. Cool it in still air to about 65°C.

4. Temper it at 565°C for four hours.

5. Cool it to near room temperature.

6. Retemper it at 650°C for four hours.

7. Cool the die in air.Source: Courtesy of ASM International.



4.12 Heat-Treating Furnaces and Equipment

• Two basic types of furnaces are used for heat treating: batch furnaces and continuous furnaces.

Blast Furnace• Box furnace is a horizontal rectangular chamber with one or two access doors through which parts are

loaded. This type of furnace is used commonly and is versatile, simple to construct and to use, and available in several sizes.




Blast Furnace

• Pit furnace is a vertical pit below ground level into which the parts are lowered. This type of furnace is particularly suitable for long parts (such as rods, shafts, and tubing), because they can be suspended by one end and (consequently) are less likely to warp during processing than if positioned horizontally within a box furnace.




Blast Furnace• Bell furnace is a round or rectangular box furnace without a bottom and is lowered over stacked parts that are to be

heat treated. This type of furnace is particularly suitable for coils of wire, rods, and sheet metal.• Elevator furnace is used by loading the parts to be heat treated onto a car platform, rolled into position, and then

raised into the furnace. This type of furnace saves space in the plant and can be especially suitable for metal alloys that have to be quenched rapidly, because a quenching tank can be placed directly under the furnace.




Continuous Furnace

• In this type of furnace, the parts to be heat treated move continuously through the furnace on conveyors of various designs that use trays, belts, chains, and other mechanisms.

Salt-bath furnaces

• Because of their high-heating rates and better control of uniformity of temperature, salt baths commonly are used in various heat-treating perations, particularly for nonferrous strip and wire.




Fluidized beds• Dry, fine, and loose solid particles (usually aluminum oxide) are heated and suspended in a chamber by an upward flow of hot

gas at various speeds. The parts to be heat treated are then placed within the floating particles—hence, the term fluidized bed.

Induction Heating• In this method, the part is heated rapidly by the electromagnetic field generated by an induction coil carrying an alternating

current, which induces eddy currents in the part.




Induction Heating• Fig 4.26 shows the types of coils used in the induction

heating of various surfaces of parts.




Furnace atmospheres• The atmospheres in furnaces can be controlled so as to avoid (or

cause) oxidation, tarnishing, and decarburization of ferrous alloys heated to elevated temperatures.



4.13 Design Considerations for Heat Treatment

• As a general guideline for part design for heat treating, sharp internal or external corners should be avoided; otherwise, stress concentrations at these corners may raise the level of stresses high enough to cause cracking.



• Commercially pure metals generally do not have sufficient strength for most engineering applications; they must be alloyed with various elements which alter their structures and properties. Important concepts in alloying are the solubility of alloying elements in a host metal and the phases present at various ranges of temperature and composition.

• Alloys basically have two forms: solid solutions and intermetallic compounds. Solid solutions may be substitutional or interstitial. There are certain conditions pertaining to the crystal structure and atomic radii that have to be met in order to develop these structures.

Concept Summary



• Phase diagrams show the relationships among the temperature, composition, and the phases present in a particular alloy system. As temperature is decreased at various rates, various transformations take place in microstructures that have widely varying characteristics and properties. Among the binary systems, the most important is the iron-carbon system, which includes a wide range of steels and cast irons. Important components in this system are ferrite, austenite, and cementite. The basic types of cast irons are: gray iron, ductile (nodular) iron, white iron, malleable iron, and compacted-graphite iron.

Concept Summary



• The mechanisms for hardening and strengthening metal alloys involve heating the alloy and, subsequently, quenching it at varying cooling rates. As a result, important phase transformations take place, producing structures such as pearlite (fine or coarse), spheroidite, bainite, and martensite. Heat treating of nonferrous alloys and stainless steels involves solution treatment and precipitation hardening.

• The control of the furnace atmosphere, the quenchants used, the characteristics of the equipment, and the shape of the parts to be heat treated are important considerations. Hardenability is the capability of an alloy to be hardened by heat treatment. The end-quench hardenability test (Jominy) is a method commonly used to determine hardenability bands for alloys.

Concept Summary



• Case hardening is an important process for improving the wear and fatigue resistance of parts. Several methods are available; among them are carburizing, nitriding, induction hardening, and laser hardening.

• Annealing includes several alternative processes (normalizing, process annealing, stress relieving, tempering, austempering, and martempering), each having the purpose of enhancing the ductility and toughness of heat-treated parts.

Concept Summary

4. metal alloys: structure and strengthening by heat treatment 2005 pearson education south asia...

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