heat treatment of tool steel aisi-h-13 and its quantitavive metallography

Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi.

1. Introduction

Tool steel range on high alloyed types of steel, intended primarily for purposes such

as plastics moulding, blanking and forming, die casting, extrusion, forging and wood

working.

Hot-work tool steels are usually delivered in the annealed state. In this condition, the

microstructure consists of a ferritic matrix with embedded globular carbides. Usual

hotworktool steels have a carbide content the annealed condition. To enable the

hardening process it is necessary to dissolve most of the carbides in the matrix. A

correct heat treatment is of great importance for the properties of tools made from

hotworksteels. In this report approximately all heat treatment of Aisi-H13 is carried

out, confirmed through mechanical testing i.e. Hardness testing because hardness is

very important property that is applicable to predict the service life according to

application. Because Hardness also give idea of other mechanical properties also. The

phases present in Aisi-H13, their distribution play important role on its mechanical

properties, therefore, the determination of specific characteristics of microstructures

will be carried out using quantitative measurements on micrographs or metallographic

images. Another attempt to be made to develop a Microsoft excel template for

statistical calculation required to obtain volume fraction, 95% confidence Level, and

Relative Accuracy (%RA).

After getting all the results of above mentioned processes result will be compared to

obtain conclusion of these treatment on basis of heat treatment operation performed

.


2. Literature Survey

2.1 Materaial

2.1.1 Hot Work Steel(AISI H13)

High-quality steels used to make tools for metal cutting and metal forming

operations are known as tool steels. These are usually complex high-alloy steels,

containing relatively large amounts of tungsten, molybdenum, vanadium, or

chromium. Such alloy contents make these steels suitable for applications requiring

high-strength, high-toughness and high-hardness.

H13 combines good red hardness and abrasion resistance with the ability to

resist heat checking.It is an AISI H13 hot work tool steel, the most widely used steel

for aluminium and zinc die castingdies. It is also popular for extrusion press tooling

because of its ability to withstand drastic coolingfrom high operating temperatures.

H13 is produced from vacuum degassed tool steel ingots. This manufacturing

practice pluscarefully controlled hot working provides optimum uniformity,

consistent response to heattreatment, and long service life.

H13 is an outstanding die steel for die casting aluminium and manganese. It is

used for zinc inlong production runs, and also employed successfully for slides and

cores in tool assemblies.

H13 in the hardness ranges from 45/52 HRC is excellent steel for plastic

moulds. It takes a high polish, making it suitable for lens and dinner ware

moulds.Consider using this grade of hot work tool steel for applications where drastic

cooling is requiredduring the operation, and where high red hardness and resistance to

heat checking are important.This grade has found wide acceptance for die casting dies

for zinc, white metal, aluminium andmagnesium. It is also widely used for extrusion

dies, trimmer dies, gripper dies, hot shear blades,casings, and other similar hot work

applications.


i) Machinablllty:- In the thoroughly annealed condition, H13 may be machined

without difficulty. Ithas a rating of 75 as compared with 1 % carbon tool steel,

which has a rating of 100.

ii) Dimensional Stability:- When air quenched from the proper hardening

temperature, H13

Generally expands 0.001 in./in. of cross section.

2.1.2 Principal alloying elements

2.1.2.1 Carbon

Carbon is by far the most important alloying element for the hardening properties of

all steel types, including tool steels. As a rule of thumb, hardenable steels should

contain at least ~0.2 wt-% carbon dissolved in the iron matrix. At carbon contents up

to ~1 wt-% the matrix hardness is continuously increasing and it reaches a maximum

of ~65 HRC (plain carbon steels).

On designing tool steels stoichiometric considerations must assure enough carbon to

provide matrix hardness and to form desired carbides such as V8C7, - carbide and

Cr7C3 (~1800- 3000 HV [8]) during heat treatment. The optimum carbon content is

attained when all alloying elements have formed carbides in a hardened and tempered

matrix [4], [5]. Carbon itself promotes formation of MC type carbides [6].

Figure 1 Hardness as a function of carbon content


2.1.2.1.1Contents Of Carbon In Tempered Martensite

Merely considering the Fe-C binary phase diagram, it is seen that ferrite (bcc) as a

maximum can contain 0.02 wt-% carbon in solid solution. This corresponds to one

carbon atom per ~500 unit cells of ferrite. However, tempered martensite (which is

the matrix constituent of all tool steel types) can be considered as ferrite which is

stable with ~0.2 wt-% carbon in solid solution [7], [5]. The reason is that tempered

martensite contains a high concentration of lath or plate boundaries originating from

the parent martensite where carbon can be localised. In lath type martensite where the

dislocationdensity is relatively high, carbon can additionally be localised at

dislocation cores.

Figure 2 the metastable iron equilibrium phase diagram

2.1.2.1.2Localization Of Carbon In Tempered Martensite

The ferrite lattice has two principal locations where carbon can be positioned i.e.

tetrahedral- and octahedral interstices; for each unit cell there are six tetrahedral

spaces and six octahedral spaces (see figure 3). At both types of interstices, the vacant

volume is less than the atomic volume of a carbon atom thus presence of carbon

strains the ferrite lattice. Since the bcc lattice is relatively weak in <100> directions

due to relatively few nearest and next nearest neighbours, carbon atoms are preferably


positioned in octahedral interstices, despite larger interstices in tetrahedral positions

[7].

Carbon atoms are randomly distributed in the non-stressed lattice (primarily at

octahedral positions). If however, an external stress is applied the distribution is

affected. An applied tensile stress parallel to the [100] direction promotes the uptake

of carbon at the positions in figure3) in order to reduce the total strain energy in the

system [9].

Figure 3Positions of carbon atoms in the ferrite bcc lattice. (a) The bcc unit cell. (b) A

carbon atom positioned in a tetragonal interstice. (c) x, y, z is octahedral interstices.

(d) Preferred site when applying an external tensile stress in the x-direction.

The reduction in total strain energy for carbon atoms positioned in the strained

octahedral sites can be understood in terms of the two closest neighbours i.e. the ―a/2-

neighbours‖ are dragged further apart, thus making more space, and thus a carbon

atom fits in better.

2.1.2.2 Chromium

In tool steels chromium will form carbides of the types Cr23C6 and some Cr7C3 during

annealing depending on the chromium content. These carbides dissolve during

austenitisation at temperatures exceeding ~900 °C and are totally dissolved at ~1100

°C. [4] Although consequently the Ms- and Mf temperatures are lowered, the addition

of chromium is found to increase the hardenability (e.g. on lowering of Ms and Mf vs.

impeding nucleation and growth of pearlite and bainite, the latter effect is the

stronger).


Chromium improves the cutting performance due to formation of wear resistant

carbides, and improvement of the tempering resistance [5], [4].

2.1.2.3Tungsten And Molybdenum

Tungsten and molybdenum exhibit similar effects, and on atomic level they are more

or less interchangeable: 1 wt-% Mo equals 1,6-2 wt-% W (same atom-%). An

important difference is that the molybdenum steel types (e.g. molybdenum high-speed

steels) have a significant greater tendency towards decarburization than tungsten steel

types at the same Weq, making heat treatment of these (molybdenum containing)

steels more difficult [8].

Both W and Mo lower the solidus temperature (The effect is more pronounced for Mo

. Likewise addition of either W or Mo narrows the domain where austenite is stable

(Mo to a greater extent -see figure 4). The secondary hardening- and cutting

performance of tool steels is enhanced proportional with Weq.

W encourages the formation of M6C type carbides [6] (M is either W, Fe and Mo or a

combination) commonly denoted as (Fe,Mo,W)6C or -carbide. These carbides

dissolve in the austenite matrix at temperatures ranging from ~1150 °C to the solidus

temperature, in practice they do not dissolve completely.

Figure 4 Equilibrium diagrams for Fe-Mo and Fe-W respectively. Note that the

division is not the same on the temperature scale.


On heat treating most tool steel types the austenitisation temperature is kept well

below 1150 °C. When an austenitisation temperature in that range is required, the

holding time is usually only a few minutes. Hence, the W and/or Mo containing

carbides present in the as delivered condition will not dissolve during conventional

heat treatments. The fraction of Mo and W bounded as carbides tights up carbon,

improves the hardenabillity by raising Ms and Mf. The fraction of W and Mo in solid

solution lowers Ms and Mf, but may benefit by slowing down pearlite and bainite

nucleation and growth.

Molybdenum promotes formation of M2C type carbides [6]. These carbides become

unstable at elevated temperatures, and at ~750 °C they transforms to M6C type

carbides by reaction with Fe. [10], [6]. Addition of both elements results in grain

refinement [5]

2.1.2.4 Vanadium

Originally vanadium was used as a scavenger to remove slag, impurities, and to

reduce nitrogen dissolved in the matrix and to act as de-oxidant during the production

of the steel [11]. Soon it was found that vanadium formed very hard and thermally

stable MCtype carbides usually 2 as isolated particles. These carbides improve the

resistance against abrasive wear and provide very good cutting performance [6], [4],

[5]. Vanadium carbides are very limited soluble in the matrix, hence addition of

vanadium will not delay the rate of diffusional decomposition of austenite but raises

the Ms- and Mf temperature by binding carbon (forming carbides), thereby improving

the hardenability. Besides adding of vanadium results in grain refinement of the

matrix [5].

2.1.2.5 Manganese

Manganese is present in most commercial steels. It increases the depth of hardening

and increases the Y/ UTS ratio. Manganese containing steels can be hardened in oil,

even though manganese augments the retained austenite content [5], [10], [12]

2.1.2.6 Cobalt

Cobalt is the only alloying element in HSSs, which can appreciably increase the

thermal stability up to ~650 °C and secondary hardness up to 67- 70 HRC [10], but it

reduces the toughness and wear resistance [4]. Addition of cobalt causes the solidus


temperature to rise. During austenitisation of cobalt containing steels it is therefore

possible to dissolve a larger fraction of the carbides and thereby enhance the

hardenability. The high austenisation temperature results in a relatively large amount

of retained austenite after quenching, but this effect is somewhat compensated by a

lower stability of austenite owing to cobalt. [4], [11].

2.1.2.7 Silicon

Alloying with silicon raises the solubility of carbon in the matrix and hence the as-

quenched hardness. It has virtually no influence on the carbide distribution [10], but it

promotes the formation of M6C type carbides [6]. During steel production up to 0.2

wt-% silicon is added, primarily to react with oxygen e.g. silicon act as a de-oxidiser.

If more than 0.2 wt-% silicon is added, it serves to improve the deep hardening

properties. Additions up to ~1 wt-% provides hardness and improves temper-stability

but reduces the ductility. At high concentration, silicon causes embitterment [5], [4],

[10]. In general silicon improves resistance against softening of martensite, and

displaces tempered martensiteembrittlement to higher temperatures [13].

2.1.2.8 Nickel

Addition of nickel increases the strength of the steel by entering into solid solution in

ferrite . It is used in low alloy steels to increase toughness and hardenability. Presence

of nickel reduces lattice distortion and cracking during quenching [12].

2.1.3Alloying Element Carbides

2.1.3.1 Formation

The carbide forming elements are substitutionally dissolved in the iron lattice (ferrite

or austenite). Generally, these elements cause local distortion of the host lattice

because their atomic radii are different from that of iron . The total strain is minimised

if these atoms diffuse to locations resulting in less total lattice distortion e.g.

dislocation cores or grain boundaries. Hence, there is a driving force for diffusion of

alloying elements to these sites [7].


Figure 5.Bright field micrograph of an AISI M3:2 type high-speed steel in

quenched and tempered condition. Different sized carbides are present in a matrix

of tempered lath martensite.

Diffusion of substitutionally dissolved elements is temperature dependent. Even in

alloys, that contain a relatively large amount of strong carbide forming elements, no

experimental evidence has bean found, that alloying element carbides or clusters of

the type X-C are formed below ~300 °C [9].

At temperatures above ~500 °C (lower than typical tempering temperatures for tool

steels) the diffusion of alloying elements becomes significant, and they will start to

develop carbides [14].

Alloy-carbide grows at the expense of cementite (Fe3C alloy-carbide), either by in

situ transformation (nucleation at cementite/ferrite interfaces followed by growth) or,

following dissolution of cementite, by separate nucleation and growth in energetically

favourable locations as described above [7].


The effect of precipitation of alloy-carbides is evident, especially in high speed steels,

where precipitation of fine and ultra-fine alloy-carbides at ~550 °C is responsible for

the secondary hardening effect giving these steels red hardness [17].

2.1.3.2 Strengthening By Carbides

Carbides contribute to strengthening of tool steels in two different ways.

1. Since especially the alloy-carbides are significant harder than the matrix, carbides

provide resistance against abrasive wear.

2. They contribute to the high yield strength of especially some tool steels by

impeding the mobility of matrix dislocations.

Ad. 1. Relatively large carbides (ranging from 1-6 μm and up to 25 μm in powder

metallurgically and conventionally processed tool steels respectively) embedded in

the matrix provides resistance against abrasive wear, especially if they are

homogeneously distributed. [18]

Ad. 2. Precipitated (alloying element) carbides provide enhanced yield strength by

hindering of dislocation movement. Precipitates/carbides intersect matrix-slip-planes

in a random fashion during growth. When a dislocation gliding in its matrix-slip-plane

meets a precipitate, it is forced to either cut through or around it, and it will choose

the route offering lowest resistance. Figure 6 shows how a dislocation typically is

effected by obstacles. By definition, obstacles are considered strong or weak

depending on whether the angle a dislocation bends in its vicinity is large or small,

respectively. An obstacle effecting the entire dislocation line to approximately the

same extent is termed diffuse, if not it is termed localised.


Figure 6.A dislocation moving from the position of the full to the dashed line. The

obstacles of mean spacing L exerts diffuse forces. The obstacles forces is strong in (a)

and weak in (b).

Dislocations from the matrix cannot move through incoherent precipitates such as

primary carbides but must bow around it. Often a dislocation passes precipitates

according to the Orowan mechanism illustrated in figure 7.. The dislocation loops

formed around the precipitates decreases the effective spacing between the

precipitates thereby increasing the so-called Orowan stress which is inversely

proportional to the distance between the precipitates [16]. Accordingly, the force that

the precipitates exert on the next dislocation that wants to pass is increased.

Figure .7.Illustration of the two principal steps in the Orowan mechanism.


It should be noted that other features than carbides (or rather precipitates) impede

movement of dislocations, e.g. substitutionally dissolved atoms and grain boundaries.

2.1.4 Tool Steel

Tool steels are alloy steels that are used to cut or machine other materials. Tool steels

contain various levels of Cr, Ni, Mo, W, V, and Co. The Categories of tool steels are

2.1.4.1 Hot-Work Steels

Hot work steels exhibit very good thermal resistance against softening, at the

(elevated) working temperature or during heat treatment [11]. All hot-work steels are

described by the prefix H in the AISI nomenclature, and contain typically a relatively

low carbon content of 0.30- 0.40 wt-% carbon. Type H steels are divided into three

groups according to the principal alloying element providing red hardness. Some of

the highly alloyed Group H steels resemble HSSs but have a lower carbon content as

well as a lower alloying element content [15].

:

M series Molybdenum high-speed steels

T series Tungsten high-speed steels

Cr series Chromium hot-work steels

H series Molybdenum hot-work steels

A series Air-hardening medium-alloy cold-work steels

D series High-carbon high-chromium cold-work steels

O series Oil-hardening cold-work steels

S series Shock-resistant steels

L series Low-alloy special-purpose tool steels

P series Low-carbon mould steels

W series Water-hardening tool steels

2.1.4.2Chromium Hot-Work Steels

These steels have compositions described by the H10 to H19 standards. They are

relatively low alloyed with contents of chromium of ~3- 5 wt-%. The principal

alloying elements are carbon, chromium, tungsten and in certain cases vanadium. The


low alloy content promotes toughness at the relatively low normal working hardness

of ~40- 55 HRC. shows that these steels have relatively high Ms- and Mf

temperatures, and they may be air hardened to full working hardness for sections up

to 300 mm in thickness [19], [5]. Chromium hot-work steels are the most widely used

for forging and die casting applications [11].

2.1.4.3 Tungsten Hot-Work Steels

These steels are called type H steels. The AISI types H21 to H26 have qualitatively

and quantitatively almost the same principal alloying elements as the HSSs, but

contain less carbon. Compared to HSSs tungsten hot-work steels exhibit higher

toughness, but otherwise similar characteristics as HSSs. In fact, type H26 is merely a

low carbon version of T15 .The rather high alloying contents of type H steels provides

enhanced thermal stability at elevated temperatures and makes them more prone to

brittleness at the normal work hardness of 40- 55 HRC relative to chromium hot-work

steels. Among the hot-work steels, H types are the hardest. Though it is possible to

air-harden these steels, they are commonly hardened in oil or salt baths as to prevent

scaling. [19], [11], [17] Examples of applications are extrusion dies for brass and

bronze and hot punches [11].

2.1.4.4 Molybdenum Hot-Work Steels

At present only two Mo-type hot work steels are in use, H42 and H43 [19]. The

principal alloying elements are carbon, molybdenum, chromium and vanadium.

Analogous to group M and T steels (HSSs), the molybdenum- and tungsten hot-work

steels show similar properties for the same value of Weq. Relative to tungsten hot-

work steels, the costs resulting from necessary precautions during heat treatment

exceed the savings due to lower initial costs. Consequently tungsten hot-work steels

are most widely applied [19], [5].


.

Table 1 Classification and description of tool steels.

Table 2 Physical data for the principal alloying elements used in tool steels.


2.2 Quantitave Metallography

After the first microscopes were created, one of the next logical questions to follow

was how big a particular feature was or how much of some constituent was present.

From these questions, quantitative microscopy had its roots. The next logical question

to arise was how to relate observations made from two dimensional fields of view to

three dimensions; this analysis is termed stereology. Initially, the procedures

developed to perform stereological measurements were based on laborious time

consuming measurements. As television and computer systems were developed, and

matured, powerful lmage Analysis Systems (I/A) were created. Today many

measurements and calculations that previously required many hours to perform can be

made in minutes or even micro-seconds.

“ The determination of specific of microstructures using quantitative measurements

on micrographs or metallographic images is called Quantitative metallography.”

P.P.Anosov first used the metallurgical microscope in 1841 to reveal the structure of a

Damascus knife [1]. Driven by natural curiosity, the very next question proposed was

probably "what are the volume fractions of constituents?"

Many of the early studies of metallography are attributed to Sorby. He traced the

images of rock onto paper by using projected light. After cutting out the different

phases present and weighing the pieces from each phase, the volume fraction of the

phases was determined. The relationship between lineal analysis and volume

fractions, LL= VV (i.e., the lineal fraction equals the volume fraction) was

demonstrated by Rosiwal in 1898 [2]. One of the first studies to correlate chemical

compositions with structure was made by AlbertSauveur in 1896 [3]. From this work,

the relationship between the carbon content of plain-carbon steel and the volume

fraction of the various constituents was discovered.


2.2.1 Point Counting Example

ASTM E 562 describes the point counting procedure for determining the amount of

second-phase constituents. A grid with systematically spaced points (e.g., 10 rows of

10 equally spaced points) is superimposed over the structure, either on an eyepiece

reticle or a plastic sheet placed over or behind a ground glass projection screen or on a

TV monitor screen. The points are usually drawn as fine perpendicular crossing lines

and the ―point‖ is the intersection of the two lines. This is done because actual points

would be very difficult to see. The optimum point density for manual point counting

is usually determined from 3/VV where the volume fraction is a fraction (not a

percent). If the volume fraction is 0.5 (50%), then the optimum grid point density is 6.

On the other hand, if the volume fraction is 0.01 (1%), the optimum point density is

300. The point fraction is the ratio of the points in the phase of interest to the number

of grid points. Some people like to use a 100 point grid for all work since the division

is unnecessary. Points falling on the interface are counted as ½ a hit. For best manual

results we need to sample more fields and do as little work as possible on each field

measurement (the adage, ―do more, less well‖). The field-to-field variability has a

greater influence on measurement precision than the counting precision on a given

field.

Figure.8 Point Counting

The microstructure above shows the beta phase in Muntz metal (Cu-40% Zn)

preferentially colored by Klemm’s I reagent


While the alpha matrix is unaffected - ideal conditions for point counting. Since there

is less β than α, we will count the number of times the points fall in the colored β

grains. The amount of α is simply 100 - %β. As you can see, we have superimposed a

64-point test grid (8 rows of 8 points) over the structure and we have 15 hits and 4

tangent hits. The point fraction (volume fraction) is 17/64 = 0.266 or 26.6%. The

point counting grid would be placed randomly over the structure a number of times so

that the point fraction is determined for a number of fields. The necessary number of

fields to yield a 10% relative accuracy varies inversely with the volume fraction (the

lower the volume fraction, the greater the number of fields, i.e., the greater the total

number of applied grid points).

2.2.1.2 Statistics

Other measurements are possible, but the ones described above represent some of the

simplest and most useful. Each can be repeated on a number of fields on the plane-of-

polish so that a mean and standard deviation can be obtained. The number of fields

measured influences the precision of the measurement. Manual measurements are

tedious and time-consuming so sampling statistics may be less than desired. Image

analysis removes most of the barriers to inadequate sampling.

A good measure of statistical precision is the 95% confidence interval (or confidence

limit). This defines a range around the mean value where, 95 times out of 100, a

subsequently determined mean will fall. For example, a mean volume fraction of 10%

± 2% implies that for 95 of 100 measurements, the mean value will be between 8 and

12%. The 95% confidence interval is determined by:

95% CI = ts/n½

Where t is the Student’s t factor (t is a function of theconfidence level desired and the

number of measurements,n, and can be found in standard textbooksand in some

ASTM standards, e.g., E 562 andE 1382) and s is the standard deviation.The relative

accuracy, RA, of a measurement isdetermined by:

%RA = 100 · (95% CI)/X

Where X is the mean value. In general, a relativeaccuracy of 10% or less is

considered to be satisfactoryfor most work.


2.3 HOW HARDENING AND TEMPERING IS DONE IN

PRACTICE

The material should be tempered immediatelyafter quenching. Quenching should be

stopped at atemperature of 50–70°C (120–160°F) and temperingshould be done at

once. If this is not possible,the material must be kept warm, e.g. in a special―hot

cabinet‖, awaiting tempering.The choice of tempering temperature is oftendetermined

by experience. However, certainguidelines can be drawn and the following factorscan

be taken into consideration:

• Hardness

• Toughness

• Dimension Change

If maximum hardness is desired, temper atabout 200°C (390°F), but never lower than

180°C(360°F). High speed steel is normally tempered atabout 20°C (36°F) above the

peak secondaryhardening temperature.If a lower hardness is desired, this means a

higher tempering temperature. Reduced hardnessdoes not always mean increased

toughness, as isevident from the toughness values in our productbrochures. Avoid

tempering within temperatureranges that reduce toughness. If dimensional stability

is also an important consideration, the choiceof tempering temperature must often be

a compromise.If possible, however, priority should be givento toughness..Distortion

due to hardening must be taken into consideration when a tool is rough-machined.

Rough machining causes local heating and mechanical working of the steel, which

gives rise to inherent stresses. This is not serious on a symmetrical part of simple

design, but can be significant in asymmetrical machining, for example of one half of a

die casting die. Here, stress relieving is always recommended.

2.3.1 How many tempers are required?

Two tempers are recommended for tool steel and three are considered necessary for

high speed steel with a high carbon content, e.g. over 1%. Note that the carbides are

partially dissolved. This means that the matrix becomes alloyed with carbon and

carbide-forming elements.


When the steel is heated for hardening, the basic idea is to dissolve the carbides

to such a degree that the matrix acquires an alloying content that gives the hardening

effect—without becoming coarse grained and brittle.

The microstructure consists of a soft matrix in which carbides are embedded. In

carbon steel, these carbides consist of iron carbide, while in the alloyed steel they are

chromium (Cr), tungsten (W), molybdenum (Mo) or vanadium (V) carbides,

depending on the composition of the steel. Carbides are compounds of carbon and

these alloying elements and are characterized by very high hardness. A higher carbide

content means higher resistance to wear. To ensure repeated use of the die (long

service life), dies are carefully heat treated and surface hardened to obtain an optimum

combination of high hardness and high-toughness.


2.4 Reference Microstructures

Figure 9 (a) (AISI H13), as annealed structure Microstructure of investigated steel (as-

delivered condition). Etched with 2%nital


Figure 9 (b)Low hardened structure (950 °C, Oil) of steel grade (AISI H13).


Figure 9 (c) Conventional hardened structure (1040°C, Oil) of steel grade (AISI H13).


Figure 9 (d) High hardened structure (1150 °C, Oil) of steel grade (AISI H13).


3. Experimental Work

3.1 Annealing To eliminate fear of any pre-existinganomalies of material

properties, all samples were obtained in mill annealed condition.

3.2 Austenitizing (Hardening) Heat the furnace to 1010C; hold for 120 min at

1010 C. to obtain austenitizing temperature throughout the cross section. This totally

depend on cross section of sample, as my samples were small enough 1-2 in. in cross

section, this much time was enough for my samples.

Figure. 10 Exploded view of Electric furnace used for Heat treating


3 Quenching. After holding for 120 minutes, holding in furnace, Take out the

samples and air-cool in still air. In the case of oil quenching, take out the samples

which are at the austenitizing temperature, submerge in oil bath and oil-quench to

room temperature.

Figure.11For Air Quenching

Figure 12 For Oil Quenching

Holding for 2 hr Air Quenched

Heating

from Room

Temperature

1010 °C

Holding for 2 hr OilQuenched

Heating

from Room

Temperature

1010 °C


3.4 Tempering

3.4.1 Single Tempering at 650 C

Tempering is carried out at 650 C, the furnace was set to the desired tempering

temperature; hardeningwas already done while the samples are being quenched. The

samples loaded inside the furnace immediately after they reach 65C (or room

temperature for oil-quenched samples); hold for 2 hours. Remove samples from the

furnace and allow them to cool to room temperature in still air.

Figure 13Single Tempering at 650 C

3.4.2 Double Tempering at 650 C

Tempering is carried out at 650 C, the furnace was set to the desired tempering

temperature; hardening was already done while the samples are being quenched. The

samples loaded inside the furnace immediately after they reach 65C (or room

temperature for oil-quenched samples); hold for 2 hours. Remove samples from the

furnace and allow them to cool to room temperature in still air.

For double tempering, samples was allow to cool for at least one hour; then placed in

furnace steadied at the same tempering temperature as before; hold for 2 hr; remove

from furnace; air cool to room temperature.

Figure 14Double Tempering at 650 C

Holding for 2 hr Air

Hardened/Water

Quenched

Heating

from Room

Temperature

1010 °C

650 °C 650 °C

Air cool

Air cool

Holding for 2 hr

Heating

from Room

Temperature

1010 °C

650 °C

Air Cool 2 hr

3min

Air

Hardened/Water

Quenched


3.4.3 Single Tempering at 550 °C

Tempering is carried out at 550 °C, Set the furnace to the desired tempering

temperature; this should be already done while the samples are being quenched. Load

the samples inside the furnace immediately after they reach 65°C (or room

temperature for oil-quenched samples); hold for 90 minutes. Remove samples from

the furnace and allow them to cool to room temperature in still air.

Figure 15Single Tempering at 550 °C

3.5 Heating to 870 °C and Air Cool

Sample loaded to the furnace and hold at 870 °C for 11 hr. after that the

sample is removed from the furnace and Air cooled.

Figure 16

Holding for 2 hr

Heating

from Room

Temperature

1010 °C

550 °C

Air Cool 1 hr 30

min

Air

Hardened/Water

Quenched

Holding for 11 hr

Air Cool


3.6 Heating to 870 °C and Air Cool, heat again to 550 °C and Air

Cool

Sample loaded to the furnace and hold at 870 °C for 11 hr. after that the sample is

removed from the furnace and Air cooled. After sample reach room temperature again

loaded to furnace at 550°C for 14 hrs. and then air cool to room temperature.

Figure 17Heating to 870 °C and Air Cool, heat again to 550 °C and Air Cool

3.7 Development of Microsoft Excel Template

Microsoft excel template is developed using formula given in ASTM-e-562 for point

counting method. This template is standardized for 100 point grid because all the

parameter put into formula are obtained from ASTM standard for 100 point grid and

5000 point counting, mean 50 fields of 100 point grid.

This template calculate mean of Pp, Volume Fraction, 95% CL (95% confidence

level), and relative accuracy (%RA).

Holding for 11 hr 550 °C

Air Cool 1 hr 30

min

Air cool

870 °C


4.Results

Table No.3 Chemical Composition

Element Carbon Silicon Manganese Chromium Molybdenum Vanadium

Contents 0.32-0.45 0.80-1.20 0.20-0.50 4.9 1.25 0.974

Table No.4 Description of heat treatment routines of the test sample

Sample No. Description

0 As Received Annealed

1 Heating to 870 °C and Air Cool for 11 hr.

2 Heating to 870 °C for 11 h and Air Cool, heat again to 550 °C for 14 h and Air

Cool

3 Oil quenched from 1010ºC

4 Air cooled from 1010ºC

5 Oil Quenched from 1010ºC and single tempered (1 h 30 min) at tempering

temperature i-e 550 ºC

6 Air cooled from 1010ºC and single tempered (1 h 30 min) at tempering temperature

i-e 550 ºC

7 Oil Quenched from 1010ºC and single tempered (2 h) at tempering temperature i-e

650 ºC

8 Air cooled from 1010ºC and single tempered (2 h ) at tempering temperature i-e

650 ºC

9 Air cooled from 1010ºC and Double tempered (2 h + 2h) at tempering temperature

i-e 650 ºC

10 Oil Quenched from 1010ºC and single (2 h + 2h) at tempering temperature i-e 650

ºC


Table No.5 Hardness Test Results

190 196.72

197.98

640.34

550.6

572.88

564.1

411.34

350.84

325

353.21

0

100

200

300

400

500

600

700

-1 0 1 2 3 4 5 6 7 8 9 10 11

Sample Number vs Hardness (Hv)

Sample Numbers

H a

r d

n e s

s

(Hv)

Figure.18 . Sample number Vs Hardness obtained after Heat treatment.

Sample No. 0 1 2 3 4 5 6 7 8 9 10

Hardness

(Hv) 190 196.7 197.9 640.3 550.6 572.8 564.1 411.34 350.8 325 353.2


Figure.19 . Volume fraction vs Hardness obtained after Heat treatment.

0

Sample #6 564.1

Sample #8 350.84

Sample #10 353.21

0

100

200

300

400

500

600

0 1 2 3 4 5 6 7 8 9 10

Hardness of Air Quenched & Tempered

Sample Numbers

H a

r d

n e s

s

(Hv)

Figure.20 . Sample number and Hardness obtained after Tempering


(Air Quenched and tempered).

Sample # 5 572.88

Sample #7 411.34

Sample #9 325

0

100

200

300

400

500

600

700

0 2 4 6 8 10

Hardness of Oil Hardened & Tempered

H a

r d

n e s

s

(Hv)

Sample Numbers

Figure.21 . Sample Number and Hardness obtained after Tempering

(Air Quenched and tempered).


Microstructural Results

Figure No. 22 (i) Sample No.0 (AISI H13), Annealed, 500×structure

Microstructure of investigated steel (as-delivered condition). Etched with

2%nital

Figure No. 22 (ii) Sample No.0 (AISI H13), Annealed, 1000× structure

Microstructure of investigated steel (as-recieved condition). Etched with 2%nital


Figure No. 22(iii) Sample no. 1 At 500 × structure Microstructure of investigated steel

(as-recieved condition). Etched with 2%nital

Figure No. 22(iv) Sample no. 2 At 1000 × structure Microstructure of investigated

steel, Etched with 2%nital


Figure No. 22(v) Sample # 03, 500× structure Microstructure of investigated steel (as-

quenched ―oil quenched―). Etched with 2%nital

Figure No. 22(vi) Sample # 03× structure Microstructure of investigated steel (as-

quenched ―oil quenched―). Etched with 2%nital


Figure No. 22 (vii) Sample # 04, 500× structure Microstructure of investigated steel

(as-quenched ―Air quenched―). Etched with 2%nital

Figure No. 22(viii) Sample # 04, 1000× structure Microstructure of investigated steel

(as-quenched ―Air quenched―). Etched with 2%nital


Figure No. 22(ix) Sample # 05, 500× structure Microstructure of investigated steel

(Tempered ―Oil Quenched―). Etched with 2%nital

Figure No. 22(x) Sample # 05, 1000× structure Microstructure of investigated steel

(Tempered ―Oil Quenched―). Etched with 2%nital


Figure No. 22(xi) Sample # 06, 500× structure Microstructure of investigated steel

(Single Tempered ―Air quenched―). Etched with 2%nital

Figure No. 22(xii) Sample # 06, 1000× structure Microstructure of investigated steel



Figure No. 22(xiii) Sample # 07, 500× structure Microstructure of investigated steel

(Single Tempered ―Oil quenched―). Etched with 2%nital

Figure No. 22 (xiv) Sample # 07, 1000× structure Microstructure of investigated steel



Figure No. 22(xv) Sample # 08, 400× structure Microstructure of investigated steel


Figure No. 22(xvi) Sample # 08, 1000× structure Microstructure of investigated steel



Figure No. 22(xvii) Sample # 9, 500× structure Microstructure of investigated steel


Figure No. 22(xviii) Sample # 9, 500× structure Microstructure of investigated steel



Figure No.22(xix) Sample # 10, 500× structure Microstructure of investigated steel

(Double Tempered ―Air quenched―). Etched with 2%nital

Figure No. 22 (xx) Sample # 10, 1000× structure Microstructure of investigated steel

(Double Tempered ―Air quenched―). Etched with 2%nital


5.Discussion

Approximately all heat treatment related to the grade Aisi-H13 perform to obtain

better understanding through mechanical testing and Quantitative metallography

In the beginning, samples that are obtained in mill annealed condition are tested for

by spectroscopy to assure the Grade composition and that elements present in what

concentration, Result obtain from spectroscopy are shown in Table No.3

All the samples heat treated at different temperature and time, mechanical testing for

hardness perform on all samples to confirm treatment performed on it, that

predetermined properties obtain or not, hardness Results are shown in Table No.5,

graph is also plotted for hardness vs. sample No. as shown in figure No.18.

These samples then taken to the optical microscope to obtain microstructure

photograph of all samples at different magnification, but to reveal structure more

clearly and study for quantitative metallography only 500× and 1000× is shown in this

report, micrograph for all samples are given from figure No.22(i) –figure no. 22 (xx).

After having all the micrographs, I have to doo quantitative metallography i.e. Manual

Point counting method, for this technique one must have very clear focused and

descriptive micro graph with feature readily available to count, but when I try to study

quenched and tempered samples, I got a constraint that feature that are present in

quenched, and quenched & tempered condition are unable to be fully revealed at

microscopic level, although I try it on 100X but I am not confident about actual phase,

so I have to decide to quantify only sample #0, sample #1, sample #2, as all these

sample containing spheroidal carbide in the matrix of ferrite, so they are easy to be

counted on 1000×.

Results obtained by manual point counting 100 point grid 0n 50 fields on

microstructure for each sample, and date is feed into the excel template developed to

eliminate time taking calculation and accuracy of statistical parameters., Table No.6 ,


Table No.7,Table No.8 is showing all results for sample including volume fraction,

95% confidence level and Relative accuracy.

After getting point counting Result, volume fraction is plotted against hardness in

graph shown in Figure No.19, this graph shows that no of particles of carbides present

are some what in same volume fraction and approximately same Hardness, although

samples treatments are totally different, these result surprised and opened up door for

further research in on this treatment. I want to provide a detail for now, as sample

no.1 is hold at 870 ºC for 11 hrs. as Material has annealing temperature in between

845-900 ºC for smaller samples lower temperature range is use, but for annealing

sample have to cool down in furnace slowly instead I take sample out of furnace and

cool in normal air because I want to perform some what normalizing type treatment as

this grade is not recommended for Normalizing, that’s why I want know what

happens to this grade on Normalizing. After seeing the micrograph of this treatment I

was surprised to see carbide in spheroidal form, as spheroidizing annealing is a costly

and time consuming treatment as material is hold in furnace for long period of time

and cool slowly in furnace. But on the basis of only one sample this treatment can not

be referred as alternative to sherardizing annealing but to get better understanding this

treatment can be studied in future to obtain better or comparable result to sherardizing

on economic basis.

In Sample No.2, I perform the same treatment as on Sample 1, except when sample

reached to room temperature, again loaded to furnace at 550 ºC for 14 hrs. this is only

to analyse the effect or lower temperature on the shape and distribution of carbide.

But unfortunately results of Sample No. 2 not showing any remarkable effect, and

features of microstructural constituent are similar to sample No.1/

After finishing Quantitative metallography effect of tempering temperature and

holding have to be analyse, for this purpose samples Air quenched and tempered are

plots separately as shown in Figure No 20, and another graph is plot for air Quenched

specimen Figure No21,

Both graph following same trend except the different reading of hardness although

this grade is air hardenable, but on oil quenching high hardness are obtained and same

are showing their effects on tempering.


Highest hardness during tempering is obtain on samples treated at 550ºC for 1 hr. and

30 min, intermediate for single tempered at 650ºC for 2 h, and lowest for sample

double temper at 650ºC for (2hr + 2hr).and same results were expected by these

treatment, so tempering temperature can be selected on the basis of application and

properties desired.


6.Conclusion

Heat treatment of tool steel require depth of knowledge of heat treatment parameters

.How these parameter effecting Microstructure and what are morphology of

constituent. It is established through this research that:

1. If you want to quantify Aisi-H13 tool steel micro structure, specially in

hardened, and hard & tempered condition you must have to use high

magnification techniques like Scanning Electron microscopy, which enables

you to classify different alloy carbide also.

2. Although volume fraction of spheroidal carbide can be quantify through point

counting Method using ASTM-E-562, Volume fraction was not varied that

much to effect over all hardness of the sample, but if the volume fraction

change considerably it must effect the overall hardness and Properties of Hot

work Tool Steel.

3. Tempering temperature can be determined using, service requirement of

particular application. Either of treatment done in this research can give

economy with required properties.

4. Microsoft Excel template, can be use to calculate statistical parameter of

Manual Point Counting Method, simply putting the incident point obtain on

100 point Grid. This makes time consuming calculation can be done in

seconds.


Appendix (i)

Practice of Point counting

P = number of points hitting profiles

=

Pt = total number of reference points

=

P/Pt = area fraction

= %


References

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heat treatment of tool steel aisi-h-13 and its quantitavive metallography

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