heat treatment of tool steel aisi-h-13 and its quantitavive metallography
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Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 1
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
.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 2
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.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 3
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
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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
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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).
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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.
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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
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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].
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 9
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].
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 10
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.
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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.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 12
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
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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].
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 14
.
Table 1 Classification and description of tool steels.
Table 2 Physical data for the principal alloying elements used in tool steels.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 15
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.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 16
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
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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.
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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.
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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.
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2.4 Reference Microstructures
Figure 9 (a) (AISI H13), as annealed structure Microstructure of investigated steel (as-
delivered condition). Etched with 2%nital
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Figure 9 (b)Low hardened structure (950 °C, Oil) of steel grade (AISI H13).
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Figure 9 (c) Conventional hardened structure (1040°C, Oil) of steel grade (AISI H13).
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Figure 9 (d) High hardened structure (1150 °C, Oil) of steel grade (AISI H13).
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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
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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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 26
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 27
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 28
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 29
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 30
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 31
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 32
(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).
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 33
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 34
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 35
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 36
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 37
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 38
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
(Single Tempered ―Air quenched―). Etched with 2%nital
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 39
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
(Single Tempered ―Oil quenched―). Etched with 2%nital
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 40
Figure No. 22(xv) Sample # 08, 400× structure Microstructure of investigated steel
(Single Tempered ―Air quenched―). Etched with 2%nital
Figure No. 22(xvi) Sample # 08, 1000× structure Microstructure of investigated steel
(Single Tempered ―Air quenched―). Etched with 2%nital
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 41
Figure No. 22(xvii) Sample # 9, 500× structure Microstructure of investigated steel
(Single Tempered ―Oil quenched―). Etched with 2%nital
Figure No. 22(xviii) Sample # 9, 500× structure Microstructure of investigated steel
(Single Tempered ―Oil quenched―). Etched with 2%nital
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 42
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
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 43
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 44
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 45
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 46
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 47
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 ,
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 48
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.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 49
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.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 50
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.
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 51
Appendix (i)
Practice of Point counting
P = number of points hitting profiles
=
Pt = total number of reference points
=
P/Pt = area fraction
= %
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 52
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 53
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 54
Laraib Sarfraz (MM-04) , MMD,NEDUET. Karachi. Page 55
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