Heat Treatment and IT’S Metallurgy
TOPICS
1. Heat Treatment –Introduction and Definitions
2. Iron Carbon Diagram and Phase Transformation
3. Time Temperature Transformation (TTT) Curve
4. Properties Of Pearlite ,Bainite And Martensite
5. Tempered Martensite
6. Classification of Heat Treatment Processes
6.1 Annealing
6.1.1
6.1.2
6.1.3
6.1.4
6.1.5
Full annealing (conventional annealing)
Isothermal Annealing
Spherodise Annealing
Recrystallization annealing
Stress Relief Annealing
6.2 Normalizing
6.3 Hardening
6.3.1 Conventional hardening
6.3.2 Martempering
6.3.3 Austempering
6.4 Surface Heat Treatment
6.4.1 Flame Hardening
6.4.2 Carburizing
6.4.3 Nitriding
7. Heat Treatment of Austenitic Stainless Steel
8. Welding Metallurgy
8.1 Preheating
8.2 Post weld Heat Treatment
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HEAT TREATMENT
1.Introduction

Heat Treatment can be defined as a combination of heating and cooling operations
applied to metals and alloys in the solid state to obtain desired conditions or properties.

The Heat Treatment of metals involves raising the temperature of a steel or alloy, often
through a prescribed thermal profile to a defined temperature. The material is then held
at temperature for a period of time before being cooled usually at a carefully controlled
rate or in a quench process to a fixed temperature or to ambient temperature.

Treatments are carried out in furnaces and ovens where gases are often used to control
the atmosphere of the process. Controlled atmospheres are used to reduce the effects
of oxidisation on the component being treated or to provide an enriching atmosphere
for surface chemistry effects.

Heat treatments can be employed to homogenise cast metal alloys or to improve their
hot workability, to soften metals prior to, and during hot and cold processing
operations, or to alter their microstructure in such a way as to achieve the desired
mechanical properties.

Thermal treatments of metallic alloys are also employed to alter the surface chemistry
of a material. This is achieved by diffusing Carbon, Nitrogen and other gaseous or solid
material in to the surface of the component.
2.Iron Carbon Diagram and Phase Transformation
A study of iron-carbon system and Phase Transformation is useful and important in many
respects for understanding Heat Treatment.
The Fe-Fe3C is characterized by five individual phases and four invariant reactions.
Five phases that exist in the diagram are: α–ferrite (BCC) Fe-C solid solution, γ-austenite
(FCC) Fe-C solid solution, δ-ferrite (BCC) Fe-C solid solution, Fe3C (iron carbide) or cementite
- an inter-metallic compound and liquid Fe-C solution.
Four invariant reactions that cause transformations in the system are namely eutectoid,
eutectic, monotectic and peritectic.
1. peritectic reaction at 1495 ℃ and 0.16%C,
δ-ferrite + L ↔ γ-iron (austenite)
2. monotectic reaction 1495 ℃ and 0.51%C,
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L ↔ L + γ-iron (austenite)
3. eutectic reaction at 1147 ℃ and 4.3 %C,
L ↔ γ-iron + Fe3C (cementite)
4. eutectoid reaction at 723 ℃ and 0.8%C,
γ-iron ↔ α–ferrite + Fe3C (cementite) [PEARLITE]






C is an interstitial impurity in Fe. It forms a solid solution with α, γ, δ phases of iron
Maximum solubility of C in BCC α-ferrite is limited (max.0.022 wt% at 727 °C) - BCC
has relatively small interstitial positions
Maximum solubility of C in FCC austenite is 2.14 wt% at 1147°C - FCC has larger
interstitial positions
Mechanical properties: Cementite is very hard and brittle - can strengthen steels.
Mechanical properties also depend on the microstructure, that is, how ferrite and
cementite are mixed.
Magnetic properties: α -ferrite is magnetic below 768 °C, austenite is non-magnetic
Eutectic and Eutectoid reactions in Fe–Fe3C-Very Important for HeatTreatment
As per Iron carbon diagram the Ferrous Materials can be broadly classified as1. Iron: less than 0.008 wt % C in α−ferrite at room T
2. Steels: 0.008 - 2.14 wt % C (usually < 1 wt % )-in α-ferrite + Fe3C at room T
3. Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)
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3.Time Temperature Transformation (TTT) Curve
Solid state transformations, which are very important in steels, are known to be
dependent on time at a particular temperature. Isothermal transformation diagram, also
known as TTT diagram, measures the rate of transformation at a constant temperature i.e. it
shows time relationships for the phases during isothermal transformation. Information
regarding the time to start the transformation and the time required to complete the
transformation can be obtained from TTT diagrams.


It is used to determine when transformations begin and end for an isothermal
heat treatment of a previously austenitized alloy
TTT diagram indicates when a specific transformation starts and ends and it also
shows what percentage of transformation of austenite at a particular
temperature is achieved.
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Temperature Range 0C
727 to 540
540 to 215
215 to 120
Microstructure
Pearlite
Bainite
Martensite
Hardness
Comparision
Martensite
>
Fine
Bainite > Coarse Bainite
> Fine Pearlite > Coarse
Pearlite
Possible transformations involving austenite decomposition
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4.Properties Of Pearlite ,Bainite And Martensite
Pearlite - It is lamellae of Ferrite and Cementite. The layer thickness of each of the ferrite and
cementite phases in the microstructure also influences the mechanical behavior of the material.
Fine Pearlite is harder and stronger than coarse pearlite. Pearlite forms in temp rage of 540 ℃
to 727 ℃.
Coarse Pearlite
Fine Pearlite
Bainite - It consists of needle/plate shaped cementite in ferrite phase. Bainite forms in temp
rage of 215 ℃ to 540 ℃.
Bainite
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Martensite – It’s grains take on a plate-like or needle-like appearance. The martensitic
transformation occurs when the quenching rate is rapid enough to prevent carbon diffusion. It
forms at temp below 215 ℃. It is Hardest microstructure in comparision to the Pearlite and
Bainite.
Martensite
5.Tempered Martensite
In the as-quenched state, martensite, in addition to being very hard, is so brittle that it
cannot be used for most applications; also, any internal stresses that may have been
introduced during quenching have a weakening effect. The ductility and toughness of
martensite may be enhanced and these internal stresses relieved by a heat treatment
known as Tempering.
Tempering is accomplished by heating a martensitic steel to a temperature below the
eutectoid for a specified time period. Normally, tempering is carried out at temperatures
between 250 ℃ to 650 ℃.The microstructure of tempered martensite consists of extremely
small and uniformly dispersed cementite particles embedded within a continuous ferrite
matrix.
Tempered martensite may be nearly as hard and strong as martensite, but with
substantially enhanced ductility and toughness.
Tempered Martensite
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6. Classification of Heat Treatment Processes
Annealing
Annealing refers to a wide group of heat treatment processes and is performed primarily for
homogenization, recrystallization or relief of residual stress in typical cold worked or welded
components. Depending upon the temperature conditions under which it is performed,
annealing eliminates chemical or physical non-homogeneity produced of phase
transformations. Few important variants of annealing are full annealing, isothermal annealing,
spheroidise annealing, recrystallization annealing, and stress relief annealing.
Full annealing (conventional annealing)
Full annealing process consists of three steps. First step is heating the steel component to
above A3 (upper critical temperature for ferrite) temperature for hypoeutectoid steels and
above A1 (lower critical temperature) temperature for hypereutectoid steels by 30-500C
(Figures 1 and 2).
The second step is holding the steel component at this temperature for a definite holding
(soaking) period of at least 20 minutes per cm of the thick section to assure equalization of
temperature throughout the cross-section of the component and complete austenization. Final
step is to cool the hot steel component to room temperature slowly in the furnace, which is
also called as furnace cooling. The full annealing is used to relieve the internal stresses induced
due to cold working, welding, etc, to reduce hardness and increase ductility, to refine the grain
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structure, to make the material homogenous in respect of chemical composition, to increase
uniformity of phase distribution, and to increase machinability.
Figure 1
Iron-carbon phase equilibrium diagram
910C
Acm
Ful
A3
lA
nne
alin
g
Full Annealing
723C
A1

T
Wt% C
0.8 %
Figure 2
Schematic representation of annealing operation
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Isothermal Annealing
Isothermal annealing consists of four steps. The first step is heating the steel components
similar as in the case of full annealing. The second step is slightly fast cooling from the usual
austenitizing temperature to a constant temperature just below A 1. The third step is to hold at
this reduced temperature for sufficient soaking period for the completion of transformation
and the final step involves cooling the steel component to room temperature in air. Figure 3
depicts the heat treatment cycles of full annealing and isothermal annealing. The terms α, γ, P,
PS and PF refer to ferrite, austenite, pearlite, pearlite starting and pearlite finish, respectively.
Isothermal annealing has distinct advantages over full annealing which are given below.
1. Reduced annealing time, especially for alloy steels which need very slow cooling to
obtain the required reduction in hardness by the full annealing.
2. More homogeneity in structure is obtained as the transformation occurs at the same
time throughout the cross section.
3. Improved machinability and surface finish is obtained after machining as compared to
that of the full annealed components.
Isothermal annealing is primarily used for medium carbon, high carbon and some of the alloy
steels to improve their machinability.
Figure 3
Heat treatment cycles of full annealing and isothermal annealing
Spheroidise Annealing
Spheroidise annealing is one of the variant of the annealing process that produces typical
microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix of
ferrite. The following methods are used for spheroidise annealing Holding at just below A1
Holding the steel component at just below the lower critical temperature (A1) transforms the
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pearlite to globular cementite particles. But this process is very slow and requires more time for
obtaining spheroidised structure.Thermal cycling around A1.
In this method, the thermal cycling in the narrow temperature range around A1 transforms
cementite lamellae from pearlite to spheroidal. Figure 4 depicts a typical heat treatment cycle
to produce spheroidised structure. During heating above A1, cementite or carbides try to
dissolve and during cooling they try to re-form. This repeated action spheroidises the carbide
particles. Spheroidised structures are softer than the fully annealed structures and have
excellent machinability. This heat treatment is utilized to high carbon and air hardened alloy
steels to soften them and to increase machinability, and to reduce the decarburization while
hardening of thin sections such as safety razor blades and needles.
Figure 4 A typical heat treatment cycle to produce Spheroidised structure
Fig (a) A medium-carbon low-alloy steel after Spheroidised at 720℃;
(b) High-speed steel Spheroidised at 820℃
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Recrystallization Annealing
Recrystallization annealing process consists of heating a steel component below A1
temperature i.e. at temperature between 6250C and 6750C (recrystallization temperature range
of steel), holding at this temperature and subsequent cooling. This type of annealing is applied
either before cold working or as an intermediate operation to remove strain hardening
between multi-step cold working operations. In certain case, recrystallization annealing may
also be applied as final heat treatment. The cold worked ferrite recrystallizes and cementite
tries to spheroidise during this annealing process. Recrystallization annealing relieves the
internal stresses in the cold worked steels and weldments, and improves the ductility and
softness of the steel. Refinement in grain size is also possible by the control of degree of cold
work prior to annealing or by control of annealing temperature and time.
Low-carbon steel (0.10% C), cold rolled 90% to a
thickness of 0.25 mm and annealed 106 s at 550 °C .
Recrystallized 10%
Same steel and cold rolling as above fig, but annealed
7 min at 550 °C Recrystallization increased to 40%
Same steel and cold rolling as above fig , but annealed
14.5 min at 550 °C .Recrystallization is 80%
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Stress Relief Annealing
Stress relief annealing process consists of three steps. The first step is heating the cold worked
steel to a temperature between 5000C and 5500C i.e. below its recrystallization temperature.
The second step involves holding the steel component at this temperature for 1-2 hours. The
final step is to cool the steel component to room temperature in air.
The stress relief annealing partly relieves the internal stress in cold worked steels without loss
of strength and hardness i.e. without change in the microstructure. It reduces the risk of
distortion while machining, and increases corrosion resistance. Since only low carbon steels can
be cold worked, the process is applicable to hypoeutectoid steels containing less than 0.4%
carbon. This annealing process is also used on components to relieve internal stresses
developed from rapid cooling and phase changes.
Stress Releiving Cycle for Fabricated Gear Cases
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Normalizing
Normalizing process consists of three steps. The first step involves heating the steel component
above the A3 temperature for hypoeutectoid steels and above Acm (upper critical temperature
for cementite) temperature for hypereutectoid steels by 30 0C to 500C (Figure 5). The second
step involves holding the steel component long enough at this temperature for homogeneous
austenization. The final step involves cooling the hot steel component to room temperature in
still air. Due to air cooling, normalized components show slightly different structure and
properties than annealed components. The same are explained in Table 1.
The properties of normalized components are not much different from those of annealed
components. However, normalizing takes less time and is more convenient and economical
than annealing and hence is a more common heat treatment in industries. Normalizing is used
for high-carbon (hypereutectoid) steels to eliminate the cementite network that may develop
upon slow cooling in the temperature range from point Acm to point A1. Normalizing is also used
to relieve internal stresses induced by heat treating, welding, casting, forging, forming, or
machining. Normalizing also improves the ductility without reducing the hardness and strength.
Figure 5 Normalizing
Table 1
The variation in the properties of the annealed and normalized components
Annealed
Less hardness, tensile strength and
toughness.
Pearlite is coarse and usually gets resolved
by the optical microscope.
Grain size distribution is more uniform.
Internal stresses are least.
Normalised
Slightly more hardness, tensile strength and
toughness.
Pearlite is fine and usually appears
unresolved with optical microscope.
Grain size distribution is slightly less uniform.
Internal stresses are slightly more.
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Fig. - Carbon steel of 0.5% C. (a) As-rolled or forged; (b) normalized.
Normalizing refines the grain of a steel that has become coarse-grained as a result of heating to
a high temperature, e.g., for forging or welding
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Test Certificate of Normalizing
MATL. GRADE. ASTM A105
HEAT NO. 17F00888
HEAT TREATMENT DETAIL- Normalized – 920°C/ Air Cool
Element
Results %
C
Si
Mn
P
S
Cr
Mo
Ni
Al
Cu
V
0.18
0.24
1.15
0.030
0.011
0.035
0.001
0.005
0.033
0.06
0.004
Microstructure & Grain size:Protocol Used: - 2% Nital
(THE MICROSTRUCTURE CONSTITUTES FINALLY RESOLVED PEARLITE (BLACK), IN A MATRIX OF FERRITE
(WHITE AREAS), WITH EVEN DISTRIBUTION.)
GRAIN SIZE
Magnification – 100X
Pre Austenic Grain Size No. -7.5 (Avg.)
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Test Certificate of Normalized 45mm IS 2062 E250BR Plate
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Hardening
Different techniques to improve the hardness of the steels are Conventional Hardening,
Martempering And Austempering.
Conventional Hardening
Conventional hardening process consists of four steps. The first step involves heating the steel
to above A3 temperature for hypoeutectoid steels and above A1 temperature for
hypereutectoid steels by 500C. The second step involves holding the steel components for
sufficient socking time for homogeneous austenization. The third step involves cooling of hot
steel components at a rate just exceeding the critical cooling rate of the steel to room
temperature or below room temperature. The final step involves the tempering of the
martensite to achieve the desired hardness. Detailed explanation about tempering is given in
the subsequent sections. In this conventional hardening process, the austenite transforms to
martensite. This martensite structure improves the hardness.
Following are a few salient features in conventional hardening of steel.
1. Proper quenching medium should be used such that the component gets cooled at a
rate just exceeding the critical cooling rate of that steel.
2. Alloy steels have less critical cooling rate and hence some of the alloy steels can be
hardened by simple air cooling.
3. High carbon steels have slightly more critical cooling rate and has to be hardened by oil
quenching.
4. Medium carbon steels have still higher critical cooling rates and hence water or brine
quenching is necessary.
Figure 6 depicts the conventional hardening process which involves quenching and tempering.
During quenching outer surface is cooled quicker than the center. Thinner parts are cooled
faster than the parts with greater cross-sectional areas. In other words the transformation of
the austenite is proceeding at different rates. Hence there is a limit to the overall size of the
part in this hardening process.
Figure 6 Heat treatment cycle
for conventional hardening
process
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Test Certificates of Hardening and Tempering
MATL. GRADE. 10725AB
HEAT NO. D6846
HEAT TREATMENT DETAIL- Hardened-1040°C/ Oil Quenched & Tempered-650°C/ Still Air
Element
Results %
C
Si
Mn
P
S
Cr
Mo
Ni
Al
Co
Cu
Nb
Ti
V
B
W*
0.209
0.202
0.837
0.0167
0.0029
12.14
0.999
0.533
0.0263
0.0303
0.103
0.0126
<0.0010
0.213
0.00069
0.932
Microstructure & Grain size:Protocol Used: -ASM Vol-9, ASTM E: 112:2013 (General microstructure shows tempered martensite.)
GRAIN SIZE
Magnification – 100X
Pre Austenic Grain Size No. -7.5 (Avg.)
Microstructure
Magnification – 500X
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Tempered Martensite
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Martempering (Marquenching)
Martempering process overcomes the limitation of the conventional hardening process. Figure
7 depicts the martempering process. This process follows interrupted quenching operation. In
other words, the cooling is stopped at a point above the martensite transformation region to
allow sufficient time for the center to cool to the temperature as the surface. Further cooling is
continued through the martensite region, followed by the usual tempering. In this process, the
transformation of austenite to martensite takes place at the same time throughout the
structure of the metal part.
Figure 7 Heat treatment cycle for martempering
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Test Certificate of Martempering
TYPICAL TEST CERTIFICATE OF HP INNER CASING (TELANGANA)
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Tempered Martensite
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Austempering
This process is also used to overcome the limitation of the conventional hardening process.
Figure 8 depicts the austempering process. Here the quench is interrupted at a higher
temperature than for martempering to allow the metal at the center of the part to reach the
same temperature as the surface. By maintaining that temperature, both the center and
surface are allowed to transform to bainite and are then cooled to room temperature.
Austempering causes less distortion and cracking than that in the case of martempering and
avoids the tempering operation. Austempering also improves the impact toughness and the
ductility of the metal than that in the case of martempering and conventional hardening.
Figure 8 Heat treatment cycle for austempering
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Surface Heat Treatments
There are some applications where it is necessary to have a hard, wear resistant surface but a
tough, shock-resistant inner core. For example, cams, gears, and shafts require hard surfaces to
resist wear but tough inner cores to resist shock. When heat treated, the high-carbon surface
layers will attain a much higher hardness than the low-carbon core. This method of case
hardening, called carburizing, is feasible if small, fast-diffusing elements, such as carbon or
nitrogen, are used that will form hard carbides or nitrides. Some of the commonly used
techniques of Surface Heat treatment such as –Flame Hardening and Case Hardening e.g.
Carburizing and Nitriding is covered in this write-up.
Flame Hardening
The objective of flame hardening is to austenitize the steel at and near the surface and then to
remove the flame and rapidly quench the work to produce martensite. The surface is heated by
a gas flame created by burning acetylene, propane, or natural gas. The relatively low thermal
conductivity of steel enables the surface regions to be austenitized using high rates of energy
input without the interior being significantly affected. Flame hardening is a very rapid and
efficient method for producing cases as deep as 6.3 mm, but the maximum hardness that can
be obtained (50 to 60 HRC) is less than can be attained with through hardening. Unless the
process is automated, it can be difficult to control the case depth, and prolonged heating can
result in a case depth deeper than desired. It is often used where small quantities of parts
require hardening, the part is large and bulky, or the heat treating facilities are limited.
Carburizing
Carburizing is conducted by heating a low carbon steel into the single-phase austenitic field,
generally between 845 and 955 ℃, where the steel has a high solubility for carbon. After
holding for the appropriate time, the part is either quenched or cooled to room temperature.
After quenching, the part is then tempered in the normal manner. Carburizing produces a wearresistant high-carbon case on top of a tough low-carbon steel core. Steels used for carburizing
usually have carbon contents of approximately 0.2 wt%, with carburized cases containing up to
0.8 to 1.0 wt% C. Carburizing produces the hard cases; hardness values in range of 50-63 HRC
are obtainable.
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Nitriding
Nitriding is a surface hardening process, where nitrogen is added to the surface of steel
parts using dissociated ammonia as the source. Gas nitriding develops a very hard case in a
component at relatively low temperature, without the need for quenching. Nitriding is carried
out at temperatures below the transformation temperature of alloy steels, so that with proper
manufacturing techniques, there is little or no distortion as a result of the process. Parts to be
nitrided are heat treated to the proper strength level, and final machined. The parts are then
exposed to active nitrogen at a carefully controlled temperature, typically in the range of 500°C
to 525°C. This temperature is typically below the final tempering temperature of the steel so
that nitriding does not affect the base metal mechanical properties. As a result, a very high
strength product with extremely good wear resistance can be produced, with little or no
dimensional change. Nitriding produces the hardest cases; hardness values as high as 70 HRC
are obtainable.
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7. Heat Treatment of Austenitic Stainless Steel
Solution Annealing
Solution annealing is the heat treatment most frequently specified for austenitic stainless
steels. The main objective of this treatment, as the name implies, is to dissolve the phases that
have precipitated during the thermomechanical processing of the material, especially the
chromium-rich carbides. As the precipitation of chromium-rich carbides occurs in the 450 to
900 ℃ temperature range, the lower temperature limit for solution annealing should be over
900 ℃. Carbides should be completely dissolved but they dissolve slowly. Grain growth limits
the maximum solution-annealing temperature. For the more conventional stainless steels, such
as AISI 201, 202, 301, 302, 303, 304, 304L, 305, and 308, recommended solution-annealing
temperatures are around 1010 to 1120℃.For higher carbon-containing steels such as the AISI
309 and 310 or steels containing molybdenum such as AISI 316, 316L, 317, and 317L, the
minimum temperature should be increased to 1040℃ whereas the maximum should be kept at
1120℃. In the case of the stabilized steels,the solution annealing temperature range should be
at a lower level, between 955 and 1065℃, for titanium-stabilized AISI 321 type, and narrower,
between 980 and 1065℃, for the niobium-stabilized AISI 347 and 348 (nuclear grade) types.
Cooling from heat treatment temperatures should be sufficiently fast to avoid chromium
carbide precipitation. In the case of nonstabilized grades such as AISI 201, 202, 301, 302, 303,
304, 305, 308, 309, 316, and 317, if distortion considerations permit, water quenching may be
utilized. In the case of the AISI 309 and 310 types that contain maximum allowed carbon
content and are susceptible to carbide precipitation, water cooling is mandatory. In the case of
stabilized AISI 321, 347, and 348 types, water cooling is not needed and air cooling is sufficient
to avoid sensitization.
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MATL. GRADE. ASTM A182 F304H
HEAT NO. CHP 9257
HEAT TREATMENT DETAIL- Solution Annealed – 1120°C/ Water Quenched
Element
Results %
C
Si
Mn
P
S
Cr
Mo
Ni
Al
Cu
N2
0.058
0.77
1.57
0.033
0.019
18.54
0.28
8.24
0.032
0.29
0.072
Microstructure & Grain size:Protocol Used: - ASTM E407 (2015) ASM HAND BOOK VOL. ‘9’ MARBLE’S REAGENT
(ANNEALED TYPE AUSTENITE MATRIX SHOW EQUIAXED AUSTENITE GRAINS, FREE FROM ANY EVIDENCE OF PRECIPITATION
OF CARBIDE PARTICLES/INERGANULAR ATTACK OF AUSTENITE GRAIN BOUNDARIES.)
GRAIN SIZE
Magnification – 100X
Pre Austenic Grain Size No. -4.5 (Avg.)
Microstructure
Magnification – 200X
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8. Welding Metallurgy
Figure - Carbon steel side of weld
metal in a weld between a carbon
steel and an austenitic stainless
steel made with a Ni-based filler
metal: (a) martensite along fusion
boundary; (b) martensite avoided
by preheating and controlling
interpass temperature.
Significance of Preheating in Welding
In general, low alloy steels are required to be preheated to some temperature (TPH), prior to
welding. TPH for any given steel should be about 10℃ above the martensite start temperature
(MS) for the particular steel being welded.
Preheating drives moisture and other contaminants off the joint; moisture, lubricants and other
contaminants are sources of hydrogen. More importantly, preheating serves to reduce the rate
at which the metal cools down from the welding temperature to TPH. This is so whether
preheating is above or below MS. Cooling rate reductions will lead to a general reduction in
residual stress magnitudes, and also allow more time for hydrogen removal,thus assisting is
avoiding Hydrogen Cracking. Furthermore, cooling rate reductions can affect austenite
transformation to products other than martensite, before reaching MS (TPH < MS) or TPH (TPH >
MS). Most low alloy steels that may be susceptible to hydrogen induced cracking transform
from austenite during cooling through the 800-500℃ temperature range .The length of time,
(seconds), a steel spends in this range during cooling, will establish its microstructure and,
hence, its susceptibility to cold cracking. To maximize cracking resistance, a microstructure that
is free of martensite is desired; that is, the austenite would have transformed to ferrite +
carbide and no austenite will be available to transform to martensite upon reaching MS.
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Significance of PWHT in Welding
PWHT is a collective term referring to the thermal treatments performed after the completion
of welding. There are two types of PWHT: Postheating and Stress Relief. In post heating, the
weld is not allowed to cool to Room Temperature (RT). Rather, it is maintained at some
elevated temperature for a period of time, then allowed to cool down to RT. Stress relief, by
contrast, is a separate heat treatment that is performed after the weld had cooled to RT. It
involves heating to some specified temperature (590-750℃ for steels), soaking for a period of
time (2-4 hrs), and then cooling to RT in air.
The microstructural constituents present in the weld metal and adjacent HAZ regions at the
conclusion of welding are: untransformed austenite; ferrite and carbide (mostly in the form of
pearlite and / or bainite) that formed during cooling from the weld temperature to TPH / TIP,
and; untempered martensite that formed between TPH and MS (only when TPH < MS).
If the weldment is allowed to immediately cool down from TPH to RT, the untransformed
austenite can transform to martensite (untempered). This transformation will generate
additional residual stresses, which will be superimposed on those generated earlier during
cooling from the weld temperature down to TPH. The increased magnitudes of residual stresses,
the presence of hydrogen, picked up during welding, the general brittleness of untempered
martensite, and the metallurgical notch resulting from the presence of a weld will all combine
to increase the risk of cold cracking.
Noting that susceptible steels typically experience cold cracking around ambient temperature
(RT), it is possible to avoid cracking by allowing the hydrogen to diffuse from the weld before
the weld cools to RT. That is, the completed weld should not be allowed to cool to RT, before
the hydrogen is given the chance to diffuse out of the weld at some elevated temperature.
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Quality Assurance & Inspection Department, EOC, NOIDA