SEA Materials Engineering
Fundamentals of Heat Treatment
for Metallic Materials
The purpose of this presentation is to provide a
basic understanding of the metallurgical
processes associated with the heat treatment of
1. Shape change of materials
Such as forging, forming, extrusion, rolling, welding
and casting (foundry).
2. No shape change of materials
Such as heat treatment and coating.
Liquid water and ice are familiar examples of how a
material can exist in various forms. Steel also exists in
various forms, including several different solid forms.
Various forms of material
What is Heat treatment?
• Answer: The controlled heating and cooling of
materials for the purpose of altering their structures
Purposes of performing a heat treatment
process on a metallic component
•Modification of the microstructure for improvement of
machining, cold forming processes.
•Obtaining the required mechanical properties, such as
strength, toughness, hardness, wear resistance and
fatigue life based on application.
•Reduction in brittleness, residual stress, dimensional
instability of components.
•Surface protection from environment, such as
oxidation, corrosion medium and stress corrosion.
Fundamentals of Heat Treatment Processing Development for
When a material is selected per design requirements and application,
the next step is how to have the designed parts satisfy these
requirements, such as mechanical properties (strength, hardness,
toughness, residual stress state and fatigue strength) and
microstructure. They are achieved through a proper heat treatment
How to develop a heat treatment process???
Tool one: Fe-Carbon phase diagram
For heating above the austenitizing temperature
Tool two: T-T-T or IT curve of selected material
Determination of cooling rate (reduction of cracking risk and
distortion) to obtain the required microstructure.
Iron-Carbon phase Diagram
Since steel is a Fe-C ally, the carbon is a key element in any grades of steel.
Most alloying elements in the steels are Cr, Mo, Ni, Si, Mn, as well
as V and W.
•All of them change the positions of the A1, A3 and Am boundaries
and the eutectoid composition will be changed.
•All important alloying elements decrease the eutectoid carbon content. For example, 1080 (0.8% C) steel is
called hyper-eutectoid steel. However, H19 (0.3% C) is hyper-eutectoid steel due to addition of Cr (2%), W (8%)
and V (1%). Eutectoid point shifts toward to left in the Fe-C diagram.
•Austenite-stabilizing elements (Mn and Ni) decrease A1 temperature, i.e., expend γ gamma zone.
•Ferrite-stabilizing elements (Cr, Si, Mo , W, V and Ti) increase A1 temperature, i.e., shrink γ gamma zone.
•The effect of combination of alloying elements on the Fe-C diagram is very complicated (may expand or shrink
Effects of alloying elements on the Fe-C phase diagram
1. 1035 steel – 1570 F (855 C)
2. 1040 steel – 1555 F (845 C)
3. 4340 steel – 1570 F (855 C)
4. 5130 steel – 1570 F (855 C)
5. 8620 steel – 1600 – 1700 F (870 – 925 C for carburizing)
6. H19 steel – 2005-2200 F (1095 – 1205 C)
7. D2 steel – 1795-1875 F (980-1025 C)
8. T2 steel – 2300-2375 F (1260-1300 C)
Examples of austenitizing temperature development for
Full TTT Diagram
The complete TTT
diagram for an
iron-carbon alloy of
Effect of the carbon content on the T-T-T- curve profile
1. Move the nose of T-T-T curve toward the lower-right direction.
2. Lowering Ms point temperature (martensitic transformation start temperature).
3. Increasing hardening ability or hardenability using same cooling rate.
Effect of the Alloying elements on the T-T-T- curve profile
0.23C 0.82Mn 1.22Cr
1. Most of them move the nose of T-T-T curve toward the lower-right direction.
2. Some of them promote the formation of two noses in T-T-T curves
3. Some of them lower Ms temperature point (martensitic transformation start temperature).
4. Most of them increase hardening ability or hardenability using same cooling rate.
1.5C 12Cr 1Mo 1V
So What’s a CCT Diagram?
• Phase Transformations and Production of
Microconstituents takes TIME.
• Higher Temperature = Less Time.
• If you don’t hold at one temperature and allow time to
change, you are “Continuously Cooling”.
• Therefore, a CCT diagram’s transition lines will be
different than a TTT diagram.
Time in region
indicates amount of
Cooling Rate, R, is
Change in Temp /
This steel is very
Martensite in ~ 1
minute of cooling!
Heat Treatment can be considered in terms
of three aspects
1. In crystallographic change
2. In microstructure change
3. In mechanical and physical property change
•In crystallographic change
From BCC to FCC to BCT
BCC – Body centered cubic
FCC – Face centered cubic
BCT – Body centered tetragonal
•In microstructure change
From pearlite to austenite to martensite
through quenching (fast cooling)
In mechanical properties, such as hardness
SAE 1050 SAE 4147
Objectives of the Full Annealing
• To improve ductility
• To facilitate cold working or machining
• To remove internal stresses completely
• To get enhanced magnetic and electrical properties
• To promote dimensional stability
• To refine grain structure
– The prolonged heat treatment cycle, involved in this process, makes it quite
• In this process, hypoeutectoid steel is heated
above the upper critical temperature (A3 -750-
900°C)and held for some time at this
• The steel is then cooled rapidly to a temperature
less than the lower critical temperature (i.e. 600 -
• After all the austenite is transformed into
lamellar pearlite, steel is cooled in air.
Adv. of Isothermal Annealing
• The time required is less compare to Full
• Hence cheaper than full annealing
• Improves Machinability and also results in
a better surface finish by machining.
• Widely used for alloy steels.
Disadv. of Isothermal Annealing
• Used for hypoeutectoid steels only
• It is suitable only for small-sized components.
– Heavy components cannot be subjected to this treatment because it is
not possible to cool them rapidly and uniformly to the holding
temperature at which transformation occurs.
• Steel is heated to a temperature below the lower
critical temperature (670-720°C), and is held at
this temperature for sufficient time and then
• To reduce hardness and to increase ductility of
cold-worked steel so that further working may be
carried out easily.
• It is an intermediate operation and is sometimes
referred to as in-process annealing.
• Mostly used in sheet and wire industries
• Spheroidising is a heat treatment process which
results in a structure consisting of globules or
spheroids of carbide in a matrix of ferrite.
• Spherodisation can take place by the following
– Prolonged holding at a temperature just below the lower critical line
– Heating and cooling alternately between temperature that are just above
and just below the lower critical line.
– Heating to a temperature above the lower critical line and then either
cooling very slowly in the furnace or holding at a temperature just below
the lower critical line.
Purpose of Spheroidising
• The majority of all spheroidising activity is
performed for improving the cold formability of
• The spheroidised structure is desirable when
minimum hardness, maximum ductility, or (in
high carbon steels ) maximum machinability is
• Low-carbon steels are seldom spheroidised for
machining, because in the spheroidised
condition they are excessively soft and gummy.
• The cutting tool will tend to push the material
rather than cut it, causing excessive heat and
wear on the cutting tip.
• It is used to relieve stresses that remain locked
in a structure as a consequence of a
• No microstructural changes occur during the
• The process of stress relieving consists of
heating steel uniformly to a temperature below
the lower critical temperature (less than 600°C),
holding at this temperature for sufficient time,
followed by uniform cooling.
• Adverse effect of internal stresses
– steels with residual stresses under corrosive environment fail by stress-
– Residual stresses will enhance the tendency of steels towards warpage
and dimensional instability.
– Fatigue strength is reduced considerably when residual tensile stresses
• WHAT IS NORMALISING?
– Normalising is an austenitising heating cycle followed by cooling
in still air or slightly agitated air.
– Typically, the job is heated to a temperature about 50°C above
the upper critical line of the iron-iron carbide phase diagram prior
to cooling. (830 - 925°C)
PURPOSE OF NORMALISING
• To improve Machinability
• To refine the grain structure
• To homogenise the microstructure in order to
improve the response in hardening operation.
• To modify and refine cast dendritic structure
• To reduce banded grain structure due to hot
NORMALISING Vs ANNEALING
• Normalised steels are harder than annealed one.
• Prolonged heat treatment time and higher energy
consumption make the annealing treatment more
expensive than normalising.
• Cooling rates are not critical for normalising as in the
case of annealing.
• Annealing improves the machinability of medium carbon
steels, whereas normalising improves machinability of
low carbon steels.
• Certain applications demand high hardness
values so that the components may be
successfully used for heavy duty purposes.
• High hardness values can be obtained by a
process known as Hardening.
• Hardening treatment consists of heating to austenitising
temperature(815 - 870°C), holding at that temperature,
followed by rapid cooling such as quenching in water, oil,
or salt baths.
• The high hardness developed by this process is due to
the phase transformation accompanying rapid cooling.
• The product of low temperature transformation of
austenite is martensite, which is a hard microconstituent
• Successful hardening usually means achieving
the required microstructure, hardness, strength,
or toughness while minimising residual stress,
distortion, and the possibility of cracking.
Selection of Quenching Medium
• Selection of a quenching medium depends on
the hardenability of the particular alloy, the
section thickness and shape involved and the
cooling rates needed to achieve the desired
– Hardenability: It is the ability of the steel to be transformed partially or
completely from austenite to martensite while quenching.
Various Quenching Mediums
• Gaseous Quenchants
– Helium, Argon and Nitrogen
• Liquid Quenchants
– Oil with some additives
– Polymer Quenchants
– Brine Water
Factors affecting Hardening
• Chemical composition of steel
• Size and shape of the steel part
• Hardening cycle (heating rate, hardening
temperature, holding time and cooling rate)
• Homogeneity and Grain size of austenite
• Quenching Media
• Surface condition of steel part.
• High hardness developed by hardening enables
tool steel to cut other metals.
• It also improves wear resistance.
• Tensile strength and Yield Strength are
improved by hardening.
• This process is frequently used for chisels,
sledge, hand hammers, centre punches, shafts,
collars and gears.
• Tempering consists of heating hardened steel
below the lower critical temperature, followed by
cooling in air or at any other desired rate.
• In the as-quenched martensitic condition, the
steel is too brittle for most applications.
• The formation of martensite also leaves high
residual stresses in the steel.
• Therefore, hardening is almost always followed
• The purpose of tempering is to relieve residual
stresses and to improve the ductility and
toughness of the steel.
• This increase in ductility is usually attained at the
sacrifice of the hardness or strength.
• Hardness decreases and toughness increases
as the tempering temperature is increased.
• Dimensional Changes
– Martensite transformation is associated with an increase in volume.
– During tempering, martensite decomposes into a mixture of ferrite and
cementite with a resultant decrease in volume as tempering
• Retained Austenite:
– In practice, it is very difficult to have a completely martensitic structure
by hardening treatment.
– Some amount of austenite is present in the hardened steel.
– This austenite existing along with martensite is referred to as Retained
• Retained austenite is converted into martensite
by this treatment.
• The process consists of cooling steel to sub-zero
temperature which should be lower than the Mf
temperature of the steel (-30 to -70°C)
• Tempering is done immediately to remove the
internal stresses developed by Sub-zero
• Increase in hardness
• Increase in wear resistance
• Increase in dimensional stability
• In the conventional hardening process, the surface and
centre cool at different rates and transform to martensite
at different times.
• In Martempering, the steel is quenched into a bath kept
just above Ms. After allowing sufficient time for the
temperature to become uniform throughout the cross-
section, it is air-cooled through the martensitic range.
• The transformation to martensite occurs more or less
simultaneously across the section.
Adv. Of Martempering
• Residual stresses developed during
martempering is lower.
• It also reduces or eliminates susceptibility
• This is a heat-treating process developed to obtain a
structure which is 100 percent bainite.
• It is accomplished by first heating the part of the
proper austenitising temperature (790 - 915°C)
followed by cooling rapidly in a salt bath, held in the
bainite range (250-400°C).
• The piece is left in the bath until the transformation
to bainite is complete.
• Upper (550-350°C)
– Rods of Fe3C
• Lower (350-250°C)
– Fe3C Precipitates in Plates of
• It is still Ferrite and Cementite!
It’s just acicular.
• Increased ductility, toughness and strength
• Reduced distortion, which lessens subsequent
machining time, stock removal, sorting,
inspection and scrap.
• The shortest overall time cycle to through
harden within the hardness range of 35 - 55
HRc, which results in savings in energy and
– Limitation on size is necessary since the part is required to attain
uniform temperature of the quenching bath rapidly.
– Therefore, only comparatively thin sections can be austempered
•Austempered Ductile Iron (ADI)
Actually, the definition is Isothermal temperature heat treatment of ductile iron (spheroidized iron)
(a) As-received ductile iron.
(B) Aus-ferrite at high T.
(C) Aus-ferrite at low T.
• There are situations in which the requirement is
such that the outer surface should be hard and
wear resistant and the inner core more ductile
• Such a combination of properties ensures that
the component has sufficient wear resistance to
give long service life and at the same time has
sufficient toughness to withstand shock loads.
• This is the oldest and one of the cheapest
methods of case hardening.
• It is carried out on low carbon steels which
contain from 0.10 - 0.25% carbon.
• Carburising is carried out in the temperature
range of 900 - 930 °C
• The surface layer is enriched with carbon upto
0.7 - 0.9 %
• In this process, carbon is diffused into steel by
heating above the transformation temperature
and holding the steel for sufficient time in contact
with a carbonaceous material which may be a
solid medium, a liquid or a gas.
• Followed by Quenching and Tempering.
• The Steel is heated in contact with carbon
monoxide and/or a hydrocarbon which is readily
decomposed at the carburising temperature.
• Temperature : 870-950°C
• Gas carburising may be either batch or
Endo-gas Generator Carburizing furnace
• Gas atmosphere for carburising is produced
from liquid (methanol, iso-propanol) or gaseous
hydrocarbons (propane and methane)
• An endo-thermic gas generator is used to supply
• Approximate composition of the gas inflow into
the furnace is
– Nitrogen 40%
– Hydrogen 40%
– Carbon Monoxide 20%
– Carbon Dioxide 0.3%
– Methane 0.5%
– Water vapour 0.8%
– Oxygen in traces
• CHEMICAL REACTIONS
– C3H8 ---> 2CH4 + C (Cracking of hyd.carbon)
– CH4 + Fe ---> Fe(C) + 2H2
– CH4 + CO2 ---> 2 CO + 2H2
– 2CO + Fe ---> Fe(C) + CO2
1. Heat and soak at carburizing temperature to ensure temperature uniformity
2. Boost step to increase carbon content of austenite.
3. Diffusion step to provide gradual case/core transition.
4. Gas pressure or oil quench
CH4 + Fe=Fe(C) +2H2
Atmosphere carburized surface
profile, showing the IGO
Vacuum carburized surface profile,
showing a clear surface (no IGO)
• Popularly known as Salt bath carburising.
• In this process, carburising occurs through
molten cyanide (CN) in low carbon steel cast pot
type furnace heated by oil or gas.
• Bath temperature : 815 - 900°C
• Salt mixture consists of
– Sodium or Potassium Cyanide
– Barium chloride
• This method of carburising is also known as
• In this process, steel components to be heat
treated are packed with 80% granular coal and
20% BaCO3 as energizer in heat resistant boxes
and heated at 930°C in electric chamber furnace
for a specific period of time depends on case
• CHEMICAL REACTION
– Energizer decomposes to give CO gas to the steel furnace
– BaCO3 ---> BaO + CO2
– CO2 + C ---> 2CO
– Carbon monoxide reacts with the surface of steel
– 2CO + Fe ---> Fe(C) + CO2
• The surface layer of the steel is hardened by
addition of both carbon and nitrogen.
• This process is carried out a lower temperatures
(in the range 800 - 870°C) in a gas mixture
consisting of a carburising gas and ammonia.
• A typical gas mixture contains about 15% NH3,
5% CH4 and 80% neutral carrier gas.
Endo-gas Generator Carbonitriding furnace
• Nitriding is most effective for those alloy
steels which contain stable nitride forming
elements such as Aluminium, Chromium,
Molybdenum, Vanadium and Tungsten.
• Nitriding is carried out in a ferritic region below 590°C.
• So there is no phase change after nitriding.
• Before nitriding, proper heat treatment should be given
to steel components.
• All machining and finishing operations are finished
• The portions which are not to be nitriding are covered by
thin coating of tin deposited by electrolysis.
• Anhydrous ammonia gas is passed into the
furnace at about 550°C, where it dissociates into
nascent nitrogen and hydrogen.
2NH3 ----> 2[N]Fe + 3H2
• The surface hardness achieved varies from 900
to 1100 HV.
• Plasma nitriding is also known as ion nitriding process.
• In this process, the steel component to be nitrided is kept
at 450°C in vacuum at a negative potential of the order
of 1000 volts with respect to chamber.
• Then an appropriate mixture of N2 and H2 is passed at a
pressure of 0.2-0.8 m bar.
• As a result, plasma formation of these gases takes
Ion (Plasma) Nitriding Equipment
(Photos Courtesy of Surface Combustion)
• Flame hardening is done by means of
• Heating should be done rapidly by the torch and
the surface quenched before appreciable heat
transfer to the core occurs.
– For large work pieces
– Only a small segment requires heat treatment
– When the part requires dimensional accuracy
• Here, an alternating current of high frequency
passes through an induction coil enclosing the
steel part to be heat treated.
• The induced emf heats the steel.
• Immediately after heating, water jets are
activated to quench the surface.
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
Distance from surface, inch
Induction hardening of crankshaft
Adv. of Induction Hardening
• Provides energy savings
• Provides much higher heating rates
• Ease of automation and control
• Reduced floor space requirements
• Quiet and clean working conditions
• Suitability for integration in a production line
PS-1: PS-1<S> HEAT TREATMENT - QUENCH AND TEMPER AND
PS-2: PS-2<S> HEAT TREATMENT - GAS CARBURIZINGD
Ps-3: PS-3<S> HEAT TREATMENT - LIQUID BATH CASE HARDENING
PS-4: PS-4<S> HEAT TREATMENT - MISCELLANEOUS
Ps-5: PS-5<S> SELECTIVE HEATING SPECIFICATIONS - HEAT STAKING,
INDUCTION BONDING, INDUCTION HARDENING & TEMPERING PROCESSES,
LASER HEAT TREATING
PS-6: PS-6<S> HEAT TREATMENT – ALUMINUM ALLOYS
PS-7: PS-7<S> HEAT TREATMENT - FLAME HARDENING
PS-8: PS-8<S> HEAT TREATMENT - CARBONITRIDING
PS-9: PS-9<S> HEAT TREATMENT-AUSTEMPERED NODULAR AND
•Chrysler Engineering Standards Related to Heat Treatment Process