Heat treatment of steel

HEAT TREATMENT
OF STEEL
Material Science &
Metallurgy

TTT DIAGRAM
 T.T.T. shows relation between temperature & time taken for
decomposition transformations to take place in a metal when the
transformation is isothermal.
 Assess decomposition of austenite in a heat treatable steel.
 Provides information for the process of austenite decomposition under
non-equilibrium conditions. (Transformation of austenite to the time &
temperature conditions.)

DIFFERENCE BETWEEN
IRON-CARBON & TTT
 Study of Fe-C diagram shows study of cooled steels under
non-equilibrium conditions.
 Doesn’t involve reaction condition during heat treatment of
steel.
 It only shows phases & resulting microstructure
corresponding to equilibrium conditions.
 Fixing to austenitizing temperature & predicting phases
eventually obtained at given % of C & temperature.
 Microstructure & properties of steel depends upon rate of
cooling.
 As cooling rate increases transformation temperatures are
lowered & metastable (non-equilibrium) phases are formed.

CONTINUE…
 At a very high rate of cooling of steel produces Martensite
(non-equilibrium phase)

STEPS TO CONSTRUCT
TTT DIAGRAM
 Obtain large number of relatively small specimens.
 Place the sample in a molten salt bath held at the austeniting
temperature of 1080°C. Specimen are kept in a salt bath for a
long period of time to form complete austenite.
 When austenitized, specimen is transferred to other salt bath
at temperature of 810°C.
 After specimen react isothermally, quenched in cold water/
iced brine.
 As the specimen is quenched in cooled water, isothermal
reaction stops & remaining austenite suddenly transforms into
martensite.
 Reaction curve forms when large no. of specimen
isothermally reacted for veriying time periods.

CONTINUE…
 Finally data obtained from a series of isothermal reaction
curves (TTT) for the whole temperature range of austenite
instability for a given composition of steel.

DECOMPOSITION OF
AUSTENITE IN
EUTECTOID STEEL

AUSTENITE TO PEARLITE
TRANSFORMATION

AUSTENITE GRAIN SIZE &
ITS CONTROL
 Concept:
 The grain size of steel refers to austenite grain size.
 Austenite grain size is not altered much by rate of cooling
to room temperature.
 Very important factor in relation to strength, usefulness &
other physical properties of steel.
 Very important in developing fundamental theories of
metallic behavior.

CONTINUE…
Importance:
 Fine Grain Size:
Increase impact toughness
Improve machining finishes &
mitigate quenching cracks, distortion in quenching
 Coarse austenite grains:
Raise hardenability, tensile strength & creep strength &
Improve rough machinability
 Is important in determining the hardening response of the
steel.

CONTINUE….
 Grain Size Measurement:
 ASTM has standardized a grain size index for austenite grain
size in steel.

CONTINUE…
 Effect of Austenite Grain Size on Properties:
 This mainly influence on the reaction characteristics during
transformation.
 Property Fine Coarse
Depth Hardening Shallower Deeper
Retained Austenite Less More
Possibility of quenching cracks Shallower Deeper
Internal Stress after quenching Less More
Embrittlement by cold working Less More
Toughness More Less
Machinability after normalizing Inferior Better

CONTINUE…
(5) Metallic & non-metallic inclusions
(6) Heat treatment processes
(7) Cold Working
 Grain size can be controlled by mechanical working
operations.
i.e. Forging, Rolling

TEMPER BRITTLENESS IN
STEEL
 Brittleness resulting from
 (i) holding certain steels within particular temperature below
the transformation range or
 (ii) by cooling slowly through this range
 Brittleness appears at or below room temperature.
 After being quenched & tempered some alloy steels lose
their impact resistance & become brittle.
 Temper Brittleness occur in a steel if after tempering, they
are slowly cooled or held for longer time in the temperature
range of 600 to 300º C.

OVERHEATED STEEL
 If steel is heated well above the upper critical temperature
large austenite grains form.
 It develops undesirable coarse grains.
 If cooled slowly to room temperature, both ductility &
toughness of steel will decrease.
 The grain structure of the overheated steel can be corrected
by
(1) Suitable heat treatment
(2) Mechanical Work
(3) A Combination of the two

BURNT STEEL
 Permanently damaged by being heated close to its melting
point or by intergranular oxidation.
 Characterized by the presence of brittle iron oxide films
which render the steel unfit for service, except as scrape for
remelting.
 Burning is caused by incorrect grinding results in
discoloration of the work piece due to the heat.

HEAT TREATMENT
 Heat Treatment:
 An operation or combination of operations which involves heating &
cooling of a metal/alloy in solid state to obtain desirable conditions &
properties.
Heat Treatment Processes
Anneling
Normalising
Hardening Tempering Martempering
Austempering
Maraging

PURPOSE OF HEAT
TREATMENT
 Heat treatment is carried out to
(1) Cause relief of internal stresses developed during cold working,
welding, casting, forging etc.
(2) Harden & strengthen metals
(3) Improve machinability
(4) Change grain Size
(5) Soften metals for further working as in wire drawing or cold
rolling

CONTINUE….
(6) Improve ductility & toughness
(7) Increase heat, wear & corrosion resistance of materials
(8) Improve electrical & magnetic properties
(9) Homogenize the structure

ANNEALING
 Annealing: Process of heating a metal which is in a metastable or
distorted structural state, to a temperature which will remove the
instability/distortion & then cooling is so that the room temperature
structure is stable &/or strain free.
 Process of heating the metal to a temperature slightly above the
critical temperature & then cooling slowly.
 Purpose:
 To produce an even grain structure
 To relive the internal stresses caused by various manufacturing
processes or by previous treatments.
 To reduce the hardness & increase the ductility.
 After annealing, the metal becomes soft which improves
machinability.

ANNEALING
Types of Annealing Processes:
(i) Stress Relieving
(ii) Process Annealing
(iii) Spherodise Annealing
(iv) Full Annealing

PROCESS ANNEALING
 Subcritical Annealing
 To remove the effects of cold work
 To soften & permit further cold work in sheet & wire
industries.
 Ferrous alloys are heated to a temperature below 723ºC in
the range of (550-650ºC) & then cooled usually in air to soften
the alloy for further cold working.
 Associated with partial recrystallisation of the distorted ferrite.
 Doesn’t involve any phase change & the constituents ferrite
& cementite remain present in the structure throughout the
process.

SPHERODISE ANNEALING
 Involves subjecting steel to a selected temperature (near the
formation range) to produce a spheroidal / globular form of
carbide in steel.
 How is it helpful ?
 Improves machinability.
 Improves surface finish during machining.
 Facilitates a subsequent cold working operation.
 Soften tool steels & some of the air hardening alloy steels.
 Prevents cracking of steel during cold forming operations.
 Obtain a desired structure for subsequent heat treatment.

CONTINUE…
 Application:
 Extensively employed for high carbon (tool) steels to
transform lamellar pearlitic cementite into spheroidal type.
 Condition:
 Heating steel & then holding it prolonged at a temperature
just below 723ºC (650-700ºC).

FULL ANNEALING
 Involves prolonged heating just above the 723ºC to produce
globular form of carbide (To improve machinability).
 Annealing a ferrous alloy to austenitic condition & then
cooling slowly in furnace through 723ºC up to a low
temperature.
 Cooling rate 25ºC to 30ºC /hr. to 600ºC.
 Advantage:
 Refines grains Removes strains (from forging &
castings)
 Induces softness Improves machinability
 Improves formability

NORMALIZING
 Heating the metal to auste nitic te m pe rature rang e & cooling
in air at room temperature.
 Purpose:
 Produces a uniform structure
 Reduces internal stresses
 Refines the grain size of steel
 Improves structures in welds
 Produces a harder & stronger steel than full annealing.
 Improves engineering properties of steels.

HEAT TREATING
TEMPERATURES FOR
CARBON STEEL

DIFFERENT
STRUCTURES WITH
DIFFERENT HEAT
TREATMENT

HARDENING (BY
QUENCHING)
 Increases hardness of steel by quenching.
 Tools & machine parts subjected to heavy duty are usually
hardened.
 hardening of steel requires the formation of martensite.
 After hardening, tempering is done to:
(i) Hardens steel to resist wear
(ii) Enables steel to cut other metals
(iii) Improves strength, toughness & ductility
(iv) Develops best combination of strength.

CONTINUE…
 Hardening Procedure:
Steel with sufficient carbon (0.35 to 0.70 %)
 Heated 30 to 50ºC above A3 line.
 Held at that temperature from 15 to 30 min.
 Cooled rapidly or quenched in a suitable medium (Water,
Brine, Oil etc.)
 Produce desired rate of cooling

CONTINUE…
 Degree of hardness depends upon:
 Composition of steel
 Nature & Properties of quenching medium
 Size of the object to be quenched
 Rate of Cooling
 Surface condition of metal (workpiece): Impurities, scale etc.

TEMPERING
 After quench hardening, heating the hardened steel to a
temperature below the lower critical temperature (A1) is called
Tempering.
 Why Tempering?
 Quench hardening produces martensite & retained
austenite
 Martensite brittle, hard & highly stressed
 After quenching, cracking & distortion occure in hardened
steel.
 Quenched hardened steel also retain austenite.

CONTINUE…
 Condition:
 Heating hardened steel below the lower critical temperature.
 Holding it at that temperature for 3 to 5 minutes.
 Cooling the steel (in water, oil or air) either rapidly or slowly.

CONTINUE…
 Purpose:
 Relieve residual stresses.
 Improve ductility
 Improve toughness
 Reduce hardness
 Increase % elongation

CASE HARDENING
 Case hardening:
 Case-hardening or surface hardening is the process of hardening the
surface of a metal object. For iron or steel with low carbon content, which
has poor to no hardenability of its own, the case-hardening process
involves infusing additional carbon into the surface layer. Case-hardening
is usually done after the part has been formed into its final shape.
Case Hardening
Flame
Induction
Hardening
Carburizing
Nitriding
Cyaniding
Carbonitriding

CONTINUE…
 Flame/Induction Hardening:
 Surface of the steel is heated very rapidly to high
temperatures then cooled rapidly (Water Cooling).
 Creates a case of martensite on surface.
 A carbon content of 0.3–0.6 wt% C is needed for this type of
hardening.

CONTINUE…
 Application:
 Where outer layer is hardened to be file resistant
 Mechanical Gears (For Toughness, Hardness & to avoid
catastrophic Failure).

CONTINUE…
 Carburizing:
 Heat treatment process in which iron or steel absorbs carbon
while the metal is heated in the presence of a carbon bearing
material, such as charcoal or carbon monoxide.
 To make metal harder
Workmetal
Properties
Effects of Carburizing
Mechanical  Increase Surface hardness
 Wear Resistance
 Increase fatigue/tensile strength
Physical  Grain growth may occur
 Change in volume may occur
Chemical  Increased surface carbon content

CONTINUE…
 Nitriding:
 Heat steel in the presence of ammonia or other nitrogenous
material so as to increase hardness and corrosion resistance.
 Diffuse nitrogen into the surface of a metal to create a case-
hardened surface.
These processes are most commonly used on low-carbon, low-
alloy steels.
Also used on medium and high-carbon steels, titanium, aluminium
and molybdenum.
In 2015, nitriding was used to generate unique duplex
microstructure (Martensite-Austenite, Austenite-ferrite), known to
be associated with strongly enhanced mechanical properties

CONTINUE…
Nitriding heats the steel part to 482–621 °C (900–1,150 °F) in
an atmosphere of ammonia gas and dissociated ammonia.
 Application:
 Gears, crankshafts, camshafts, cam followers, valve parts,
extruder screws, die-casting tools, forging dies, extrusion dies,
firearm components, injectors and plastic-mold tools.

CONTINUE…
 Cyaniding:
 Cyaniding is a case-hardening process that is fast and
efficient; it is mainly used on low-carbon steels.
 The part is heated to 871-954 °C (1600-1750 °F) in a bath of
sodium cyanide and then is quenched and rinsed, in water or
oil, to remove any residual cyanide.
 This process produces a thin, hard shell (between 0.25 -
0.75 mm, 0.01 and 0.03 inches) that is harder than the one
produced by carburizing

CONTINUE…
 Application:
 bolts, nuts, screws and small gears
 Drawback:
 cyanide salts are poisonous.

CONTINUE…
 Carbonitriding:
Carbonitriding is similar to cyaniding except a gaseous
atmosphere of ammonia and hydrocarbons is used instead of
sodium cyanide. If the part is to be quenched, it is heated to
775–885 °C (1,427–1,625 °F); if not, then the part is heated to
649–788 °C (1,200–1,450 °F).

Heat treatment of steel

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