Heat treatment of materials

1. HEAT TREATMENT
Thermal processing is also known as heat treatment
Purpose of heat treatment
 To refine grain structure/size
 To impart phase changes,
 Improvement in ductility
 Relieving internal stresses
 Increase of strength
 Improvement in machinability, toughness; etc.

2. REASONS FOR HEAT TREATMENT
 Softening: Softening is done to reduce strength or
hardness, remove residual stresses, improve toughnesss,
restore ductility, refine grain size or change the
electromagnetic properties of the steel.
 Restoring ductility or removing residual stresses is a
necessary operation when a large amount of cold working
is to be performed, such as in a cold-rolling operation or
wiredrawing.
 Hardening: Hardening of steels is done to increase the
strength and wear properties. One of the pre-requisites for
hardening is sufficient carbon and alloy content. If there is
sufficient Carbon content then the steel can be directly
hardened. Otherwise the surface of the part has to be
Carbon enriched using some diffusion treatment hardening
techniques.

3. FACTORS AFFECTING HEAT TREATMENT
 Temperature up to which material is heated
 Length of time that the material is held at the
elevated temperature
 Rate of cooling, and the surrounding atmosphere
under the thermal treatment.
 Material,
 pre-processing of the material’s chemical
composition
 size and shape of the object
 Final properties desired
 Material’s melting point/liquidus, etc.

4. TYPES OF HEAT TREATMENT
 Annealing
 Normalising
 Hardening and quenching
 Tempering
 Martempering
 Austempering

5. ANNEALING
 Heat treatment process in which the material is
taken to a high temperature(meta stable) to remove
the instability.
 kept there for some time and then cooled.
 High temperatures allow diffusion processes to
occur fast.
 The time at the high temperature (soaking time)
must be long enough to allow the desired
transformation to occur.
 Cooling is done slowly(mainly in furnace) to avoid
the distortion (warping) of the metal piece, or even
cracking, caused by stresses induced by differential
contraction due to thermal in homogeneities .

6. BENEFITS OF ANNEALING
 Relieves internal stresses
 Increase softness, ductility and toughness
 Produce a specific microstructure
 Removing gasses
 Improving machinability

7. TYPES OF ANNEALING
 Process annealing
 Stress relief annealing
 Full annealing
 Spheroidizing Annealing
Process annealing
 Is primarily applied to cold worked metals to negotiate
the effects of cold work.
 During this heat treatment, material becomes soft and
thus its ductility will be increased considerably.
 It is commonly sandwiched between two cold work
operations. During this, recovery and recrystallization
are allowed whereas grain growth was restricted.
 Normally heated just below the transformation range
 Ferrous alloys are heated in the range of 550-650oC

8. Stress relief
 Removes the stresses that might have been
generated during quenching, casting, machining,
welding, cold working etc..
 Unless removed, these stresses may cause
distortion of components. Temperature used is
normally low such that effects resulting from cold
working are not affected.
Full annealing
 Is normally used for products that are to be
machined subsequently, such as transmission gear
blanks.
 After heating(to austenitizing temperature) and
keeping at an elevated temperature, components
are cooled in furnace at the rate of about 20 ºC/hr
in a furnace to about 50 ºC into the Ferrite-
Cementite range. Typically, the product receives
additional heat treatments after machining to

9. Spheroidizing Annealing
 Is an annealing process used for high carbon steels
(Carbon > 0.6%) that will be machined or cold formed
subsequently.
 Mainly improves machinability.
 Heat the part to a temperature just below the Ferrite-
Austenite line, or below the Austenite-Cementite line,
essentially below the 727 ºC . Hold the temperature for
a prolonged time and follow by fairly slow cooling for
the formation of spheroidal or global form of carbide in
steel. For tool and alloy steels heat to 750 to 800 ºC
(1382-1472 ºF) and hold for several hours followed by
slow cooling.

10. NORMALIZING(AIR QUENCHING)
 Normalizing is a type of heat treatment applicable to ferrous
metals only.
 The purpose of normalizing is to remove the internal stresses
induced by heat treating, welding, casting, forging, forming, or
machining
 It is the process of raising the temperature to over 60 º C , above
Austenite range. It is held at this temperature to fully convert the
structure into Austenite, and then removed from the furnace and
cooled at room temperature under natural convection.
 It is used to refine the grains and produce a more uniform and
desirable size distribution. It involves heating the component to
attain single phase (e. g : austenite in steels), then cooling in
open air atmosphere.
 This results in a grain structure of fine Pearlite with excess of
Ferrite or Cementite.
 Cooling rate is faster than full annealing
 The resulting material is soft (free from residual stress); the
degree of softness depends on the actual ambient conditions of
cooling & Produces a uniform structure.

11. QUENCHING
 Quenching is heat treatment process where
material is cooled at a rapid rate from elevated
temperature to produce Martensite phase. This
process is also known as hardening.
 Increases the hardness.
 Rapid cooling rates are accomplished by immersing
the components in a quench bath that usually
contains quench media in form of either water or oil,
accompanied by stirring mechanism.
 Quenching media- water, oil ,salt water, Brine(Nacl)

12. TEMPERING PROCESSES
 Tempering is a process done subsequent to quench
hardening. Quench-hardened parts are often too brittle.
This brittleness is caused by a predominance of
Martensite. This brittleness is removed by tempering.
Tempering results in a desired combination of hardness,
ductility, toughness, strength, and structural stability.
 Tempering is the process of heating martensitic steel at
a temperature below the eutectoid transformation
temperature to make it softer and more ductile. During
the tempering process, Martensite transforms to a
structure containing iron carbide particles in a matrix of
ferrite.
 Relieves internal stress
 Reduces hardness
 Increases strength

13. INTERRUPTED QUENCHING
 Two stage quenching process.
 The piece is first quenched in a medium which cools
rapidly.
 After it is furher quenched in a second medium (but
slowly than the first (medium) to avoid bainite
transformation.
 Eg : Martempering, Austempering

14. AUSTEMPERING
 (i) Heated to above the critical range to make it to
austenite.
 (ii) Quenched into salt bath at temperature until
formation of bainite.
 (iii) Subsquently cooling the work piece in air.
 As the part is held longer at this temperature, the
Austenite transforms into Bainite. Bainite is tough
enough so that further tempering is not necessary, and
the tendency to crack is severely reduced.
 Allowed to cool to room temperature.
 The biggest advantage of Austempering over rapid
quenching is that there is less distortion and tendency to

15. MARTEMPERING
 (i) Heated to above the critical range to make it to
austenite.
 (ii) Quenched into salt bath at temperature above Ms.
 (iii) Subsquently cooling the work piece in air.
 Martempering is similar to Austempering except that the
part is slowly cooled through the martensite
transformation. The structure is martensite, which needs
to tempered just as much as martensite that is formed
through rapid quenching.
 First quenched in salt bath.
 Subsequently cooling the workpiece in air through
martensite range.

16. BAINITE
 Is an acicular microstructure (not a phase) that forms in
steels at temperatures from approximately 250-550°C
depending on alloy content.
 It is one of the decomposition products that may form when
austenite (the face centered cubic crystal structure of iron) is
cooled past a critical temperature of 727 °C.
 A fine non-lamellar structure, bainite commonly consists of
cementite and dislocation-rich ferrite. The high concentration
of dislocations in the ferrite present in bainite makes this
ferrite harder.
 Bainite forms needles or plates.

17. CASE HARDENING
 Case hardening is a process in which an alloying
element, most commonly carbon or nitrogen, diffuses
into the surface of a monolithic metal. The resulting
interstitial solid solution is harder than the base material,
which improves wear resistance without sacrificing
toughness.
 Case hardening is specified by hardness and case
depth.

18. CARBURIZING
 Is a process of adding Carbon to the surface.
 This is done by exposing the part to a Carbon rich
atmosphere at an elevated temperature and allows
diffusion to transfer the Carbon atoms into steel.
 This diffusion will work only if the steel has low carbon
content,(0.1%-0.25%).
 If the steel had high carbon content to begin with, and is
heated in a carbon free furnace, such as air, the carbon
will tend to diffuse out of the steel resulting in
Decarburization.

19. PACK CARBURIZING:
 Parts are packed in a high carbon medium such as
carbon powder or cast iron shavings and heated in a
furnace for 12 to 72 hours at 900 ºC
 At this temperature CO gas is produced which is a
strong reducing agent. The reduction reaction occurs
on the surface of the steel releasing Carbon, which is
then diffused into the surface due to the high
temperature.
 The Carbon on the surface is 0.7% to 1.2% depending
on process conditions. The hardness achieved is 60 -
65 RC. The depth of the case ranges from about 1 mm
up to 2 mm .
 Some of the problems with pack carburizing is that the
process is difficult to control as far as temperature
uniformity is concerned, and the heating is inefficient.

20. CARBURIZING MEDIUM CONSISTS OF 80% OF CHARCOAL WITH COKE
AND 20% OF BARIUM CARBONATE

21. GAS CARBURIZING:
 Gas Carburizing is conceptually the same as pack
carburizing, except that Carbon Monoxide (CO) gas
is supplied to a heated furnace .
 This processes overcomes most of the problems of
pack carburizing. The temperature diffusion is as
good as it can be with a furnace.
 The only concern is to safely contain the CO gas.
 The depth of the case ranges from about 0.2 mm
up to 0.5 mm

23. LIQUID CARBURIZING:
 The steel parts are immersed in a molten carbon rich
bath. In the past, such baths have cyanide (CN) as the
main component. However, safety concerns have led to
non-toxic baths that achieve the same result.
 Medium will be fused salt bath composed of sodium
cyanide, sodium chloride, barium chloride.
 The depth of the case ranges up to 0.08 mm

25. NITRIDING
 Produces the hardest surface of any of the hardening
processes.
 It differs from the other methods in that the individual
parts have been heat-treated and tempered before
nitriding.
 The parts are then heated in a furnace that has an
ammonia gas atmosphere(NH3).
 No quenching is required so there is no worry about
warping or other types of distortion.
 This process is used to case harden items, such as
gears, cylinder sleeves, camshafts and other engine
parts, that need to be wear resistant and operate in high-
heat areas.
 Heated in a furnace for 40 to 100 hours at 550 ºC

27. CYANIDING
 Diffusion of both carbon and nitrogen
 This process is a type of case hardening that is fast and
efficient.
 Preheated steel is dipped into a heated cyanide bath and
allowed to soak(30%NaCN,40%Na2Co3,30%sNaCl).
 Used at temperatures 787-898oC
 Upon removal, it is quenched and then rinsed to remove
any residual cyanide. This process produces a thin, hard
shell that is harder than the one produced by carburizing
and can be completed in 20 to 30 minutes vice several
hours.
 The major drawback is that cyanide salts are a deadly
poison.
 Nitrogen increases hardenability

29. CARBONITRIDING
 Is most suitable for low carbon and low carbon alloy steels.
In this process, both Carbon and Nitrogen are diffused into
the surface.
 The parts are heated in an atmosphere of hydrocarbon
(such as methane or propane) mixed with Ammonia (NH3)
gas. The process is a mix of Carburizing and Nitriding.
 Carburizing involves high temperatures (around 900 ºC)
and Nitriding involves much lower temperatures (around
600 ºC). Carbonitriding is done at temperatures of 760 -
870 ºC which is higher than the transformation
temperatures of steel that is the region of the face-centered
Austenite.
 After carbotonitriding quenching is done followed by
tempering
 Applied for nuts, bolts and gears etc.

31. FLAME HARDENING -
 A heat treat method used to harden the surface of some parts
where only a small portion of the surface is hardened and
where the part might distort in a regular carburizing or heat
treating operation.
 The operation consists of heating the surface to be hardened by
an acetylene torch to the proper quenching temperature
followed immediately by a water quench and proper tempering.
 Generally wrought or cast steels with carbon contents of .30 to
.40%, low alloy steels, and ductile and malleable cast irons are
suitable for flame hardening.
 After hardening stress relieve is required
Types
 Stationary
 Progressive- tool moves over work piece.
 Spinning- Tool is stationary, w/p rotates
 Progressive spinning

33. INDUCTION HARDENING
 Is a form of heat treatment in which a metal part is
heated by induction heating and then quenched.
 It is done by an alternating magnetic field to a
temperature above or within a specific transformation
range
 The quenched metal undergoes a martensitic
transformation, increasing the hardness and brittleness of
the part.
 This procedure of hardening can be specifically applied
to both full as well as surface hardening methods. This is
the kind of heat treatment process in which the metal part
is heated and later it is quenched.
 Typical applications are power train, suspension, engine
components and stampings.

35. HARDENABILITY
 Hardenability is the ease with which the hardness can be
attained in the depth direction of an object.
 Hardness is the measure of resistance to plastic
deformation(by indentation).
 The ability of the material to harden deeply upon
quenching, and takes into consideration the size of the
part and the method of quenching.
 The test used to determine the hardenability of any grade
of steel is the Jominy end quench Test.
 Hardenability is a material property, dependent on
chemical composition and grain size, but independent of
the quenching medium or quenching system (cooling
rate).

37. PRECIPITATION OR AGE HARDENING
 Hardening process applicable especially for non ferrous
alloys
 Objective is to create a dense and fine dispersion of the
particles
 Step I: Heated to a (high)temperature between the solvus
and solidus temperature(near austenite) for a short period
of a time .
 Step II: Quenching is done
 StepIII: Lowering of energy is done by heating at a low
temperature for a long period of time and cooling at a very
slow rate(room temperature) .

38. PRECIPITATION HARDENING PROCESS