heat treatment

1. HEAT TREATMENT
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g

2. 1.1. Introduction: Heat Treatment
• An operation or combination of operations, involving heating and
cooling of a metal or alloy in its solid state with the objective
of changing the characteristics of the material.
Purpose
• To improve machinability.
• To change or refine grain size.
• To relieve the stresses of the metal induced during cold or hot
working.
• To improve mechanical properties.
• To improve magnetic and electric properties.
• To increase resistance to wear, heat and corrosion.
• To produce a hard surface on a ductile interior.
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3. 1.2. Constituents of Iron & Steel
Microscopic Constituents
1. Ferrite
2. Cementite
3. Pearlite
4. Martensite
5. Austenite
6. Troostite
7. Sorbite
Allotropic Constituents
1. Pure Iron
2. Graphite
3. Slag
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4. 1.2.1. Ferrite
• Contains little or no carbon.
• Very soft and ductile.
• A.k.a alpha iron.
• Trace presents in almost all
range of steels.
• Doesn’t harden when cooled
rapidly.
• Forms smaller crystals when
cooled from a bright red
heat at rapid rate.
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5. 1.2.2. Cementite
• Definite carbide of iron
Fe3C.
• Extremely hard; harder than
ordinary hardened steel or
glass.
• Increases with the
proportion of carbon
present.
• Hardness and brittleness of
cast iron is due to
cementite.
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• 6.7 % carbon contain in
globular, network or massive
forms.
• Magnetic below 25oC.
• Decreases tensile strength
but increases hardness &
cutting qualities.

6. 1.2.3. Pearlite
• 87.5 % ferrite & 12.5 %
cementite.
• Alternate layers of ferrite
and cementite in steel.
• Thickness and distance
between alternate layers is
governed by rate of cooling.
• Its eutectoid of steel.
• Appearance like mother of
pearl.
• Seen in soft steel along with
ferrite.
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• Hardness increases with its
proportion.
• Hard steel are mixtures of
pearlite and cementite.

7. 1.2.4. Martensite
• Hard brittle mass of fibrous
or needle like structures.
• Chief constituent of
hardened steel.
• Seen to be produced by the
rapid quenching of high
carbon steel from a slightly
higher temperatures.
• Not as tough as austenite
• Unlike austenite its magnetic
in nature.
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8. 1.2.5. Austenite
• Stable only within particular
range of composition &
temperature.
• Non-magnetic by nature.
• When cooled below 700oC it
completely transforms into
ferrite & cementite to form
eutectoid pearlite.
• Austenite steels cannot be
hardened by normal heat
treatment methods.
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9. 1.2.6. Troostite
• Structure in steel consisting
of very finely divided iron
carbide i.e. alpha iron.
• Produced by tempering
martensite steel between
250 to 450oC or by
quenching steel at a speed
insufficient to fully suppress
thermal change points.
• Second method’s product
structure can be termed
very fine pearlite.
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10. 1.2.7. Sorbite
• Structure consisting of
evenly distributed carbide
of iron particles in a mass of
ferrite.
• Formed when fully hardened
steel is tempered at
between 550 and 650oC.
• Characterized by strength &
a high degree of toughness.
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11. 2. Heat Treatment Process
• Annealing
• Normalizing
• Hardening
• Tempering
• Surface Hardening
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12. 2.1. Heat Treatment Process: Annealing
Objectives:
• To soften the metals.
• To increase machinability.
• To refine grain size due to phase recrystallization.
• To increase ductility of metals.
• To prepare steel for sufficient treatment.
• To modify electrical & magnetic properties.
• To relieve internal stresses.
• To remove gases.
• To produce a definite microstructure.
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13. 2.1.1. Annealing: Full
• Heating steel to a temperature
above transformation range,
holding for 1-2 hours and cooling
at a predetermined rate to
obtained desired microstructure.
• Considerable degree of
transformation of phase i.e. from
austenite to pearlite and ferrite
in hypo eutectoid region, pearlite
in eutectoid steels and pearlite
and cementite in hypereutectoid
steels.
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14. 2.1.1. Annealing: Full
• Degree of transformation is highly dependent upon the rate of cooling
i.e. slower rate of cooling means better annealing.
• Results in recrystallization hence grain become refined.
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% carbon content Annealing temperature oC
Less than 0.15 875 to 930
0.15 to 0.45 (mild steel) 840 to 870
0.45 to 0.50 (medium) 815 to 840
0.50 to 0.80 (medium) 780 to 810
0.80 to 1.50 (high carbon
steel)
760 to 780

15. 2.1.2. Annealing: Process/Sub-
critical/incomplete
• Heating steel to a temperature
under transformation range,
holding for 2-4 hours and air
cooling.
• Results softening of steel.
• Less scaling and warping can be
controlled.
• Improves machinability and
relieve stresses from cold works.
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16. 2.1.3. Annealing: Spheroidisation
• Heating to a temperature just
above the critical and cool very
slowly (about 6oC per hour).
• Causes practically all carbides in
the steel to agglomerate in the
form of small globules or
spheroids.
• Wide range of hardness is
imminent as size of globules is
directly related to hardness.
• Used for all steels with >0.6% C
subjected to machining and cold
forming.
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17. 2.1.4. Annealing: Diffusion/Homogenizing
• Heating to a temperature
sufficiently above the critical
one (1000-1200oC)and is held at
this temperature for prolonged
periods usually 10-20 hrs
followed by slow cooling.
• Used to remove structural non-
uniformity. These defects
promote brittleness and reduce
ductility and toughness of steel.
• Also know as Malleabilising.
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18. 2.2. Heat Treatment Process: Normalising
Objectives:
• To eliminate coarse grain structure obtained during forging, rolling and
stamping.
• To increase strength of medium carbon steel.
• To improve machinability of low carbon steel.
• To improve the structure of welds.
• To reduce internal stresses.
• To achieve desired results in mechanical and electrical properties.
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19. 2.2. Heat Treatment Process: Normalising
• Process of heating the steel
approximately 4oC above critical
temperature followed by cooling
below this range in still air.
• The steel produced is harder
and stronger but less ductile
than annealed steel having the
same composition.
• Used to refine grain structure
and to relieve stresses set up in
castings, forgings, etc.
• Used to wipe out the effects of
previous heat treatments.
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20. 2.3. Heat Treatment Process: Hardening
• Heating to a temperature above
critical point held at this
temperature and then rapidly
cooled (Quenched) in water, oil
or molten salt baths.
• Followed by tempering to
• Reduce brittleness
• Relieve the internal stresses
• Obtain pre-determined
mechanical properties.
• Results in formation of
martensite.
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21. 2.3.1. Hardening Methods: Single
Quenching
• Results in a very high rate of
cooling owing to large
temperature gap between the
job piece and surrounding.
• Results in hardening cracks,
distortion and other
unstabilities in the grain
structures of the job piece.
• Hardness obtained is fairly high
compared to in other methods.
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22. 2.3.2. Hardening Methods: Double
Quenching
• Job piece is quenched in water
in a temperature of 300oC to
400oC and quickly transferred
to a less intensive quenching
medium.
• Taps, dies, milling cutters are
subjected to this type of
treatments.
• The purpose of second
quenching is to reduce internal
stresses associated with
austenite to martensite
transformation.
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23. 2.3.3. Hardening Methods: Self
Tempering
• Job piece is quenched upto a
predetermined time and
withdrawn.
• The core possess a temperature
higher than the surface.
• When job piece reaches
tempering temperature it is re-
immersed in quenching liquid
• Employed in chisels, sledge
hammers, hand hammers,
punches, and other tools.
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24. 2.3.4. Hardening Methods: Stepped
Quenching or Mar tempering
• Heating to a hardening
temperature then quenched in a
medium of 150oC to 300oC.
• The job piece is kept here until
it reaches medium’s temperature
and than cooled further to room
temperature in air or oil.
• Produces martensite with
minimum distortion and residual
stresses.
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25. 2.3.5. Hardening Methods: Isothermal or
Austempering
• Similar to martempering but
with longer holding time at hot
bath to ensure complete
austenite transformation to
bainite.
• Gives greater ductility in carbon
steel than full hardening and
tempering.
• Minimizes crack and distortion.
• Gives good impact resistance.
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26. 2.3.6. Hardenability
• Ability of steel to develop its
maximum hardness when
subjected to normal hardening
heating and quenching cycle.
• Depth of hardening is dependent
upon:
• Hardenability of parent
steel.
• Severity of quench used.
• Size and shape of piece.
• Surface condition and
austenite grain size.
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27. 2.4. Tempering
• Heating quenched, hardened
steel, steel in martensite
condition, to some pre-
determined temperature
between room temperature and
critical temperature of the steel
for a certain length of time,
followed by air cooling.
• Carried out to:
• Increase toughness.
• Decrease hardness.
• Stabilize structure.
• Relieve Stresses
• Change Volume.
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28. 2.4.1. Low Temperature Tempering
• Performed in the range from
150oC to 250oC.
• Purpose is to reduce internal
stresses and to increase
toughness without an
appreciable loss of hardness.
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29. 2.4.2. Medium Temperature Tempering
• Performed in the range from
350oC to 450oC.
• Purpose is to attain high elastic
limits in conjunction with high
toughness in laminated springs,
coil springs, etc.
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30. 2.4.2. High Temperature Tempering
• Performed in the range from
500oC to 650oC.
• Eliminates almost all internal
stresses and provides
sufficiently favorable rate of
strength and toughness for
steel structures.
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