Post on 04-Apr-2018
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Heat Treatment
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Heat treatment may be defined as an
operation or combination of operations
involving heating and cooling of a metal/alloy
to obtain desirable conditions, e.g., that of
relieved stresses, properties, e.g., better
machinability, improved ductility,
homogeneous structures, etc
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19
Adapted from
Fig. 10.27,
Callister 6e.
SUMMARY: PROCESSING OPTIONS
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Bainite
coarse fine
Austenite
Martensite
Moderate cooling (AS)
Isothermal treatment (PCS)
Tempered
Martensite
Pearlite
AS: Alloy Steel
PCS: Plain-carbon Steel
Slow
Cooling
Rapid
Quench
Spheroidite
Re-heat
Re-heat
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Slow Cooling
Time in regionindicates amount ofmicroconstituent!
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Medium Cooling
Cooling Rate, R, isChange in Temp /Time C/s
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Fast Cooling
This steel is veryhardenable 100%
Martensite in ~ 1minute of cooling!
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Classification of heat-treatment processes
1. annealing
2. Normalizing
3. Hardening 4. Tempering
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Hardenability
Whereas hardness is a measure of resistance to plasticdeformation (by indentation), hardenability is the easewith which hardness may be attained in the depthdirection of an object.
Hardenability may also be remembered as the ability ofa steel to become unifoirmily hard or to harden indepth direction.
It should be remembered that hardenability is not anindication of the hardeness of a steel, rather
hardenability is an index of the depth to whichmartensite can be formed in a given steel aas the resultof quenching
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Hardenability value for a given steel is the
diameter in inches of a cylindrical bar that will
form 50% martensite t the centre during an
ideal quench.
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Factors affecting hardenability
1. Composition & menthod of manufacture
2. Quenching media and method of quenching
3. The mean composition of the austentite before
quenching, including the nature and amount ofalloying elements.All alloying elements, except Co, tend to increase thehardenability.
As compared to carbon steels, alloy steels harden to
a considerably larger depth due to the high stabilityof the supercooled austentite and the correspondinglower critical cooling rate
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The size of austentite grains before quenching.
The larger the grains prior, greater is the
degree of hardenability.
The homogeneity of the austentite before
quenching.
Method to determine the Hardenability
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Jominy/ End Quench Test
A 25-mm dia by 100-mm long bar is properly
austenitized and quenched on the end in a
standardized way as shown in figure
Softest
Hardest
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Case Hardening & Surface Treatment
Many industrial application such as cams, gears,
etc., require a hard wear resistant surface called
the case and a relatively soft, tough and shock
resistant inside, called the core. No carbon steel can posses both these
requirements at the same time.
Low carbon steel 0.1% C will be tough and Highcarbon steel 0.9%C will possess adequate
hardness when suitably heat treated
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However, both these requirements may be met byemploying a low carbon steel with suitable coreproperties and then adding (or penetrating) Carbon, N or
both to the surface of the steel part in order to provide ahardened case (or layer) of definite depth. Thesetreatments are known as Case hardening.
Processes used to create hardened cases are
1. Carburizing2. Nitriding
3. Cyaniding
4. Carbonitriding
5. Flame hardening
6. Induction hardening
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Assignment (Submission date- 9/4/2010)
1. Write about
(a) Surface Hardening
(b) Case Hardening(c) Age Hardening
(d) Precipitate Hardening
2. Elaborate Jominy Hardening Test.