Heat Treatment .Part2

7/30/2019 Heat Treatment .Part2

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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.

Heat Treatment .Part2

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