Heat Treatment

• 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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Austenite ()

Bainite ( + Fe3C plates/needles)

Pearlite ( + Fe3C layers + a proeutectoid phase)

Martensite (BCT phase diffusionless

transformation)

Tempered Martensite ( + very fine

Fe3C particles)

slow cool

moderate cool

rapid quench

reheat

Str

ength

Duct

ility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends

Adapted from Fig. 10.27, Callister 6e.

SUMMARY: PROCESSING OPTIONS

Bainite

coarse fine

Austenite

Martensite

Moderate cooling (AS)Isothermal treatment (PCS)

Tempered Martensite

Pearlite

AS: Alloy SteelPCS: Plain-carbon Steel

Slow Cooling

RapidQuench

Spheroidite

Re-heat

Re-heat

Slow Cooling

Time in region indicates amount of microconstituent!

Medium Cooling

Cooling Rate, R, is Change in Temp / Time °C/s

Fast Cooling

This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

Classification of heat-treatment processes

• 1. annealing• 2. Normalizing• 3. Hardening• 4. Tempering

Hardenability

• Whereas hardness is a measure of resistance to plastic deformation (by indentation), hardenability is the ease with which hardness may be attained in the depth direction of an object.

• Hardenability may also be remembered as the ability of a steel to become unifoirmily hard or to harden in depth direction.

• It should be remembered that hardenability is not an indication of the hardeness of a steel, rather hardenability is an index of the depth to which martensite can be formed in a given steel aas the result of quenching

• 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.

Factors affecting hardenability

1. Composition & menthod of manufacture2. Quenching media and method of quenching3. The mean composition of the austentite before

quenching, including the nature and amount of alloying elements.

All alloying elements, except Co, tend to increase the hardenability. As compared to carbon steels, alloy steels harden to a considerably larger depth due to the high stability of the supercooled austentite and the corresponding lower critical cooling rate

• 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

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

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 High carbon steel 0.9%C will possess adequate hardness when suitably heat treated

• However, both these requirements may be met by employing a low carbon steel with suitable core properties and then adding (or penetrating) Carbon, N or both to the surface of the steel part in order to provide a hardened case (or layer) of definite depth. These treatments are known as Case hardening.

• Processes used to create hardened cases are1. Carburizing2. Nitriding3. Cyaniding4. Carbonitriding5. Flame hardening6. Induction hardening

• Surface treatments involve applying coatings to the surface of metals/alloys..

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.