HEAT TREATMENT OF STEEL

MATERIAL SCIENCE

Introduction

“A combination of heating and cooling operation, timed and applied to a metal or alloy in the solid state in a way that will produce desired properties”

IRON-CARBON DIAGRAM

TRANSFORMATION IN STEEL

ANNEALING

It is the process of softening of hard metal by heating to a high temperature followed by slow cooling.

PROCESS

Heating the steel at 300 to 500 C above the upper critical temperature for hypo-eutectic steel and by the same temperature above lower critical temperature for hyper-eutectic steel.

Holding it for some period of time at that temperature so that internal changes may take place.(1-2min/mm)

Allowing it to cool down slowly in furnace.

Steel when heated it becomes Austenite and then when it is maintained at that temperature then transformation of Austenite into soft pearlite, mixed with ferrite or cementite is formed.

Slow cooling after high temperature makes the structure as well grains to refine, the stresses to relieve and trapped gases to escape.

SPHERODISING

1. Heat to just below Lower Critical Temperature. (about 650-700 0C)

2. Cool very slowly in the furnace.3. Structure will now be spheroidite, in

which the Iron Carbide has ‘balled up’.

4. Used to improve the properties of medium and high carbon steels prior to machining or cold working.

NORMALISING

TEMPERING

HARDENING