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Heat Treatment of Steel

Steels can be heat treated to produce a great variety of microstructures and

properties. Generally, heat treatment uses phase transformation during

heating and cooling to change a microstructure in a solid state.

In heat treatment, the processing is most often entirely thermal and

modifies only structure. Thermomechanical treatments, which modify

component shape and structure, and thermochemical treatments which

modify surface chemistry and structure, are also important processing

approaches which fall into the domain of heat treatment.

The iron-carbon diagram is the base of heat treatment. Typical heat

treatment operation is presented in Fig. 1.

Fig. 1. Thermal history of heat treatment operation.

According to cooling rate we can distinguish two main heat treatment

operations:

• annealing – upon slow cooling rate (in air or with a furnace)

• quenching – upon fast cooling (in oil or in water)

annealing - produces equilibrium structures according to the Fe-Fe3C

diagram

quenching - gives non-equilibrium structures

Among annealing there are some important heat treatment processes like:

• normalising

• spheroidising

• stress relieving

Normalising

The soaking temperature is 30-50°C above A3 or Acm in austenite field

range. The temperature depends on carbon content. After soaking the alloy

is cooled in still air. This cooling rate and applied temperature produces

small grain size. The small grain structure improve both toughness and

strength (especially yield strenght).

During normalising we use grain refinement which is associated with

allotropic transformation upon heating γ→α (Fig. 2).

Fig. 2. Influence of temperature on an eutectoid steel grain size

Important: austenite does not change grain size during cooling!!

Spheroidising

The process is limited to steels in excess of 0.5% carbon and consists of

heating the steel to temperature about A1 (727°C). At this temperature any

cold worked ferrite will...