STRESS RELIEVING | Heat treatment for steels

Heat Treatment for Steel :

The metal and alloy is limited by its properties, these can be varies with in limits by several methods like mechanical working, alloying, grain size control and heat treatment.

● The heat treatment processes affects mechanical properties either by allotropic transformation or by changing relative solubilities of element in the base metal.

● Properties of steel can be controlled and varied over a wide range by heat treatment. Thus a through understanding of heat treatment of steel is very important.

Stress Relieving 

Stress Relieving process is employed to relieve internal stresses, without any changes of microstructure of metal. It differs from other subcritical treatment in which structural improvement takes place.

Internal Stresses :

● Internal stresses are those stress which can exist within a body in the absence of external forces. These are also known as residual stresses or locked in stresses. Internal stresses occurs during casting, welding, case hardening, electroplated coating, precipitation and phase transformation.

● Internal stresses under certain conditions can have adverse effects.
Example :  steel with residual stresses under corrosive environment fail by stress corrosion cracking, where as failure by stress corrosion cracking occurs under the combined action of corrosion and externally applied stresses.

● Those stresses also enhance the tendency of steels towards warpage and dimensional instability. Fatigue strength is reduced considerably when residual tensile stresses are present in the steel. The problem associate with internal stresses are more difficult in brittle materials than ductile materials.

Stress relieving - heat treatment of steel
Stress Relieving

Stress Relieving : 

The process stress relieving consists of heating steel uniformly to a temperature below the lower critical temperature, holding at this temperature for sufficient time by uniform cooling.

● Uniform cooling is of almost important as non uniform cooling will itself result in the development of internal stresses. Thus the very purpose of stress relieving will be lost.

● For plain carbon steel and low alloys steels, maximum stress relieving temperature is generally limited to about 600℃. Higher temperature are employed for high alloy steels. Hardened and tempered steels, if service condition demand, can be stress relieved by heating up to a temperature which is lower than tempering temperature by 25 - 30℃. The extent to which stresses can be relieved depend on the temperature employed and holding time.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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