Residual Stresses in Heat Treatment

There are two kinds of stresses that an object may be subjected to:

(a) Applied Stresses - which is due to external forces
acting on the object.
(b) Residual Stress - which remains in the object
after all the applied forces have been removed.

The basic cause of residual stress is non-uniform plastic flow due to previous operations. Some specific causes are heat treatment, welding, mechanical operations such as cold working, grinding & so on. Residual stresses must form a balanced force system within the object, which
implies that a residual compressive stresses in one part of an object must be balanced by residual tensile stress in another part.

Residual stresses may be harmful or beneficial. When tensile in nature, it adversely affects the fatigue prop-erties in particular & ductility in general. When it is compressive in nature, as for example, in carburizing or after induction/flame hardening, the fatigue properties improve. Another example of beneficial compressive stress is the steel reinforced prestressed concrete.
Residual and applied stresses add algebraically, as long as their sum does not exceed the elastic limit. It is therefore necessary for a designer to know the level and nature of the residual stress so that he could prescribe
safe levels of applied stress for an engineering component.