Question

What is the difference between internal stress and residual stress during forming metal

Answer 1

Residual stresses are locked-in stresses within a metal object, even though the object is free of external forces.  These stresses are the result of one region of the metal being constrained by adjacent regions from expanding, contracting, or releasing elastic strains.  Residual stresses can be tensile or compressive.  In fact, tensile and compressive residual stresses co-exist within a component.
Causes of residual stress

Residual stresses arise whenever a component is stressed beyond its elastic limit and plastic deformation occurs.  Plastic deformation occurs when the stress exceeds a metal’s yield strength (discussed in Tensile Testing).  This can be as a result of…

Non-uniform plastic deformation during mechanical processing, such as that during rolling, forming operations (bending or drawing), machining, and mechanical surface treatments (shot peening and roller burnishing).

Phase transformations during cooling from elevated temperatures

Non-uniform plastic deformation during heating or cooling

Heterogeneity of a chemical or crystallographic order (nitriding or case hardening)

Various surface treatments (enameling, electroplating PVD and CVD coating)