Residual Stress in Metals

Residual stresses are locked-in elastic stresses present in a metal that is free of external forces. Elastic stresses are less than a metal’s yield strength and cause elastic deformation of the metal. The deformation is associated with stretching of the bonds between a metal’s atoms.

Residual stress can be tensile or compressive. In many cases, tensile and compressive residual stresses can co-exist within a component.

Residual stresses can be harmful or beneficial, depending on whether the stress is tensile or compressive. For fatigue and stress corrosion cracking, the stress at the surface of a component is important. Surface residual compressive stress reduces the effects of applied tensile stresses, improving resistance to fatigue and stress-corrosion cracking. Surface tensile residual stresses add to an applied tensile stress, reducing resistance to fatigue and stress-corrosion cracking

Residual stress arises from:
Non-uniform plastic deformation
Non-uniform contraction during cooling
Phase transformations during heat treating
Surface treatment, e.g. carburizing, coating, shot peening
 
Residual stresses arise when metal plastically deforms and the deformation is not uniform throughout the metal’s cross-section or along the length of the metal. This can occur during room temperature metal shaping processes such as bending, drawing, and rolling.
  
When a metal cools metal from elevated temperatures, the outer portion of the metal cools first and contracts, compressing the hotter inner metal. As the inner portion of the metal cools, the metal tries to contract, but is constrained by the already cooled outer portion. Consequently, the inner portion will have a residual tensile stress and the outer portion of the metal will have a residual compressive stress. This occurs during heat treating and welding.
 
A phase transformation is a change in the metallurgical phases present in an alloy. The transformation from austenite to martensite in steel during through hardening is a phase transformation. Volume differences between the final and initial metallurgical phases causes expansion or contraction of the metal. For phase transformations that occur during cooling from an elevated temperature, the outer portion of the metal cools first and undergoes the phase transformation before the metal’s interior. As the interior of the metal cools the volume difference between the initial and final phases will cause the interior metal to change volume. However, the volume change of the metal interior will be constrained by the cooler outer layer of metal that already transformed. As a result, residual stresses will appear within the exterior portion of metal and the interior portion of metal.
 
Mechanical treatment, stress relief heat treatment, control of heat treating processes, and alloy selection are used to control the type and magnitude of residual stress in a component.