Beyond post-weld heat treatment, explain how mechanical peening effectively reduces tensile residual stresses on the surface of a weld and enhances fatigue life.

Mechanical peening is a cold working process where the surface of a material is impacted by small, hard objects, like shot or hammer blows. This process is used to reduce tensile residual stresses and enhance fatigue life, extending beyond post-weld heat treatment, which primarily aims to relax stresses through thermal cycles. Welding inherently introduces tensile residual stresses. During welding, localized heating and subsequent cooling cause the material to contract. Because this contraction is constrained by the surrounding cooler material, internal stresses are created. These are called “residual stresses” because they remain in the material even after the welding heat source is removed and no external loads are applied. These tensile residual stresses on the surface can act as initiation points for cracks under cyclic loading. When the surface of a weld is mechanically peened, the impacts from the shot or hammer blows plastically deform the material. Plastic deformation means the material undergoes a permanent change in shape; it does not fully spring back to its original form after the impact force is removed. This localized plastic deformation stretches the surface layers. As these surface layers attempt to expand due to the plastic deformation, they are constrained by the underlying, undeformed material. This constraint forces the plastically deformed surface layer into a state of compression. These newly induced “compressive residual stresses” effectively counteract and often replace the pre-existing tensile residual stresses from welding in the critical surface region. The overall stress balance within the material is maintained, meaning the tensile stresses are redistributed deeper into the component or into unaffected areas. Fatigue life refers to the number of stress cycles a material can withstand before failing due to fatigue, which is the progressive and localized structural damage that occurs when a material is subjected to repeated loading. Fatigue cracks typically initiate at the material surface, especially at points of stress concentration or microscopic defects. Tensile stresses at the surface promote the opening and growth of these micro-cracks. By introducing compressive residual stresses on the surface through peening, any externally applied tensile stresses from cyclic loading must first overcome this existing compression before the material surface experiences a net tensile stress. This effectively reduces the *mean stressexperienced by the material at potential crack initiation sites. Furthermore, compressive stresses tend to close potential cracks rather than allow them to open and propagate. This resistance to crack initiation and the slowing of crack propagation significantly extend the fatigue life of the welded component, making it more resistant to failure under repeated stress cycles.