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....