A study done through the SAE Fatigue Design and Evaluation Committee showed that this process increases the fatigue life of the welds(and springs). What is the process we are talking about??

Shot peening is a cold working process that bombards a surface with small spherical media to induce compressive residual stresses. These compressive stresses counteract tensile stresses during service, significantly delaying fatigue crack initiation and propagation in welds and springs. The other options (normalizing, spheroidizing, martempering) are heat treatments that primarily alter microstructure, not surface stress state.