Effects of different peening techniques on residual stress and microstructure in nickel-base alloy 600 and 690

To evaluate their peening effects, nickel-based alloy 600 and 690 were subjected to treatments including water jet peening (WJP), underwater laser peening (ULP), air laser peening (ALP), and ultrasonic nanocrystal surface modification (UNSM). These methods were analyzed for their influence on roughness, residual stress, microhardness, and microstructure. Among them, UNSM resulted in the most substantial compressive stress and the most extensive plastic deformation, creating a highly refined surface grain structure of approximately 100 μm thickness. In contrast, ULP failed to induce grain refinement at the surface, likely due to surface erosion and thermal exposure caused by laser-plasma interaction. Hardness measurements showed that UNSM-treated surfaces had the highest nanoindentation hardness, while the ULP-treated surfaces had the lowest because the initial plastic deformation layer had been removed, consistent with the observed microstructure. The multiple peening treatments influenced the residual stress trends differently. ALP showed increasing compressive residual stress with repeated treatments, whereas ULP exhibited a decline caused by thermal relaxation.