Mechanical Surface Treatments of AISI 304 Stainless Steel: Effects on Surface Microrelief, Residual Stress, and Microstructure

The surface roughness, residual stress, and microstructure of AISI 304 stainless steel specimens after laser shock peening (LSP), water jet cavitation peening (WjCP), water jet shot peening (WjSP), and multi-pin ultrasonic impact treatment (UIT) were studied in this work. Compared to the initial state, the surface roughness (Ra) was, respectively, decreased by approx. 5.5, 7.8, 38.2, and 91.1% after the LSP, WjCP, WjSP, and UIT processes. The volume fraction of epsilon-martensite of similar to 3-5% was observed in all treated specimens except for the LSP-treated ones. The volume fraction of alpha '-martensite was increased in the following sequence: WjCP (similar to 5%), LSP similar to 5%), WjSP (similar to 25%), UIT (similar to 50%). The studied mechanical surface treatments promote a significant reduction in grains size of both austenite (similar to 15-20 nm) and martensite (similar to 20-37 nm) leading to essential hardening. All studied processes result in the formation of compressive residual stresses (- 377 horizontal ellipsis 693 MPa) and the improvement in the bearing curve parameters. The microhardness estimated accounting for the contributions of different hardening mechanisms to the yield strength magnitude correlates well with the experimental data. The grain boundary hardening and dislocation hardening are concluded to be the most influential mechanisms.