Gradient structure and mechanical behavior induced by multiple laser peening in 304 austenitic stainless steel

Austenitic stainless steel was subjected to multiple laser peening (LP) with high coverages and the gradient structures and corresponding mechanical behaviors were investigated in detail. Body-centered cubic alpha ' phase and hexagonal-close packed epsilon phase martensite induced by peening deformation accumulate in multiple LPed 304 austenitic stainless steel surface layer with the increasing coverage. In samples with 240 layers multiple LP, high density martensite in block and twins can be observed. The EBSD results show that the gamma -> epsilon -> alpha ' martensitic transformation can be noticed to occur in grain boundaries and twins. Martensitic transformation volume fraction, twin volume fraction and hardness of multiple LPed 304 SS exhibit gradient characteristic from the top surface to substrate. The hardness of the specimens increased with the peening coverage near the top layers, and the influence of laser peening on hardness gradually vanished until 2 mm in depth, the tensile strength increased while the tensile elongation decreased, resulting from a peak power density of 8.19 GW/cm(2). Multiple laser peening can be utilized as a solution to modify the mechanical performances by tailoring the gradient structure. Among the investigating range, the optimal multiple LP layers is suggested as 80 where both a relative high strength and ductility can be obtained.