Laser peening without coating induced phase transformation and thermal relaxation of residual stresses in AISI 321 steel

Laser peening without coating (LPwC) was performed on austenitic stainless steel AISI 321 with a power density of 5.97 GW cm(-2) and pulse densities of 1600, 2500 and 3900 pulses cm(-2). A large compressive residual stress resulted for 1600 pulses cm(-2). Surface roughness was found to have increased to a relatively smaller extent with lesser number of valleys or peaks. Micro-hardness increased after the process with the depth of hardened layer extending up to 900 pm. LPwC induced martensitic phase transformation (about 18% in volume fraction) was observed near treated surface. Microstructures confirmed the phase transformation effect. The grains were not refined as a result of single LPwC scan. Thermal relaxation of residual stresses at 700 degrees C for 2 h were 41% in longitudinal and 140% in transverse (relative to LPwC scanning) directions. The influential factors on stress relaxation were discussed. No significant reduction in hardness was seen after thermal treatment. Followed by the thermal treatment, fully austenitic phase was recovered. (C) 2016 Elsevier B.V. All rights reserved.