Effect of overlaps on microstructure and corrosion performance of large area laser-treated 304L stainless steel

In large area laser surface treatment, overlapping of individual tracks corresponding to the width of the laser beam is often required. This involves the re-heating and re-melting within overlapped areas with microstructural changes that may adversely influence corrosion properties. The purpose of this work was to investigate the influence of overlaps on microstruture and corrosion behaviour of laser-melted 304L stainless steel. Laser surface melting (LSM) was carried out using either a 2 kW CO2 laser with a spot beam in a Gaussian distribution or a 3 kW CW Nd:YAG laser with a line beam profile. The microstructures of the melt pool, in the respect of re-heated regions and re-melted regions, were characterised using SEM/EDX and XRD. The resistance to pitting corrosion was evaluated using potentiodynamic anodic polarisation in deaerated 3.5% NaCl solution. It was found that the re-heating caused transformation of δ-ferrite formed in the previous track back into γ-austenite, and the re-melting promoted the p...