Study on laser-surface melting to enhance intergranular corrosion resistance of SUS 304 weld

A laser-surface melting method was studied with the aim of increasing resistance to intergranular corrosion of welded SUS 304 stainless steel. An Nd:YAG laser beam at a laser beam size of about 1 mm and power of 170 Wm was employed and the laser power density was varied to optimize the depth of the laser melted layers. The microstructures of TIG welded and laser-surface melted regions were measured via optical, scanning electron, and transmission electron microscopy. In addition, a comparative evaluation of the intergranular corrosion properties of as-TIG welded and as-LSM surfaces of the SUS 304 weld was carried out using a doubleloop electrochemical potentiodynamic reactivation(DL-EPR) polarization method in a 1 L aqueous solution of 0.5 M H2SO4 and 0.01 M KSCN. According to the test results, the maximum melted depth was obtained at a beam scan rate of 600 mm/min and laser power density of 20 J/mm2. The laser-surface melted (LSM) region was observed to have a very fine, homogenous, and cellular microstructure compared to that of the TIG welded region. Grain growth in the laser-surface melted region from the substrate occurred epitaxially. The absence of Cr depletion along the grain boundary in the LSM region, which would result in increased resistance to intergranular corrosion of the welded SUS 304, was confirmed using an energy dispersive X-ray spectroscope attached to the electron microscope.