Effects of laser surface melting on corrosion resistance in weld metals of Mo containing austenitic stainless steels

Laser surface melting (LSM) for the weld metals of Mo containing austenitic stainless steels has been investigated. Laser surface melting was conducted using 2.5 KW CO2 laser with variation of beam traveling velocities in order to achieve different cooling rates. Corrosion resistance was evaluated by the critical current density for passivation, the pitting potential and the density of pit after FeCl3 solution immersion test. The pitting resistance in the weld metals severely degraded in comparison with the base metals, whereas the laser-treated weld metals exhibited such improved corrosion resistance as almost same as or rather better than those of the base metals. SEM observation revealed that pits which had preferentially formed at the dendrite cores in the weld metal where Mo and Cr were depleted due to microsegregation during solidification and however, they had reduced in number and also in size after laser treatment. The elements distribution across the solidification cell in the weld metals was found to become more homogeneous by the laser treatment. The theoretical consideration has demonstrated that the change in the element distributions in the laser-treated weld metals can be explained by the Kurz-Fisher model, which suggests that the homogenization of element distributions can be attributed to the rapid solidification effect by laser surface melting. Thus enhanced concentrations of Mo and Cr at the dendrite cores were considered to improve corrosion resistance in laser-treated weld metals.