Study on Effect of Weld Cooling Rate on Fusion Zone Microstructure and Solidification Cracks in 316L Austenitic Stainless Steel

A study on effect of cooling rate on mode of solidification and microstructure was carried out on austenitic stainless steel welds. A tube and plug of 316L stainless steel was joined using Gas Tungsten Arc Welding (GTAW) and laser welding processes. The welds were characterized using optical and Scanning Electron Microscope (SEM). The results indicate that cooling rate of the weld has significant effect on solidification mode, microstructure and solidification cracking. 316L weld joints prepared using GTAW process shows duplex microstructure of vermicular ferrite and austenite in the fusion zone. Whereas, the fusion zone of laser joint shows only single phase austenite microstructure. From these observations, it is clearly understood that the changes observed in the fusion zone microstructures of GTAW and laser welds are due to change in the mode of solidification as a result of change in the weld cooling rates. The predicted mode of solidification for GTA welds for 316L composition used in this study was Austenite-Ferrite (AF) and it was also confirmed through the microstructural observations. In laser joint, the weld has solidified in fully austenitic mode which deviates from the mode of solidification predicted by the conventional constitutional diagrams and hence modified weldability diagram was used. From this investigation, it was also found that the rapid solidification during laser welding is not completely partition less because segregation of sulphur was found using Scanning Electron Microscope – Energy Dispersive Spectroscope (SEM-EDS) along the dendrite boundaries of laser welds. High cooling rate during weld solidification which influences fully austenitic mode of solidification and micro segregation of impurities along the grain boundaries contribute to solidification cracking of welds in laser joints.