Effect of Laser Heat Treatment and Nitrogen Content in Shielding Gas on Precipitation of Widmanstatten Austenite in Lap Laser Welds of Duplex Stainless Steels

Duplex stainless steel attracts attention because of its excellent mechanical properties and corrosion resistance, which characterize both the ferrite and the austenite phases. However, it's well known that the welds are easily affected by the amount of heat input and that the performance may be deteriorated due to the imbalance of the phase. In particular, when laser welding is used, this phenomenon becomes more pronounced due to the effects of rapid solidification. This study investigated the efficiency of laser post weld heat treatment by laser beam, focusing on the austenite stabilizing effect of nitrogen, adding different nitrogen contents to the shielding gas for various lap laser welding. The effects of the two methods of promoting the precipitation of austenite and the influence on the precipitation morphology are compared. In addition, the tensile test was used to summarize the relationship between the amount of austenite precipitated and mechanical strength. After that, the Widmanstatten austenite in the laser beam welds was observed stereoscopically by etching and continuous sectioning.