Secondary Austenite Morphologies in Fusion Zone of Welded Joint after Postweld Heat Treatment with a Continuous Wave Laser

In order to improve the weldability of duplex stainless steels, obtaining more secondary austenite in the weld metal is an effective way. Therefore, optimizing the secondary austenite by changing its morphology, volume fraction and stability may be expected to enhance the ductility of the weld. The secondary austenite morphologies in the fusion zone of the laser continuously heat treated welds of 2205 duplex stainless steel were investigated. The secondary austenite morphologies were found to be influenced by different laser power level. The secondary austenite with penniform, freely grown and dendritic shape appeared in the course of 4, 6 and 8 kW continuous heat treatment, respectively. It was found that there were three kinds of morphologies of secondary austenite in the fusion zone treated by different power, i.e., widmannstätten austenite, grain boundary austenite and intragranular austenite. The results demonstrated that the mechanism of the secondary austenite formation was a displacement mechanism during the initial austenite lath formation and a diffusion mechanism during cooling. The nitrides provided the nitrogen for the transformation and at the same time acted as nucleation sites for the secondary austenite.