Effects of austenite regeneration heat treatment on the TLP bonding of duplex stainless steel UNS S32750 using Fe-B-Si insert metal

This study discusses microstructural evolution during transient liquid phase (TLP) bonding and the effect of austenite regeneration heat treatment with regard to nitrogen-containing duplex stainless steel UNS S32750. TLP bonding was performed between the temperature range of 1423-1473 K for 0-1000 s under a vacuum of 5 × 10−5 Torr using an amorphous Fe-B-Si insert metal. The results showed that the volume fraction of austenite (γ) decreased with an increased bonding temperature and holding time. In the case of a prolonged holding time, a depleted area of the γ phase was observed in the base metal adjacent to joints. In order to prevent this phenomenon, an austenite regeneration heat treatment process was performed. The austenite volume fraction of the base metal and joint area increased with an increase in austenite regeneration heat treatment time; deviation of the austenite volume fraction between the base metal and joint area decreased. In this study, homogenization behaviors of joint microstructures were treated through a diffusion model. Bonding and austenite formation processes at the joints were divided into 4 steps: base metal dissolution, boride disappearance and joint widening, austenite growth to the joint, and homogenization.