Role of austenite in weld toughness of super duplex stainless steel

Microstructure control for welding super duplex stainless steel SAF2507 was carried out on a welded joint by GTA welding. The toughness of the bond region in the heat-affected zone (HAZ) of the advanced SAF2507 stainless steel was much lower than that of the base metal. The microstructure of the bond region for the as-welded sample was examined and the extreme grain growth of ferrite and the lowering of the amount of austenite phase were observed. In order to improve the toughness of the bond region, microstructure control was carried out using a cooling rate control process during welding. Various cooling times from 1 673 K to 1 073 K in the bond region were selected, which corresponded to the heat input from 1 kJ/mm to 6 kJ/mm. For the ferrite grain growth, the cooling time from 1 673 K to 1 473 K, that is, Δt16-14 was controlled using a Gleeble simulator. The ferrite grain size increased with increasing cooling time Δt16-14. For austenite phase reformation, the cooling time from 1 473 K to 1 073 K, Δt14-10 was selected, since austenite phase reformation occurs within that temperature range. The amount of austenite increased with increasing Δt14-10. Increasing the cooling rate caused both ferrite grain growth and an increase of the austenite phase. Improvement of the toughness was accomplished up to 60 s in the cooling time from 1 473 K to 1 073 K, however hardly any change in toughness was accomplished at the cooling time of 120s, because the slow cooling rate caused both ferrite grain growth and an increase of the austenite phase.