Effect of Ferrite/Austenite Phase Transformation on 475 °C Embrittlement in Duplex Stainless Steel Weld

Abstract

The change in hardness due to 475°C embrittlement was investigated in the melt-run GTA welds of type 329J4L duplex stainless steel. A ferritic phase was hardened with ageing at 673-773K due to the phase decomposition into Fe-rich and Cr-rich phases, while an austenitic phase was barely hardened with ageing. Hardness in a ferritic phase was rapidly increased with ageing in the base metal (BM) region, and the hardening rate was reduced in the order of BM, weld metal (WM) and heat affected zone (HAZ). The ferrite/austenite fractions in HAZ and WM were higher than that in BM, furthermore, Cr content in a ferritic phase was lowered and Ni content in it was contrarily heightened in the same order of BM, WM and HAZ. A computed phase diagram suggested that the chemical composition of a ferritic phase in each region was located in the nucleation/growth region not spinodal decomposition region, which was situated between the spinodal and binodal lines. Computer simulation of phase decomposition phenomena in a ferritic phase using phase field model revealed that phase decomposition was accelerated with an increase in Cr content and a decrease in Ni content. It followed that Cr would enhance the 475°C embrittlement and Ni would inhibit it, because of the increased/decreased driving force of the phase decomposition. The difference in 475°C embrittlement behaviour at each region could be attributed to the difference in the chemical composition of a ferritic phase caused by the ferrite/austenite phase transformation during welding.