Modelling of sigma phase precipitation in super duplex stainless steel weld metal

Abstract

In the weld metal the time-temperature-precipitation(TTP) diagram called as TTP curve of sigma phase in duplex stainless steel was investigated to clarify the difference from that in base metal or heat affected zone. Intermetallic phase such as sigma phase is harmful for mechanical properties of duplex stainless steels. The increasing of alloying elements of chromium and molybdenum effective for improving of corrosion resistance promotes the sigma phase precipitation in weldment. The weld metal consists of the different microstructure from the base metal. Because that consists of as solidified microstructure followed by transformation of a part of ferrite to austenite phase during cooling in welding process. Employing 25%Cr duplex stainless steel weld metal bearing 3% of Mo, 2% of W, 0.3% of N and various level of Ni, the time-temperature-precipitation(TTP) curve of each weld metal was obtained by the microstructure observation test with optical microscope after heated at various temperature. The TTP curve of weld metal is shifted to the direction of longer time comparing with that of the base metal with the almost same chemical composition. The mechanism of that shift was explained from a point of the difference of partitioning of alloying elements such as nickel and chromium between the phases by using the physical model proposed.