Abstract
The effect of tungsten in duplex stainless steels on the precipitation behavior of inter-metallic compound and its mechanism were investigated to clarify the advantage of tungsten alloying for the weldment properties. The fine precipitates of sigma phase were observed in the electron micrograph of simulated heat affected zone (HAZ) of the 25%Cr-7%Ni-0.3%N steel containing 4%Mo, while the fine precipitates of chi phase were observed in that containing 3%Mo and 2%W. The tungsten addition delayed the sigma phase precipitation in aged steel at 850°C to 950°C for 600 s. In the steel containing below 8% tungsten aged at 800°C to 900°C sigma phase and austenite phase were identified, while in the aged steel containing over 12% tungsten sigma, chi and austenite phase were identified. Using these results and chemistry data of each phase obtained with electron probe microanalyzer (EPMA), the phase diagram at 900°C was derived experimentally. Chi phase precipitated with the lower contact angle to the matrix than sigma phase, therefore chi phase was considered to nucleate preferentially. However, the growth rate of chi phase can be low due to lower diffusion rate of tungsten. It was considered that tungsten can delay the sigma phase precipitation by the mechanism that tungsten should promote the nucleation of other phase than sigma, which has lower growth rate comparing sigma phase.
