Effects of tungsten on precipitation behaviour of intermetallic compounds in the HAZ of duplex stainless steel: Study of weldability of high-tungsten duplex stainless steel (2nd Report)

Abstract

Summary This paper describes an investigation of the effects of tungsten on the precipitation behaviour of intermetallic compounds in the HAZ of duplex stainless steel and the mechanism of these effects to clarify the advantage of having some Mo replaced by W. Fine precipitates of sigma phase are observed in electron micrographs of the simulated HAZ of a 25% Cr-7% Ni-0.3% N sample steel containing 4% Mo, whereas fine precipitates of the chi phase are observed in that containing 3% Mo and 2% W. The addition of tungsten delays sigma phase precipitation in steel aged at 850–950 °C × 600 sec. The sigma phase and austenite phase are identified in steel containing less than 8% W aged at 800–900°C, whereas the sigma, chi, and austenite phases are detected in steel containing more than 12% W. Based on these results and the chemistry data of the respective phases determined with an electron probe microanalyser (EPMA), the phase diagram at 900°C is experimentally derived. The chi phase is precipitated with a smaller contact angle to the matrix than the sigma phase, a feature suggesting that the chi phase is preferentially nucleated. However, the growth rate of the chi phase may be low due to the lower diffusion rate of tungsten. Tungsten can delay sigma phase precipitation by a mechanism that promotes nucleation of phases other than the sigma phase which have lower growth rates than the sigma phase.