Effects of Alloying Elements on Sigma Phase Precipitation in Duplex Stainless Steel (2) - Effects of Alloying Chromium, Molybdenum and Tungsten on C-curve of Sigma Phase Precipitation in Duplex Stainless Steel

The time-temperature-precipitation (TTP) diagram called as C-curve of the sigma phase in duplex stainless steel was investigated to clarify the effect of each amount of alloying elements. Because of the trade-off between the improvement for corrosion resistance and avoiding the sigma phase precipitation regarding the increase of Cr and Mo, it is important to clarify the effect of each alloying elements on the nose temperature and time of the TTP curve which indicates the critical heating condition to be free from harmful phase precipitation.By using 25/28%Cr duplex stainless steel containing various levels of Cr, Mo and W, the TTP curve was obtained experimentally via the microstructure observation test with an optical microscope after heating at various temperatures. The calculated TTP curves were also obtained from the physical model of sigma phase growth rate which has been proposed in the previous work by the authors of this work. Those calculated TTP curves almost agreed with the experimental results. It was possible to explain the influence of alloying elements of Cr, Mo and W on the TTP curve from the effect on the parameters in the kinetic model proposed.