Effects of alloying elements on C-curve of sigma phase precipitation in duplex stainless steel

(Effects of alloying elements on sigma phase precipitation in duplex stainless steel)

Abstract

The time-temperature-precipitation (TTP) diagram called as C-curve of sigma phase in duplex stainless steel was investigated to clarify the effect of chemistry and its mechanism. 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. It is meaningful to clarify the effect of the alloying elements on the nose temperature and time of TTP curve which indicate the critical heating condition to be free from harmful phase precipitation.
Employing 25%Cr duplex stainless steel containing various level of molybdenum, tungsten and nickel contents, The time-temperature-precipitation (TTP) curve was obtained by microstructure observation test with optical microscope after heated at various temperature. The nose time of C-curve was shifted to shorter and nose temperature was to higher by the increase of molybdenum or nickel. The nose temperature was also shifted to higher by 2% tungsten addition but the nose time was almost same. The TTP curve of each steel tested was also calculated from the physical model of growth rate considering the effect of alloying elements.
Those calculated TTP curves almost fitted the experimental results. The influence mechanism of chemistry on the TTP curve was analyzed by this model.