The segregation and precipitation behavior of Alloy 690 containing 0.001-0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 ℃ have been investigated using optical microscopy (OM),electron probe microanalysis (EPMA) and transmission electron microscopy (TEM).The results indicate that the volume fraction of TiN-type nitride formed during isothermal solidification increases with the nitrogen content of Alloy 690.Segregation of Ti and Cr exists in samples solidified at 1370 and 1355℃.The Ti content in the residual liquid markedly decreases and the concentration of Cr increases when the nitrogen content of Alloy 690 increases.Furthermore,N and S also show segregation to some extent in the residual liquids at 1355℃.Accompanying by the segregation of Cr,Ti,C,N and S,sulfides and chromium nitrides form.In a low nitrogen content Alloy 690,sulfur segregates and precipitates in the form of Ti 4 C 2 S 2 and (Cr,Ti)S,but in the form of (Cr,Ti)S or CrS in a high nitrogen content Alloy 690.(Cr,Ti)N-type nitrides with an fcc crystal structure have been identified in a sample with 0.11 wt% nitrogen.
Rong Jiang Bo Chen Xianchao Hao Yingche Ma Shuo Li Kui Liu
