Micro-segregation and Precipitation of Alloy 690 during Isothermal Solidification: the Role of Nitrogen Content

Abstract

The segregation and precipitation behavior of Alloy 690 containing 0.001-0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 degrees C 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 degrees C. 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 degrees C. 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 Ti4C2S2 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.