J Mater Sci Technol ›› 2012, Vol. 28 ›› Issue (5): 446-452.

• High Temperature Structural Materials • Previous Articles     Next Articles

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

Rong Jiang, Bo Chen, Xianchao Hao, Yingche Ma, Shuo Li, Kui Liu   

  1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
  • Received:2011-04-20 Revised:2011-08-19 Online:2012-05-30 Published:2012-05-29
  • Contact: Yingche Ma
  • Supported by:

    the National Natural Science Foundation of China (No. 50901076)

Abstract: The segregation and precipitation behavior of Alloy 690 containing 0.001-0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 °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 °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 °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.

Key words: Alloy 690, Nitrogen content, Solidification, Micro-segregation, Precipitate