Thermodynamic analysis of precipitates in modified 9Cr-1Mo heat resistant steel

摘要

Precipitation in heat resistant steels usually has a significant influence on their stress-rupture properties at elevated-temperatures, which is important for an adequate control of chemical compositions and heat treating process of those steels. The dependence of MX and M_(23)C_6 precipitates upon temperature and chemical composition was estimated in mod.9Cr-1Mo heat resistant steel using Thermo-Calc software and its OTCFE database. The equilibrium phase calculations indicate that dissolution of Nb-rich MX carbonitride is partially retarded up to about 1500 K, and considerable amounts of dissolved M_(23)C_6 carbides occur below 1123 K. Thermo-Calc predicts that a change in temperature results in a change in the compositions of precipitates in mod.9Cr-1Mo steel, and that a reduction in the nitrogen content or carbon content markedly reduces the mass contents of MX carbonitrides and M_(23)C_6 carbides, respectively, which is in agreement with the available experiments for mod.9Cr-1Mo.