Effect of nitrogen concentration on creep strength and microstructure of 9Cr-ODS ferritic/martensitic steel

Abstract

9Cr oxide dispersion strengthened steels with slightly different nitrogen concentrations (0.0034 − 0.029 wt%) were prepared and their creep property at 973 K was investigated with microstructural characterization before and after the creep test. The creep strength decreased significantly as the nitrogen concentration increased. Microstructural observation revealed that, in the higher nitrogen concentration specimen, coarse Y-rich inclusions were found along the boundary between transformed ferrite region and residual ferrite region. The solubility difference of nitrogen in α and γ phase would induce the localized increment of nitrogen concentration in the boundary region during the austenitizing process, resulting in the thermodynamic destabilization and subsequent coarsening of the dispersed oxide particles. The rows of creep voids were found near the rupture part of the crept specimen, and their fraction increased with increasing nitrogen concentration. This suggests that the coarse inclusions were one of the main starting points of creep void formation, leading to premature fracture in the higher nitrogen specimen.