Enhanced disperse Ta(C, N) precipitation in reduced activation ferritic/martensitic steels by optimized two-step austenitizing

Abstract

High-density MX precipitates block the movement of dislocations during creep at high temperature and increase sink strength to point defects under irradiation. Therefore, the precipitation control of MX phase is crucial for the advancement of reduced activation ferritic/martensitic (RAFM) steels. In this paper, an optimized two-step austenitizing strategy was proposed to enhance Ta(C, N) precipitation in RAFM steels, and the temperature effects of the first and second austenitizing on the Ta(C, N) precipitation enhancement were studied. During the first austenitizing, Ta(C, N) particles with a small radius (∼12 nm) and a high number density (∼8.5 × 1019 m−3) precipitated by heterogeneous nucleation on the M23C6 phase boundaries retained by the comparatively low temperature (900 °C). Then in the second austenitizing, enhanced Ta(C, N) precipitates kept good size and thermal stability below 1050 °C. Compared to RAFM steels treated by conventional normalizing and tempering, the strength and creep performance were significantly improved by the pinning effect of the enhanced dispersed Ta(C, N) particles. Furthermore, the optimized two-step austenitizing strategy was validated in the other two RAFM steels, showing broad application prospects in RAFM and heat-resistant steels.

Keywords

• RAFM steels
• MX phase
• Precipitation control
• Heat treatment
• Creep resistance