Comparative study on the microstructure and mechanical properties of a modified 9Cr–2WVTa steel by normalizing-tempering and quenching-partitioning treatments

Abstract

Ferritic/martensitic (F/M) steels have been proposed as important candidates for structural materials of nuclear reactors due to their good mechanical properties and radiation resistance. Compared with traditional normalizing and tempering (N&T) or quenching and tempering (Q&T) processes, recent studies have shown that a quenching and partitioning (Q&P) process can significantly improve the strength of steels due to the multi-phase microstructure. In this study, we developed a Q&P process that significantly improved the ultimate tensile strength/yield strength of a modified 9Cr–2WVTa steel from ~752 MPa/~644 MPa to ~1210 MPa/~927 MPa at room temperature, maintaining a total elongation of ~21.4%. Additionally, the strength of the steel at elevated temperatures (25–600 °C) was significantly improved by the Q&P process. The strengthening effect is attributed to the combined effects of the different microstructure characteristics, i.e., more solute carbon atoms within the matrix, higher dislocation density in the martensite, refined structural units (martensite packets/ultra-fine martensite laths) and finer precipitates (M23C6, MX, and θ-carbide). The relationship between the microstructures resulted from the two processes and the corresponding mechanical properties has also been comparatively discussed, which provides guidelines for tailoring the microstructure and mechanical properties of F/M steels by a Q&P process.