Effect of Si Content on the Microstructure and Mechanical Properties of a 9Cr Ferritic/Martensitic Steel after a Quenching and Partitioning Process

Abstract

Ferritic/martensitic (F/M) steel is considered as one of the most promising cladding materials for Lead-cooling fast reactors due to its favorable mechanical properties and radiation resistance. Quenching and partitioning (Q&P) process can further improve the mechanical properties of the F/M steels compared to conventional normalizing and tempering or quenching and tempering process. In this study, the Q&P process was employed to 9Cr F/M steels with different Si content (0, 0.4, 0.7, 1 wt.%), and the effect of Si content on their microstructure and mechanical properties was studied. The microstructure was characterized through SEM, EBSD, and TEM. The mechanical properties at room and elevated temperatures (500, 600, 700 °C) were tested. The results indicated that the addition of Si (0 to 1 wt.%) leads to a slight decrease in the ultimate tensile strength (UTS)/yield strength (YS) of the F/M steel at room temperature from 1328.3/986.5 MPa to 1300.7/950.7 MPa, respectively. Meanwhile, the fracture elongation (FE) maintains at about 16%. Additionally, as the Si content increases from 0 to 1 wt.%, the UTS and YS of the samples at elevated temperatures show a similar trend. However, the FE of the samples initially increases at 500 °C, noticeably decreases at 600 °C, and eventually rises at 700 °C. Only the FE at 600 °C was lower compared to that at room temperature. Furthermore, the UTS and YS of the samples decrease with increasing temperature. Although Si addition brings a higher solid solution strengthening effect, it is offset by the coarsening of block size and the formation of δ-ferrite. As a result, a little change in the strength of the steel is obtained.