Microstructure Stability and Mechanical Properties of Reduced Activated Ferritic Martensitic Steel during Thermal Aging at 550 °C for 5000 h

In this paper, the influence of thermal aging on the microstructure and mechanical properties of reduced activated ferritic/martensitic is investigated in air at 550 °C for 5000 h. The microstructure evolution and precipitated phases following thermal aging are investigated via optical microscopy, scanning electron microscopy, and transmission electron microscopy. The tensile and impact properties are tested at different temperatures. The results indicate that the alloy microstructure exhibits good stability during aging. Yttrium has little effect on the evolution of Laves phases and carbides. Laves phases preferentially nucleate at the original austenite grain boundary, and the growth mechanism is diffusion-controlled. After aging for 5000 h, the growth rate of M23C6 decreases to about 75%, and that of MX decreases to about 85%. Moreover, the yield strengths of A (without Y) and B (with Y) Steels are 550 and 587 MPa at room temperature and 289 and 307 MPa at 650 °C, respectively. The alloys have good mechanical properties at both room and high temperatures. The ductile–brittle transition temperatures of A and B Steels are increased to − 60 and − 63 °C, respectively. Grain coarsening is the main causative factor for the decrease in alloy toughness.