Microstructural Stability and Mechanical Property of a Medium-Si 12%Cr Reduced Activation Ferritic/Martensitic Steel at High Temperatures

The present work investigated the microstructural stability and mechanical property of a medium-Si 12%Cr reduced activation ferritic/martensitic steel (Fe-0.21C-11.76Cr-1.41W-0.17Ta-0.2V-0.63Si-0.51Mn-0.015N, wt.%) at high temperatures. Alloy samples were normalized at 1100 °C for 1 h and tempered at 700 ºC for 1 h, and finally aged at 550 and 650 °C for 5-500 h. In the tempered state, Cr 23 C 6 and MC carbides were precipitated into the full martensitic matrix, which is consistent with the thermodynamic calculated results. This alloy exhibited a higher microstructural stability during aging at 550 ºC without any obvious widening of martensitic laths and coarsening of carbides. But the martensitic laths were widened when aged at 650 ºC, as evidenced by the increase in lath width from ~ 124 nm in tempered state to ~ 275 nm in 500 h-aged state. Also, the Cr 23 C 6 carbides were coarsened with the length increasing from ~ 132 nm in tempered state to ~ 240 nm in 500 h-aged state along the lath interface. While MC carbides have a much higher stability and the particle size keeps constant (90-110 nm) even after 500 h aging at 650 ºC, which is ascribed to the different diffusion coefficients of carbide-forming elements. This aged alloy exhibited prominent mechanical properties with high yield strength of σ YS = 616-689 MPa at room temperature and σ YS = 507-577 MPa at 550 ºC.