Precipitate phases in ferritic/martensitic steel P92 after thermomechanical treatment

Thermomechanical treatment (TMT) can effectively improve the mechanical properties of 9-12% Cr ferritic/martensitic (F/M) steels. To understand the effect of TMT on the creep properties of the steels, precipitate phases in the steels produced by TMT require investigations. A TMT process, including austenization at 1373 K (1100 °C) for 2 h, followed by warm-rolling at 923 K (650 °C) plus tempering at 923 K (650 °C) for 1 h, was applied on a commercial F/M steel P92. Precipitate phases in the steel after the TMT were investigated by transmission electron microscopy. Nano-sized Cr-rich M2(C,N) carbonitride with a high number density was the dominant phase. Large-sized Nb-rich M(C,N), (Nb0.8V0.13Cr0.05Mo0.02) (C,N), and nano-sized Nb-V-rich M(C,N) carbonitride phases with the same f. c.c. crystal structure were identified. Identified Cr-rich M23C6 carbides with a typical composition of (Cr0.68Fe0.25W0.04Mo0.03)23C6 have a large size and less amount. Modified Z-phase, Cr(V,Nb)N, with a body-centered tetragonal crystal structure, and Cr-rich M7C3 carbide with a simple orthorhombic crystal structure were also identified. The amount of Z-phase and Cr-rich M7C3 carbides was the least compared to other precipitate phases.