The nature of nano-sized precipitates in ferritic/martensitic steel P92 produced by thermomechanical treatment

Abstract Thermomechanical treatment (TMT) can effectively improve the mechanical properties of high-Cr ferritic/martensitic (F/M) steels, which has been mainly attributed to a dense dispersion of nano-sized precipitates. Precipitate phases in high-Cr F/M steels produced by TMT require further investigations. Precipitates in commercial F/M steel P92 produced by a TMT process, warm-rolled at 650 °C plus tempered at 650 °C for 1 h, were investigated by transmission electron microscopy. Nano-sized precipitates with a high number density in the steel after the TMT were found to be Cr-rich M 2 (C,N) carbonitride, rather than MX or M 23 C 6 phase. The M 2 (C,N) carbonitride has a hexagonal lattice with the lattice parameters about a / c  = 0.299/0.463 nm. These M 2 (C,N) carbonitrides with a typical composition of (Cr 0.85 V 0.06 Fe 0.06 Mo 0.03 ) 2 (C,N) have an average diameter smaller than 30 nm, and mainly distribute on dislocations and at the boundaries of equiaxed ferrite grains in the TMT steel. The TMT process inhibits the precipitation of M 23 C 6 and M 5 C 2 phases. Enhanced creep properties of the P92 steel after the TMT, as reported previously, were considered to be mainly attributed to plenty of nano-sized Cr-rich M 2 (C,N) carbonitrides produced by the TMT rather than to MX and M 23 C 6 precipitates.