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 degC plus tempered at 650 degC 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 M2(C,N) carbonitride, rather than MX or M23C6 phase. The M2(C,N) carbonitride has a hexagonal lattice with the lattice parameters about a/c 0.299/0.463 nm. These M2(C,N) carbonitrides with a typical composition of (Cr0.85V0.06Fe0.06Mo0.03)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 M23C6 and M5C2 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 M2(C,N) carbonitrides produced by the TMT rather than to MX and M23C6 precipitates.