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

TEM micrographs of precipitates on extraction carbon replicas prepared from ferritic/martensitic (F/M) steel P92. (a) After conventional heat treatment, normalized at 1050 °C for 30 min plus tempered at 765 °C for 1 h. (b) After a thermomechanical treatment (TMT), warm-rolled at 650 °C plus tempered at 650 °C for 1 h. Nano-sized precipitates with a high number density in the steel produced by the TMT were found to be Cr-rich M2(C,N) carbonitride, rather than MX or M23C6 phase. The TMT process inhibits the precipitation of M23C6 and M5C2 phases. Enhanced creep properties of the steel by the TMT are mainly attributed to plenty of nano-sized Cr-rich M2(C,N) carbonitrides, rather than to MX and M23C6 precipitates. It is suggested that plenty of nano-sized Cr-rich M2(C,N) carbonitrides produced by the TMT are more helpful to improve the creep properties of the steel than M23C6 plus MX precipitates obtained by the conventional heat treatment.516