Influence of tungsten on transformation characteristics in P92 ferritic–martensitic steel

Abstract

Present study demonstrates by experiments and computation that a fully martensitic microstructure does not form in 9 Cr steels containing 1.9 wt% W (P92). The presence of a magnetic transition prior to the martensitic transformation during fast cooling supports the co-existence of ferrite and martensite. The formation of diffusional ferrite at high cooling rates is precluded by a systematic variation of three parameters namely cooling rate, austenitising temperature and hold times. However, complete martensite formation has not been experimentally feasible in P92 steel at all for all cooling rates (1–90 K min−1 in DSC and water quenching) employed from austenitisation temperatures (950–1150 °C), unlike in P91 steel where complete martensite forms during cooling at the rate of 30 K min−1 or higher. The experimental results have been substantiated by Thermo Calc simulation for varying W contents in P91 and P92 steels. Thermo Calc simulation revealed that P91 steel remains in the single phase γ domain during austenitisation at 950–1150 °C (1223–1423 K). It is observed that γ phase field gets shifted towards the right with addition of W in P91. P92 steel remains in the three phase domain of α + γ +MX region at 950–1150 °C (1223–1423 K), which explains the presence of ferrite above Ac3 temperatures.