Influence of Chemical Compositions on Lower Ferrite-Austenite Transformation Temperatures in 9Cr Steels

Computational thermodynamics approach was used to predict the ranges of the lower ferrite-austenite transformation temperatures, A₁’s, in three 9% Cr steels. The predicted A₁ ranges were: 766–856°C for SA387 Grade 91, 775–863°C for SA213 Grade T92, and 676–862°C for the weld metal SFA-5.23 B9 (2004). For Grade 91 and Grade T92 using the highest tempering temperature permitted by ASME Code, 800°C, would permit certain alloys conforming to the chemical composition specification to be tempered above their A₁, thereby risking the formation of untempered martensite. Similar circumstances exist for weld metal conforming to the SFA-5.23 B9 specification. Linear regression analyses were performed to develop simplified expressions capable of representing the thermodynamically predicted relationships between chemical compositions and A₁’s. These are, Grade 91/SFA-5.23 B9 (2004): 805°C + 2.5(%Cr) + 18.1(%Mo) + 19.1(%Si) + 37.1(%V) + 19.2(%Nb) − 63.7(%C) − 130.6(%N) − 60.5(%Mn) − 72.3(%Ni); Grade T92: 778°C + 4.9(%Cr) + 22.6(%Mo) + 10.8(%W) + 22.9(%Si) + 43.6(%V) + 20.2(%Nb) − 80.6(%C) − 150.7(%N) − 55.1(%Mn) − 68.0(%Ni).