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).

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