Comparison of α-ferrite to ɣ-austenite transformations in 9%Cr steel alloys measured by neutron powder diffraction, dilatometry, and differential scanning calorimetry

This study explored phase transformations and thermal expansion coefficients in 9%Cr steels; P91 and P92 base metals, and P91 welds, on heating to 1000 °C. Neutron powder diffraction was employed to determine transformation temperatures, lattice parameters, and phase fractions. Austenite onset (A c1 ) and completion (A c3 ) temperatures were 800–805 °C and 885–890 °C for P91 base metal, and 814–816 °C and 928–930 °C for P92. The weld metals were evaluated in the as-welded condition. At room temperature, weld metals contained ferrite, martensite, and residual austenite. Residual austenite in weld 1 was retained on heating until fresh austenite formed at 765–770 °C, whereas in weld 2 it disappeared at 655–660 °C. The A c1 and A c3 temperatures of weld 2 were 815–820 °C and 920–922 °C. P92 transformed at lower temperatures and thermal expansion during the α-BCC-to-austenite transformation than P91. Weld 1 showed higher transformation temperatures and expansion than the base metal. Dilatometry measured transition temperatures of 817 °C and 880 °C for P91 base metal, and 836 °C and 923 °C for P92. As-welded curve exhibited multiple transitions between 697–904 °C. Calorimetry showed endothermic α-ferrite-to-austenite transitions with A c1 and A c3 at 824 °C and 874 °C for P91 base metal, 839 °C and 893 °C for P92, and 831 °C and 876 °C for weld 1. This study demonstrates the advantages of integrating neutron diffraction, dilatometry, and calorimetry to improve transformation analysis in 9%Cr steels; neutron diffraction measured A c1 up to 20 °C lower than dilatometry, indicating current post-weld heat treatment temperature ranges may be insufficiently conservative. Residual austenite in weld metal and its behaviour on heating may also influence weld heat-treatment response.