Effects of Alloying and Processing Modifications on Precipitation Behavior and Elevated Temperature Strength in 9% Cr Ferritic/Martensitic Steels
Two low-carbon 9-Cr ferritic-martensitic steels were designed here with the aim of decreasing M 23 C 6 and maintaining or increasing MX phase fraction. A low-carbon (LC) alloy and a low-carbon, zero-niobium (0 Nb) alloy were fabricated, their designs based upon the P92 alloy system. Solutionizing temperatures to maximize V and Nb in solution while avoiding δ-ferrite were determined to be 1170 °C for the P92 alloy and 1050 °C for LC and 0 Nb, significantly lower than predicted using ThermoCalc ® modelling. As was intended, the M 23 C 6 phase fraction was reduced for LC and 0 Nb alloys after both heat treating and aging relative to the base P92 alloy, as determined by wide angle x-ray scattering (WAXS) analysis. Dislocation density measurements from x-ray line broadening in P92 and LC suggest these alloys had more stable dislocation substructures than 0 Nb at lower temperature and shorter time aging conditions. While LC exhibited lower microhardness than P92 at room temperature, the tensile properties were comparable at 650 °C, suggesting that elevated temperature strength can be achieved with lower carbon contents. Aging studies showed that P92 had a more stable microstructure for higher temperature and longer time aging conditions. The P92 alloy also had a longer stress rupture life, implying that the M 23 C 6 precipitate contribution to thermal stability is important. Evidence of Z-phase was discovered for the LC alloy aged 10,000 h at 650 °C, corresponding to decreased strength and increased ductility. Overall, the stress rupture lives of the modified heat-treatment variations of P92 and LC compare favorably to literature values for 9% Cr steels with conventional heat treatments.