Creep Performance of Advanced 9Cr–1Mo Steels in Steam Environment

A new test rig was developed to conduct creep evaluation in steam. The isothermal creep performance for ferritic 9Cr and ferritic/martensitic Gr.91 steels was superior at 650 °C in steam than in air. Microstructure characterization revealed the presence of an adherent and continuous (Fe,Cr)-rich inner oxide layer because of crack healing by re-oxidation. Stress calculation based on a composite structure showed that the longer lifetimes in steam were likely due a transfer of load from the alloy to the inner scale. However, a significant decrease of the Gr91 creep lifetime was observed in steam at 650 °C when the specimens were thermally cycled every 10 h. This could be explained by localized cracking of the load-bearing inner layer.