Rejuvenating the creep life of modified 9Cr–1Mo steel using a short-term heat treatment
摘要
Modified 9Cr–1Mo (P91) steel components that are used in thermal power plants undergo creep degradation during long-term service at 600 °C. This study examines the effect of a short-term rejuvenation treatment (RT), involving normalizing at 1050 °C for 2 h followed by tempering at 750 °C for 2 h, on the creep life enhancement of partially crept P91 steel at 600 °C and 135 MPa. The creep response of rejuvenated P91 steel is compared with its as-received counterpart, which is creep-tested until rupture. Results reveal that the short-term RT erases the creep history in partially crept P91 steel and enhances creep life by approximately 4 times compared to its as-received counterpart. Microstructural analysis reveals that creep leads to lath coarsening, deviates the orientation relationship (OR) between laths and prior austenite grain (PAG) from the ideal Kurdjumov–Sachs (K–S) OR, Oswald ripening of M23C6 precipitates, and a reduction in dislocation density, ρ. RT on the partially crept P91 leads to a complete microstructure reversal, wherein the microstructural features match well with the uncrept as-received sample subjected to the same treatment. Microstructural evolution during creep, involving the competition between dislocation climb-assisted lath coarsening and lath boundary pinning by M23C6 precipitate, is discussed and numerically estimated from the continuum damage model (CDM). Correlations are established between the initial lath width, lath coarsening kinetics, steady-state lath width, and creep rate. Implications of these results on extending the life of P91 components and the effectiveness of RT compared to similar rejuvenation treatments on other high-temperature alloys are discussed.