Phase transformation and mechanism on enhanced creep-life in P9 Cr–Mo heat-resistant steel

This work explores mechanical properties, structural evolution, and mechanism of creep-life enhancement for widely used P9 heat-resistant steel. The 17-year-on-site used P9 alloy exhibit a higher tensile strength and a smaller elongation than the new P9 alloy from room temperature to 700 °C. The P9 alloy also displays a typical ductile feature with a significantly necking profile. The P9 alloy shows phase transformation sequences of α-Fe (bcc) →Ac1∼858 °Cα + γ -Fe (bcc + fcc) →Ac3∼894 °Cγ-Fe (fcc) upon heating and γ-Fe (fcc) →Ms∼352 °Cmartensite (bct) →Mf∼300 °Cmartensite (bct) upon cooling. The new P9-alloy tube mainly contains ∼73.5% ferrite phase (α-Fe) and ∼26.5% carbide M3C. However, the used P9-alloy tube shows four crystalline phases including ∼45.9% ferrite, ∼14.5% martensite, ∼37.5% cementite (M3C) and ∼2.7% carbide M23C6. The creep test indicates that the used P9-alloy tube has a longer creep-life (or better anti-creep ability) than the new tube. Activation energies of atomic diffusion for the new and used tubes are respectively 252.45 and 345.87 kJ/mol, indicating a decreased diffusion capability in the used tube. This work suggests that martensite laths, lath boundaries, and precipitates (such as carbides) play important roles to inhibit creep-deformation in the P9-alloy steel.