Microstructural evolution and change in hardness in type 304H stainless steel during long-term creep

The microstructural evolution and the change in hardness have been investigated for 18Cr–8Ni (type 304H) stainless steel during long-term creep. Creep and creep-rupture tests were carried out at temperatures between 550 (823) and 750°C (1023 K) for up to 180 000 h. The hardening behavior during creep depends on the stress level, as well as the precipitation of M23C6 carbides and σ phase. At a high stress of 177 MPa, the hardening during creep is much larger than the age hardening, indicating that the hardening during creep is mainly caused by the strain hardening due to creep deformation. At a later stage of creep, the softening occurs due to the recovery of excess dislocations, which becomes more significant with decreasing stress and increasing test duration. The strain hardening disappears with decreasing stress level and increasing test duration. At a low stress of 61 MPa or less, the hardening during creep can be approximately given by the age hardening under no stress, except for the final stage of creep.