Microstructure evolution in HR3C austenitic steel during long-term creep at 650 °C

The creep behavior of HR3C austenitic steels was investigated at 650 °C and over the stress range from 150 to 250 MPa for up to 13,730 h. The corresponding microstructure evolution was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the initial stage of the creep process, the creep-resistance of HR3C steel is enhanced by the precipitation of second-phases particles in the grain and at the grain boundary. Compared with the precipitates inside the grain, the higher nucleation and growth rate of precipitates at the grain boundary is related to the higher interfacial energy and diffusion rate of atoms. The high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) results show that the precipitates inside the grain may initially nucleate at dislocation pile-up sites, and the interface coherency between the precipitate and the matrix can be destroyed after a long-term creep process. The TEM morphology indicates that the agglomerated tiny particles interact with the dislocations, contributing mostly to the precipitation strengthening inside the grain during the long-term creep process at 650 °C, while the growth of chain-like M23C6 precipitates at the grain boundary increases the tendency of intergranular cracking as the creep time increased.