Deformation-mechanism-based creep model and damage mechanism of G115 steel over a wide stress range

The effects of applied stress (150–250 MPa) on the creep deformation behavior and creep damage (stable and unstable deformation) were studied systematically for a novel tempered martensite ferritic steel G115 at 650 °C. A creep model involving three deformation mechanisms (grain boundary sliding, dislocation glide, and dislocation climb) was applied to fully understand the creep deformation behavior of G115 steel over a wide range of applications. Subsequently, the deformation-mechanism-based creep model was validated for G115 steel over a wide range of stresses (120–250 MPa) and temperatures (625–675 °C). Further, in the stable deformation regions, the martensite laths have no significant change under high-stress conditions, whereas the martensite laths obviously coarsen and micro-cavities formed under low-stress conditions. Four types of precipitates can be characterized after creep. M23C6 carbide and MX carbontride are pre-existing in the initial microstructure. Cu-rich phase was precipitated at 650 °C after very short-term creep (6.87 h), whereas Fe2W Laves phase formed after long-term creep (4404.78 h). The Cu-rich phase particles are cut by a dislocation shearing mechanism and become too small to be stable, leading to the dissolution of the Cu-rich phases after long-term creep. The fine Cu-rich phase particles can significantly retard the recovery of martensite laths in the early stage of creep or during short-term creep. In the unstable deformation (necking) regions, martensite cracking and martensite fracture are the main micro-damage features during creep at high stresses, whereas the growth of micro-cavities and micro-cracks are the main features at low stresses. Ductile fracture is the dominant fracture mode of G115 steel at 650 °C above 150 MPa.