Creep strength breakdown and microstructure evolution in a 3%Co modified P92 steel

Microstructure and hardness in the grip and gage sections of ruptured 3%Co modified P92 steel specimens were examined after creep tests under applied stresses ranging from 200 to 100 MPa at T=650 °C. Under long-term aging, the depletion of W from solid solution leads to precipitation of the Laves phase. In addition, coarsening of MX carbonitrides occurs after 34 h of aging. The final dimensions of these particles are independent of further aging time. A breakdown of applied stress vs time to rupture dependence takes place at a stress of 140 MPa or a rupture time of 1828 h at which the W content in the ferritic matrix almost attains a thermodynamical equilibrium level. Under long-term creep conditions, the strain-induced coarsening of M₂₃C₆ carbides and the Laves phase takes place. No strain-induced coarsening of MX carbonitrides was found. However, V-rich MX carbonitrides transform to Z-phase in the gage section. Zener drag force restraining migration of grain/subgrain boundaries decreases, and with long-term creep, the transformation of the tempered martensite lath structure (TMLS) to subgrain structure occurs, whereas short-term creep conditions and long-term aging result in the retained lath structure.