Microstructure Aging,Property Degradation And Residual Creep Life Evaluation Of High-Chromium Martensitic Heat-Resistant Steel

High Cr martensitic heat-resistant steel is widely used to manufacture the key components in ultra-supercritical units?for power plant?like overheating and reheating pipes.The pipes are exposed at high temperature and high pressure in the course of long-term service,resulting in microstructure aging and mechanical performance degradation and thus raising the safety issuefor power plant.Till now,there is no perfect realtime and non-destructive method to evaluate the aging state and residual life of the pipes.In addition,on account of the limited running time of ultra-supercritical units,aged specimens are rare for relevant research,therefore the microstructure evolution mechanism of the in-service heat-resistant steel remains in obscurity.In this work,the mechanisms of aging and degradation of P91 and P92 steels have been systematically investigated using the accelerated aging method,and a rapid non-destructive method to predict the aging microstructure and residual life based on room temperature hardness has been established.First,based on the thermodynamics and kinetics of P91 and P92 steels,the accelerated aging method was established to simulate the actual long-time low-temperature conditions and samples at different aging stages were prepared.Reliability of the accelerated aging method was verified,and results showed that both 790oC+620oC aging combination for P91 steel and 800oC+650oC for P92 steel guaranteed similar or identical microstructures and properties of those after long term service under the actual working condition.In the early aging period of P91 and P92 steels,massive dislocations with opposite directions in the martensitic laths got annihilated by slipping and climbing.The laths coarsened via the motion of"Y"type lath boundaries and the disappearance of parallel lath boundaries,and sub-grains began to nucleate.At this stage,the mechanical properties of P91 and P92 steels deteriorated obviously with rapid decrease in hardness and room/high temperature strength.In the middle and late aging stages,equiaxial sub-grains completely replaced the martensitic laths and further merged to form recrystallization nuclei.The recrystallized nuclei continued to grow via merging bowing until they took up the original austenite grains and eventually the matrix was composed of large ferrite grains without distortion.Further reduction in the hardness and tensile strength took place during this process,but the decrease gradually slowed down.The microstructure evolution of P91 and P92 steels during aging could be divided into five stages:?I?dislocation recovery and annihilation stage;?II?martensite lath broadening and subgrain nucleation stage;?III?large numbers of equiaxial sub-grains formation stage;?IV?the sub-grains growth stage;?V?recrystallization nucleation and growth stage.The Ostwald ripening process of the M₂₃C₆ carbides in P91 and P92 steels is subjected to the law of 1/4 order and the control mechanism is the diffusion of solute in grain boundary.The M₂₃C₆ carbides in the P92 steel have a lower ripening rate than those in the P91 steel due to the low interfacial energy of the M₂₃C₆ carbides and the matrix resulting from their coherent relation,and the reduction of the solute atoms migration rate caused by W element.In addition,the precipitation strengthening of M₂₃C₆ carbides is limited after ripening,especially under the aging data of 790oC/800 h for P91 steel and 800oC/1000 h for P92 steel,the precipitation strengthening effect is only about 13MPa.Laves phase precipitated preferentially in the vicinity of the M₂₃C₆ carbides and some precipitated separately coherently with the grains and grew in a non-coherent manner to the adjacent grains,with a significant"cluster"phenomenon.The solute elements would migrate as the Laves phase continuously grew up;the Mo content in P91 steel and the W content in P92 steel both increased significantly after long term aging.There was a rapid decrease in toughness with the increase of the average size and content of the Laves phase.Laves phase would promote the occurrence of brittle fracture and deteriorate the impact toughness due to the high ripening rate,the"cluster"precipitation and merging with the M₂₃C₆ carbide into a larger one for those precipitated along the carbides.The results of creep experiments of different aging microstructures showed that the creep performance of P91 and P92 steels would significantly decrease due to the microstructure aging,especially the recovery and annihilation of dislocations and the migration of martensite laths.Meanwhile,the logarithm of parameter A and B in the isothermal extrapolation method shows good linear relationship with the high temperature tensile strength,and the room temperature hardness of the aging material has such relationship with the high temperature tensile strength as well.Therefore,an evaluation method has been established based on thehigh temperature tensile strength to predict the residual life of P91 and P92 steels under a certain temperature?550-650oC?and stress by the simple room temperature hardness test.This assessment method can quantitatively characterize the effect of the aging material on the creep performance,thus effectively avoiding the overestimation of the residual creep life of the material due to the neglect of the microstructure evolution.In addition,based on the corresponding relationship among the room temperature hardness,microstructure aging and residual life,the aging state of the material can be conveniently assessed by room temperature hardness measurement.