Experimental Study On Creep-fatigue Of P92 Steel Based On Damage Evolution Mechanism

P92 steel is one of the main material of thermal power and nuclear power generator units.It has long been subjected to the interaction of creep and fatigue due to the harsh service environment.Creep-fatigue failure is one of the main failure modes of high temperature parts.Such failures are mainly affected by factors such as temperature,holding time,fatigue damage,and so on.At present,the research on the effects of the above factors on the creep-fatigue behavior of P92 steel is not enough to fully support its structural safety assessment.Therefore,the creep-fatigue test of P92 steel based on the material damage evolution mechanism was carried out in this paper.The creep-fatigue behavior of P92 steel was revealed macroscopically,and the evolution mechanism of microcracks and microvoids was observed microscopically.Provide guidance for engineering practice,which has great engineering significance.Creep-fatigue interaction experiments were carried out at a high temperature of550-650?.The effects of temperature,holding time and pre-fatigue damage on the creep-fatigue behavior of P92 steel were investigated.With the increase of temperature,the creep-fatigue life of P92 steel is significantly reduced,the cyclic softening is faster,and the stress relaxation phenomenon during holding time is more obvious.The holding time mainly affects the stress relaxation behavior of the material.The longer the load-holding time,the more pronounced the stress relaxation.Pre-fatigue damage replaces the initial phase of the creep-fatigue cyclic stress response and substantially reduces the duration of the stabilization phase.However,pre-fatigue damage does not affect the creep-fatigue behavior of the subsequent stable phase of P92 steel.The life prediction of P92 steel was carried out by linear cumulative damage model,strain range partitioning model and strain energy density exhaustion model.The applicability of the above three models to P92 steel was compared and analyzed.Optical microscopy and SEM observations were carried out on the specimens and fractures before and after the creep-fatigue test,and the damage evolution mechanism of the creep-fatigue interaction was explained from the perspective of microstructure changes.The microstructure of the P92 steel base material is lath tempered martensite,and a large amount of M₂₃C₆ carbide is dispersed at the grain boundary.M₂₃C₆ carbide has poor thermal stability,which will cause coarsening and connection after long-term creep and reduce anti-creep ability.Fracture morphology analysis shows that the short-time holding time creep-fatigue specimen fracture has obvious fatigue fracture characteristics.It can be divided into three areas:fatigue source zone,crack propagation zone and fracture zone.Secondary cracks and fatigue strips perpendicular to the crack propagation direction can be observed microscopically.The fracture morphology of the creep-fatigue specimen for long-term holding time has the characteristic of obvious creep fracture.In addition,the Chaboche plastic constitutive model and the strain-enhanced creep model are used to simulate the creep-fatigue cycle characteristics of P92 steel.The finite element results are close to the experimental data and it can simulate the creep-fatigue cycle deformation behavior of P92 steel to a certain extent.