A Study On Creep Damage Mechanism And Constitutive Relation Of Chinese Steel SA508-? Over Phase Transision Temperature

The safety of nuclear power has been paid much attention by the state and the public.Under the condition of In-Vessel Retention,microstructral damage occurrs in the lower head of Reactor Pressure Vessel(RPV)endured by the combined action of high temperature and load.However,the study on the creep test and microstructure of Chinese steel SA508-III is still in its infancy stage.The research results over the phase transition temperature are particularly scarce.Forthermore,the traditional creep constitutive equation did not involve the microscopic creep damage and its evolution mechanism over the phase transition temperature.In this paper,the microscopic creep damage mechanism of steel SA508-III over the phase transition temperature is systematically analyzed.The parameters describing inhomogeneity and evolution of microscopic damage are defined respectively,and they are introduced into damage evolution equation and creep constitutive equation.The new creep constitutive equation considering microscopic damage mechanism is established,which lays a theoretical foundation for the high temperature structural integrity analysis of RPV suffered extreme accidents.The main research work and conclusions are as follows:Through high temperature tensile tests,some key mechanical properties of Chinese steel SA508-III at different temperatures were obtained.The critical temperature for phase transition is 740°C determined by the DSC curve.Multiple sets of creep tests were performed over the phase transition temperature to obtain creep performance parameters.Over the phase transition temperature,creep specimens with different creep time by conducting interrupted creep test and high-temperature aging specimens with different holding time were obtained.Finally,the slices of interrupt-creep specimens and the high temperature aging specimens were observed by using SEM and TEM,the microscopic damage and evolution of the material in the process of creep were analized.The creep microscopic damage mechanism analysis shows that over the phase transition temperature,the creep quickly enters the second stage.Under the effect of heat induction from high temperature,the precipitates(second phase particles)in some crystals are incorporated into the matrix,and segregated gradually to grain boundaries and subgrain boundaries by dislocation motion,which made a contribution on grain boundaries coarsening,as well as second phase particles.Under stress induction,vacancies diffuse toward grain boundaries and converge into voids.More and more voids initiate continuously at the junction of three crystals,coarse grain boundaries,and second phase particles with high stress.At the third stage of creep,with the emergence of more coarsening second phase particles,voids continue to iniatiate and grow rapidly.The adjacent voids connect to each other to form micro-cracks or larger cavities.With creep processed,both the volume fractions of voids and second phase particles show an approximate linear growth trend.By observing the microstructure of creep specimens,it was found that the coarsening of second phase particles and the initiation and growth of voids are the main causes of creep damage.The time-normalized evolution equations for coarsening second phase particles and voids in the process of creep are proposed.Based on the concept of meso-mechanics,a mesoscopic three phase complex composed of nondestructive phase,void phase,and coarse second phase particle phase was established as a representative volume unit.The K-R creep damage constitutive model reflecting the damage evolution characteristics of microstructures was deduced.The ABAQUS user subroutine CREEP of the creep damage constitutive equation over the phase transition temperature is established,and the meso-structure simulation calculation corresponding to the interrupted creep test is performed by using this program.The mechanism about the stress and strain evolution of the matrix around coarsening second phase particles and voids during creep process was analyzed.It shows that with the coarsening of second phase particles,the multiaxial stress increases significantly,and the stress triaxiality at the phase boundary along the loading direction is higher than the others.The uniaxial stress near the creep cavity is more prominent and promotes the growth of the voids.High stress triaxiality arises in the wall of cavities perpendicular to the loading direction and the adjacent matrix,which will cause the emergence of small voids in these areas,aggravating creep damage and promoting cavities' growth.Based on the above experimental research and theoretical analysis,the creep microdamage mechanism of the Chinese steel SA508-III over the phase transition temperature was revealed.The evolution mechanism of coarse second phase particles and voids during the creep is expounded.A K-R creep damage constitutive model reflecting the damage evolution characteristics of microstructure was established,which laid a theoretical foundation for the RPV high-temperature structural integrity assessment in the implementation of severe accident mitigation techniques.