Creep Mechanism And Constitutive Model Of A RPV Steel SA508-Ⅲ Under Accident Conditions

The nuclear safety is crucial for Chinese energy market,since the nuclear power plays an important role in the electricity production of China.The In-Vessel Retention（IVR）is considered to be a mitigation strategy in severe nuclear accidents.The idea of IVR is to remove the heat of melting core in reactor pressure vessels（RPV）by external cooling water and maintain the integrity of RPV,preventing the leakage of radiative materials into environment.In IVR conditions,the phase transformation will inevitably occur in RPV materials and creep fracture will become the primary failure mode.However,there is no sufficient research on the creep behaviour of RPV material under the accident conditions,especially the temperature range of phase transformation.The physical creep constitutive model is necessary to predict the creep deformation and fracture life of RPV materials.In current work,a typical RPV mater,SA508-Ⅲsteel,is selected to study its phase transformation and creep behaviours in accident conditions.The influence of phase transformation on the tensile and creep properties and relevant damage mechanisms is clarified by the high-temperature tensile and creep tests.The deformation-mechanism based true-stress（DMTS）creep model and modified DMTS model,which concerns the effects of grain size and threshold stress in the model,are used to give a full description of the creep behaviours in different stages of phase transformation.The predicted time to specific creep strain or rupture is achieved by the efforts of DMTS and modified DMTS models.Based on the results of differential scanning calorimeter（DSC）and heat-treatment experiments,the temperature range of phase transformation is determined to be700⁸00°C by Scanning Electron Microscopy（SEM）,Electron Back-Scattered Diffraction（EBSD）and Transmission Electron Microscopy（TEM）.According to the microstructure analyses,the major microstructural evolutions caused by phase transformation are the formation of austenite and the dissolution of cementite.In term of the tensile tests of SA508-Ⅲsteel ranging from room temperature to 1000°C,the yield strength and ultimate tensile strength are found to be monotonously decreased with the elevated temperature,while the ductility is significantly increased at 750°C and then tends to be steady.By means of the fracture morphology and microstructure examinations,the preferential sites of voids and cracks are changed due to the occurrence of phase transformation:the cementite-aggregated areas and the interfaces between the cementite and matrix（before phase transformation）,austenite-ferrite interfaces（during phase transformation）and austenite grain boundaries（after phase transformation）.The decline in strength and growth in ductility above the phase-transformation temperature are mainly related to the formation of austenite and dissolution of cementite induced by the phase transformation.The high-temperature creep tests of SA508-Ⅲsteel at 650⁸50°C reveals that the fracture life is obviously shortened and the minimum creep rate is enhanced at 750°C when these two results are compared to that at at 700 and 800°C with the same applied stress.The relatively low fracture life at phase-transformation temperature is mainly related to the decline in dislocation strengthening and precipitate strengthening mechanisms,causing by the formation of austenite and dissolution of cementite during the phase transformation.The coexistence of ferrite and austenite at phase-transformation temperature produces the local strain concentration at interfaces of two phases,leading to the premature creep failure.The evolution of creep deformation mechanism with the process of phase transformation is determined by the microstructural analyses and calculations of stress exponents.The stress-dependent multiple creep deformation mechanisms are dominant before the phase transformation.The controlled creep deformation mechanisms during the phase transformation are dislocation gliding and grain boundary sliding.The predominant creep deformation mechanism becomes grain boundary sliding after the phase transformation.The Orr-Sherby-Dorn（OSD）parameter method fails to predict the time to specific strain(t_3%and t_5%)in the testing range controlled by multiple creep deformation mechanisms（450⁷50°C and 10⁴00 MPa）,attributing to the single deformation mechanism in OSD method.In order to overcome the limitations of predictions of creep deformation and life by single creep deformation mechanism,the creep constitutive models of SA508-Ⅲsteel are built at different stages of phase transformation,according to the microstructural evolution caused by the phase transformation.The deformation-mechanism-based true-stress（DMTS）creep model is used to describe the creep behaviors before the phase transformation（450⁶50°C）.The modified DMTS creep model,which consideres the effects of grain size and threshold stress,is utilized to depict the creep behaviors after phase transformation（800¹000°C）.The unified creep model,which is weighted by the volume fractions of austenite and ferrite,is built for the phase transformation stage（700⁷50°C）.The obtained creep models achieve an accurate prediction on the creep behaviors at different stages of phase transformation.The predicted time to certain creep strain(t_3%and t_5%)and fracture life（t_r）,which are calculated based on above models,are in good agreement with the experimental results.In present study,the high-temperature tensile and creep data of SA508-Ⅲsteel are obtained and the relationship between the mechanical properties,damage mechanisms and microstructural evolutions triggered by phase transformation is revealed.The creep models are achieved towards the different stages of phase transformation.The current work will provide an in-depth insight for the optimization of IVR strategy and the integrity of RPV in severe accident conditions.