Effects Of Phosphorus And Cerium On Low-temperature Embrittlement And Hot Ductility Of SA508-4N Nuclear Pressure Vessel Steel

The nuclear power plants are developing towards large-scale and integration due to increasing demand of green energy.The new generation advanced reactor pressure vessel SA508-4N steel with appealing comprehensive properties has attracted extensive attention from the researchers from the whole word.During the service,the non-hardening embrittlement of reactor pressure vessel steel is caused by the grain boundary segregation of impurity atoms irradiated by neutron,resulting in the occurring of major and sudden disaster by brittle fracture.The base material and coarse grain heat affected zone of reactor pressure vessel is vulnerable to irradiation embrittlement during long-term operation under high temperature,high pressure and intense irradiation.Due to the limitation of testing in irradiatied reactor pressure vessel steel,low-temperature embrittlement caused by impurity atoms grain boundary segregation is used to predict the tendency of non-hardening irradiation embrittlement in the research.At the same time,impurity elements can cause the occurring of transverse cracks during straightening operation in continuous casting process,which directly influences the productivity and quality of cast steel and reduce the recycling rate of scrap.At present,thermal simulation tensile test is usually used to approximate restore the microstructure of steel during straightening operation in the continuous casting process,and the possibility of transverse cracking occurring is predicted by hot ductility of the material.In addition,the addition of cerium can significantly reduce the low temperature embrittlement tendency and improve the hot ductility of steel.However,the influence and mechanism of phosphorus and cerium on the low-temperature embrittlement and hot ductility of SA508-4N steel is also lack.In this paper,thermal aging simulation method,thermal simulation welding test,thermal simulation tensile test and impact test were used to specifically explore the influence of phosphorus and cerium on the low-temperature embrittlement of based materials,coarse grain heat affected zone and hot ductility in the SA508-4N reactor pressure vessel steel,which making up for the blanks in relevant fields.Combined with Auger electron spectroscopy(AES),field emission gun transmission microscopy(FEGSTEM),scanning electron microscopy(SEM)and optical microscopy(OM),the mechanism on the effect of phosphorus and cerium on the low-temperature embrittlement and hot ductility of SA508-4N steel was clarified in this paper.Mainly studies the following aspects in this paper:The thermal aging method was used to simulate the non-hardening embrittlement caused by phosphorus grain boundary segregation.In this paper,the influence of phosphorus on the low-temperature embrittlement in the base material of SA508-4N steel was investigated and the tendency of radiation embrittlement of steel was predicted by low-temperature embrittlement.The results show that there is an obvious linear relationship between the ductile-brittle transition temperature and phosphorus equilibrium grain boundary segregation concentration i n the base material of SA508-4N steel.As well,the prior austenite grain size influences the phosphorus segregation free energy in the based material of SA508-4N steel,resulting in the difference of low-temperature embrittlement.When the prior austenite grain size is relatively larger,the phosphorus equilibrium grain boundary segregation concentration increases significantly caused by the higher phosphorus segregation free energy.Combining with the equilibrium segregation kinetic model and the linear relationship between the ductile brittle transition temperature and the phosphorus equilibrium grain boundary segregation concentration,the temperature-time embrittlement diagram of P-SA508-4N steel is drawn in this paper,which provides an effective theoretical basis and guiding significance for practical production.In order to effectively reduce the embrittlement tendency of SA508-4N reactor pressure vessel steel caused by phosphorus equilibrium grain boundary segregation,the effect and mechanism of Ce-doped on the low temperature embrittlement of base material in the P-SA508-4N reactor pressure vessel steel were studied in this paper.The result shows that there is a position competition relationship between cerium atom and phosphorus atom:the cerium grain boundary segregation can lower the low-temperature embrittlement by significantly inhibiting the phosphorus grain boundary segregation,as to ensure the safety operation during service of based material in P-SA508-4N reactor pressure vessel steel by effectively reducing the low-temperature embrittlement.Baed on the research on the effect of phosphorus and cerium on the low-temperature embrittlement and hot ductility in the base material of SA508-4N steel,the influence of cerium and phosphorus on the low-temperature embrittlement in the coarse grain heat affected zone of SA508-4N steel was in detail investigated by using welding thermal simulation and thermal aging method in this paper.The results show that the ductile-brittle transition temperature is increased by higher phosphorus equilibrium grain boundary segregation in the coarse grain heat affected zone of SA508-4N steel.Also,the ductile-brittle transition temperature of the coarse grain heat affected zone is higher than based material in the P-SA508-4N and P+Ce-SA508-4N steels and the tendency of low-temperature in the coarse grain heat affected zone is greater.In addition,cerium equilibrium grain boundary segregation can reduce the phosphorus equilibrium grain boundary segregation concentration.As well,the interaction coefficient of Ce-P is less than zero.Hence,the addition of trace cerium can effectively reduce the low-temperatue embrittlement tendency of coarse-grain heat-affected zone in the P-SA508-4N steel,as to restrain the irradiation embrittlement.At last,the thermal simulation tensile test was used to investigate the influence of trace elements phosphoru and cerium on the hot ductility of SA508-4N steel.The results show that phosphorus can effectively improve the hot ductility of SA508-4N steel by restraining the grain boundary sliding during the hot deformation.Hence,straightening in the P-SA508-4N steel could operation lower 750 ~oC.At the same time,the addition of cerium is able to facilitate the occurrence of the dynami c recrystallization of the steel,lowering the initial temperature of dynamic recrystallization and thereby enhancing the hot ductility performance without hot ductility curve trough.Furthermore,addition of trace P and Ce could further improve the hot ductility of SA508-4N steel.