Study On The Microstructural Evolution And Property Improvement For Reactor Pressure Vessel Heavy Forgings Of SA508 Gr.3 Steel

SA508 Gr.3 steel has been extensively used to fabricate the large forgings for nuclear pressure vessels that play a crucial role on the safety operation of nuclear power station,such as the shell of nuclear reactor pressure vessel,steam generators and compressors.The property of the steel,especially the impact toughness will be deteriorated because of the high temperature and pressure of the service condition and fast neutron irradiation.To compensate for the material degradation,the material with high initial toughness and low initial ductile-to-brittle transition temperature should be adopted for nuclear pressure vessels.Study the microstructure evolution of SA508 Gr.3 steel during heat treatment is an effective method to make the nuclear pressure vessels obtain excellent performance.In this dissertation,the kinetics of bainite transformation,microstructure evolution during tempering and the effect of chemical segregation have been studied.The relationship among heat treatment process-microstructure-properties of SA508 Gr.3 steel is established.Moreover,the effect of microstructure evolution during heat treatment on the mechanical property of nuclear pressure vessels were evaluated.First,the bainite isothermal transformation in the SA508 Gr.3 steel has been studied.The bainite formation range,relationship between maximum volume fraction and isothermal transformation temperature and TTT curve for the bainite transformation have been obtained by the method of thermodynamic calculation,dilatometric experiments and microstructure observation.The results of thermodynamic calculation and microstructure observation show that the austenite transformed into carbide-free bainite in the temperature range of 551??520?.And the transition temperature of the upper and lower baintie is about 440?.The results of dilatometric experiments and quantitative metallographic analysis show that the maximum volume fraction has a relationship with the temperature as follows:f_max=1-exp[-?×(551-T)],and the=0.04.The kinetics analysis results show that TTT curve for the bainite transformation has a C shape,and the nose temperature is about 490?.Besides,there is a obvious acceleration of bainite transformation when the temperature close to the Ms.It can be concluded that the bainite transformation in the large forging of SA508Gr.3 steel is complex during heat treatment.Second,the characteristic of retained austenite(RA)decomposition and its effect on the impact toughness were investigated.The relationship between characteristic of RA decomposition and tempering temperature has been obtained by location observation and dilatometric experiments.And the characteristic of RA decomposition has been used to optimize the tempering process.The results of magnetic measurement show that there are about 9%austenite retained in the material after quenched to room temperature with a rate of 0.75?/s.The RA and its decomposition product were localization observed in terms of SEM observation.The results show that the RA decomposed in to martensite,bainite and mixture of long rod carbides and ferrite during tempering at temperature range of 190??247?,247??461?and 461??650?,respectively.The results of impact test show that long rod carbides which formed from RA decomposition with a big size in its length direction decrease the critical cleavage stress for initiation of micro-cracks.The impact toughness of the steel in this study can be dramatically improved by pre-tempering at 400?before heating to 650?during tempering.Finally,the effect of chemical segregation on the microstructure evolution and mechanical properties of SA508 Gr.3 steel have been assessed.The relationship between chemical segregation and microstructure evolution,mechanical property have been obtained by method of electron probe microanalysis(EPMA),microstructure observation and property tests.The EPMA results show that the segregation in this material is mainly related to the inhomogeneous distribution of alloying elements Mn,Ni and Mo,while the segregation of carbon is not observed between positive and negative segregation zones.Martensite and baintie formed during quenching respectively in the regions where solutes are enriched and diluted.Carbon-enriched Martensite-Austenite(M-A)islands also have been found in the negative segregation zone.The M-A islands decomposed into a mixture of carbides and ferrite during tempering.These carbides accumulated together at the original position of M-A islands.The relationship between toughness and chemical segregation is considered to be significantly associated with the content of M-A islands in the negative segregation zone.The accumulated carbides formed from M-A decomposition were regarded as the main sources of crack initiation and deteriorated the toughness of the material.The chemical inhomogeneities have been substantially reduced by homogenization treatment.The homogeneity of the microstructure and mechanical properties have been improved remarkably by means of homogenization treatment.In this dissertation,the effect of RA decomposition and chemical segregation on the microstructure and mechanical property of SA508 Gr.3 steel have been analyzed.On this basis,the heat treatment process of nuclear pressure vessel has been optimized.The microstructure and mechanical property of nuclear pressure vessels have been improved.