Study On Low Temperature Impact Toughness Of SA508-3 Steel For Heavy Section Forging In Reactor Pressure Vessel

Reactor pressure vessel and steam generator are the key components of nuclear power unit,which have a demanding requirement for the mechanical propereties of material.SA508-3 steel as a low-carbon low-alloy steel has been widely used in nuclear pressure vessels' forgings for its excellent strength and low temperature impact toughness.However,with the power of the nuclear power plant increasing,the wall thickness of the heavy forgings,such as conical shell increases,which always induces the lower and fluctuating phenomenon of impact toughness at low temperature.In this research,the microstructure of a conical shell at different position along the thickness as well as the phase transformation of SA508-3 steel in the continuous cooling process have been investigated systematically.Different ways of cooling have been used in lab experiments to simulate the cooling conditions of different position in the real forgings.The microstructure and mechanical properties were compared between the lab experimental results and real conical shell forging.And the reasons inducing the poor impact toughness of the material has been analyzed according to the experimental results.Finally,a new heat treatment has been designed to improve the mechanical properties of SA508-3 steel by optimizing the M-A constituents in granular bainite without changing the cooling rate of the forging.The main contents of this research are:1.Microstructure and mechanical properties analyses of a heavy SA 508-3 steel forgingA heavy SA508-3 steel forging,i.e.conical shell forging,has been analyzed from three aspects of purity,microstructure and mechanical properties.It is found that the chemical composition,content of gas elements and inclusions of actual forging have been controlled well.It is martensite and lower bainite appearing in the surface of the actual forging that has a good mechanical performance in strength and low temperature impact toughness,and the appearance of granular bainite in the central part of forging is the main reason that results in lower and fluctuating phenomenon of low temperature toughness.2.The phase constitution and transformation behavior of SA508-3 steelThe equilibrium phase diagram and the precipitated phase component diagram have been calculated by the Thermo-Calc thermodynamic calculation software.The equilibrium phase transformation point and the continuous cooling transformation phase diagram of SA508-3 steel are measured by thermal dilatometer and microhardness tester.Additionally,the microstructure of SA508-3 steel after quenching with different cooling rates have been observed detailedly by OM and SEM.Different cooling medium in heat treatment were used in lab experiments to simulate cooling conditions of different position in actual forging,and the granular bainite appearing in the center of actual forging were analyzed and characterized systematically.It is found that the KSI carbide,cementite,MC_SHP and M7C3 are the main precipitates in SA508-3 steel in equilibrium condition.The microstructure was bainite when cooling rates were during 0.35~20?/s,and granular bainite,upper bainite and lower bainite apperaed successively with the increase of cooling rates.The impact toughness of samples after sand cooling,which has a granular bainite microstructure,were significantly lower than those cooled by water.Granular bainite consists of bainitic ferrite matrix and M-A constituents in irregular blocky or thin bar shape.3.Study on intercritical quenching and tempering process of SA508-3 steelEffect of intercritical treatment on phase transformation of SA508-3 steel has been analyzed.It is found that the hardenability of the material is improved,when the austenitization temperature is decreased to the intercritical temperature region.The microstructure and mechanical properties of samples after sand cooling in lab,which is used to simulate the central part of actual forging,have been studied in different intercritical quenching temperatures and tempering temperatures.It is found that the size and amount of M-A constituents in granular bainite decreased obviously.The low temperature impact toughness of the material has been optimized significantly after 800? intercritical quenching + 650? tempering heat treatment.And the strength of the material has not been influenced remarkably.Therefore,the mechanical properties of SA508-3 steel are improved by optimizing the M-A constituents in granular bainite without raising the cooling ability of forging.