Investigation On Strengthening And Toughening Factors Of Deposited Metal In A508-Ⅲ Nuclear Pressure Vessel Steel Welds

A508-Ⅲ steel is mainly used for manufacturing Nuclear Pressure Vessel (NPV)which services in extreme cruel conditions. Being part of Reactor Pressure Vessel(RPV), thewelds, enduring high temperature, high pressure and high intensity irradiation which make thewelds embrittlement, show a significant decrease of the fracture toughness and a increase oftemperature in the ductile-brittle transition region. Deposited metal is required to havesufficient plasticity and toughness in order to guarantee the working safety of RPV throughthe life span.The strengthening and toughening mechanism of A508-Ⅲ steel had been lucubratedwhile the deposited metal’s was less reported. The submerged arc welding deposited metal ofA508-Ⅲ steel was studied in this paper. The relationship among Post Weld Heat Treatment(PWHT), chemical compositions, metallography, microstructures and mechanical propertieswas investigated by means of impact test machine, tensile test machine, OM, SEM and TEM.The crack growth behavior of deposited metal was discussed.620℃×24h wasan optimum PWHT of deposited metal, the tensile strength and theyield strength was605MPa and530MPa respectively, and the upper platform impact energywas234J, ductile-brittle transition temperature was-55℃. The metallography of depositedmetal was the combination of small amount of proeutectoid ferrite+acicular ferrite+smallamount of bainite.Deoxidation products Al2SiO5and Mn2TiO4were observed in the as-weldeddeposited metal, M3C precipitates and needle shape Mo2C precipitates were observed in thedeposited metal after24h and40h PWHT respectively, the needle shape Mo2C precipitatesresulted secondary hardening. Superabundant precipitates would cause the increase of tensileproperty and the decrease of the fracture toughness.The morphology of fracture surface after Charpy impact test at room temperaturewas dimples, the fracture type was ductile fracture. The splitting of ductile creaks was mainlyinfluenced by inclusions. The fracture toughness of deposited metal at room temperaturewould improve by the method of controlling the diameter, reducing the amount of inclusionsand making them distribute uniformly in matrix. The fracture surface morphology at-70℃was dimples and quasi-cleavage, the fracture type was ductile fracture and brittle fracture. Thesplitting of brittle creaks was mainly influenced by acicular ferrite grain size. Refining theferrite grains would improve fracture toughness of deposited metal at-70℃.Tensile strength and yield strength of deposited metal with different alloying elements content combinations were increased with the value of carbon equivalent. Excessivecarbon equivalent led to the decrease of impact toughness. This relationship could be used asthe basis for the design of the composition and mechanical property.