| The steel used in the heavy forging of reactor pressure vessel(RPV),which runs in a high-temperature,high-pressure and strong neutron-irradiation environment,should have excellent integrated mechanical properties,weldability and irradiation resistance.The SA508Gr.3 steel,extensively used as the heavy forging of RPV,have reached its ultimate strength and would not meet the requirements for large-scale and integrated forgings.In this situation,the SA508GR.4N steel,with better strength and toughness characteristics as compared to SA508Gr.3 steel,may gradually take the place of SA508Gr.3 steel for industrial application.However,only a few reports on heavy forging of SA508GR.4N could be found.In this study,isothermal hot compression tests were conducted using a Gleeble-3800 thermal mechanical simulator in the tempreture range of 800-1200?and at different strain rate of 0.001-10 s-1,for describing the hot deformation behavior of SA508Gr.4N steel during hot compression.The quenching and tempering treatments of SA508Gr.4N steel were researched using a specially designed thermal-simulation furnace which can control the cooling rate.Scanning electron microscopy(SEM,Hitach-S6100),transmission electron microscopy(TEM,Hitach-800)and chemical phase-analysis were used for examinations of the typical microstructure and precipitation characteristics after experiment processes.Microstructural evolutions and corresponding control factors,mechanical properties and the associated strengthening and toughening mechanisms of SA508Gr.4N steel were meticulously analyzed and systematically discussed.The primary results of this study are summarized as follows:The critical temperature of transformation was found to drop and the temperature range was lengthened with decreasing the heating rate.The continuous-heating-transformation(CHT)curves was established according to the thermal expansion curve.Ac1 and Ac3 of SA508Gr.4N were measured to be 710?and 790?,respectively.Microstructural evolution of SA508Gr.4N steel with cooling rate decreasing can be summarized up as following:fully martensite?mixture of martensite and bainite?fully bainite-?mixture of proeutectoid ferrite and bainite microstructures.The continuous-cooling transformation(CCT)curves of SA508Gr.4N steel was improved based on these test results.The max amount of bainite transformation at various temperatures was obtained,and the time-temperature-transformation(TTT)curves was established.Processing map for hot working of SA508Gr.4N steel was developed in the temperature range 800-1200? and strain rate range 0.001-10 s-1.The map exhibited a pronounced bimodal domain distribution with the peak efficiency of approximately 0.4 occurring at 1023-1175? and 0.001-0.005 s-1.Zener-Hollomon parameter(Z)was also measured with respect to the variations of dynamic recrystallization conditions.It is found that dynamic recrystallization was completed when the parameter InZ was less than 37.44.A prediction model was established to describe the flow stress of SA508Gr.4N steel.The flow curves calculated with the proposed model has been found to have very good agreement with the experimental data.The present steel has a high content of Al,was prone to suppress grain growth,owing to the pinning effect of AIN particles.Two technical approaches were designed and developed to avoid the structure inheritance which usually occurred in SA508Gr.4N steel heavy forgings.After isothermal annealing,intercritical normalizing and normalizing treatments,the mean grain size was decrease from ASTM Grade 2-9 to Grade 7.5-8,and the structure inheritance of SA508Gr.4N steel could be successfully eliminated.Moreover,increasing the quenching temperature had a minor effect on mechanical properties and microstructure of the quenched and tempered SA508Gr.4N steel.With increasing the tempering temperature,the lath size of ferrite increased and the amount of fairly uniform distributive fine M2C and M3C carbides decreased,which result in the reduction in strength.The preset steel exhibits outstanding combination of tensile strength and impact toughness after the optimum quenching treatment at 860?,together with the optimum tempering treatment at the temperature range from 600? to 620?.All of the mechanical properties of the present SA508Gr.4N steel in the above-mentioned optimum heat-treated conditions meet the requirements of ASME standard. |
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