Research On Microstructure Evolution Of 25Cr2Ni4MoV Steel During Forging And Heat Treatment Process For A Large Nuclear-power Rotor

The large nuclear power rotor is one of the key components of nuclear power equipment.The rotor with the weight of 350 tons is produced by integral forging from large cast ingot about 650 tons.It has high requirements for chemical composition,mechanical properties and defects detection,which belongs to the field of extreme manufacturing process.The manufacturing of rotors has the problem of unstable quality,long production cycle and high cost.It is necessary to use numerical simulation to predict the shape and microstructure evolution during forging and heat treatment.Therefore,the hot deformation behavior of 25Cr2Ni4MoV steel and microstructure evolution during hot manufacturing are researched in this paper.The constitutive equation,hot processing map and models for microstructure evolution are established.The integral simulation software for forging and heat treatment is developed,and it is applied to predict the microstructure evolution for forging and heat treatment of large nuclear power rotor.The research result can guide the practical production of large nuclear power rotor.The main research content is as follows:Based on hot compression tests,the true stress strain curves for 25Cr2Ni4MoV steel were obtained.The deformation activation energy was calculated about 356.79 KJ/mol.The strain compensated Arrhenius model was used to fit the constitutive equation.The hot processing map indicated that when strain was lower than 0.5,strain,deformation temperature and strain rate influence the power dissipation map and the instability map significantly.When strain is higher than 0.5,the influence of strain on the processing map is small.The optimum hot deformation condition for the studied material is 1105-1150?/0.0067-0.01 s-1.Microstructure evolution characterizations,including incomplete DRX,mixed grain,fine grains and coarse grains were studied by combined analysis of hot processing map and microstructure.The instability region caused by the incomplete DRX with uneven microstructure distribution occurs at high strain rates.Fine grains area with the highest efficiency of power dissipation locates between mixed grains and coarse grains area.When the temperature is higher than 1200?,mixed and coarse grains are observed.The static grain growth of 25Cr2Ni4MoV steel for large nuclear power rotor was investigated.The grain was coarse at 1250? for holding long time.Therefore,it should be avoided that the rotor was kept without deformation for long time at high temperature.The dynamic recrystallization model and static recrystallization model were established respectively.When the initial grain size is small,the reconstituted microstructure by DRX shows no refinement.Static grain growth occurs when static recrystallization is complete.The speed of grain growth after static recrystallization is much larger than that of grain growth during holding at constant temperature.Therefore,it is suggested that the rotor should not be kept for long time at high temperature after deformation.Heating speed,normalizing temperature and holding time have influence on the grain size during cyclic normalizing.The optimum temperature for the normalizing of the first cycle is 930?.The normalizing temperature can be lower with the number of cycle increasing,which can also have a good effect on grain refinement.The grain size model considering the influence of normalizing cycles was established.The transformation condition for different microstructure evolution statuses was determined.The models for static grain growth,dynamic recrystallization,static recrystallization and grain growth after static recrystallization were developed into the FE simulation software,then the simulation software for microstructure evolution during hot forging was completed.The microstructure evolution software was used to simulate the influence of anvil width,reduction and deformation temperature on the rotor deformation.The microstructure evolution was studied during continuous loading and discontinuous loading.Compared with continuous loading,discontinuous loading results in lower dynamic recrystallization volume fraction and more region with incomplete DRX,which means continuous loading is recommended for microstructure refinement.The integral simulation software of microstructure evolution during forging and heat treatment is built.The microstructure evolution is complicated during the last pass and retention period at room temperature.The microstructure simulation results of the forging process were imported into the heat treatment,the grain size variation during five cyclic normalizing was simulated,and the phase content was predicted after quenching of large nuclear power rotor.The research results provide important references for process optimization and mechanical property prediction for heavy forgings.