Microstructure Evolution During Forging Process And Forging Process Optimization For SA508Gr.4N Steel Heavy Forgings

With the increasingly prominent energy and environmental problems,the country has formulated the strategic plan of carbon peak,and nuclear energy,a clean,efficient and stable energy,has attracted more and more attention.Tubesheet,head plate and other large cake forgings are important parts of nuclear power plants.The yield of forgings has become an important factor restricting the production cost and cycle of nuclear power.In this paper,the hot deformation behavior of SA508 Gr.4N steel for nuclear power and the large cake forgings of ?4000 mm × 1000 mm were taken as the research objects,and the microstructure evolution in the deformation process,pass interval and heating process of the material were studied,and the corresponding mathematical model was established.A forging scheme for grain refinement in the center of large forgings during the final multi-pass forging was proposed.The preforming and final forming forging processes and equipment of large cake forgings were optimized by Deform-3D software.The main research work of this paper was as follows:The effects of heating process parameters(holding temperature and time)on grain growth of SA508 Gr.4N steel were studied by isothermal heat treatment furnace and high temperature confocal microscope,and the grain growth model was established.The austenite grain growth of SA508 Gr.4N steel was dynamically observed by hightemperature high-focus microscope,which was essentially a grain boundary migration process.The effects of deformation parameters on the flow stress and microstructure evolution of SA508 Gr.4N steel were studied by single pass hot compression test.With the increase of deformation temperature and the decrease of strain rate,the flow stress,dynamic recrystallization volume fraction and grain size after complete dynamic recrystallization increased gradually.The high-temperature flow stress and dynamic recrystallization grain size model of SA508 Gr.4N steel were established?The static and metadynamic recrystallization behavior of SA508 Gr.4N steel was studied via two-pass hot compression experiment.The effects of deformation parameters(deformation temperature,strain rate,strain,initial grain size and pass interval time)on static and metadynamic recrystallization of SA508 Gr.4N steel were studied,and the static and metadynamic recrystallization kinetics and grain size models were established.When the initial grain size ? 256 ?m,the grain size did not change after metadynamic recrystallization.By controlling the final forging temperature between 1050 °C and 1150 °C,and adopting the three-step forging scheme with step increase(5 %,8 %,13 %),the grain size grade of the central part coulde be controlled less than grade 3.Based on the results of numerical simulation analysis(effcetive strain field,stress field,temperature field and grain size distribution),the preforming and final forming forging equipment and process were modified.In the preforming process,5° frustum anvil was used for upsetting,and narrow flat anvil outer edge rotary forging(rotation angle of 60°)was used for flatting end face.The final forming process,outer edge rotary forging(rotation angle 60°)was used,and the pass reduction was 100 mm.