| Large forgings are becoming more and more important in the life of Chinese residents.The performance of large forgings not only affects the overall operation of large equipment,but also its production capacity is a concentrated reflection of the comprehensive level of heavy machinery industry in a country.Due to the large size,in the process of plastic forming of large forgings,the distribution of temperature and stress inside and outside the workpiece is not uniform,so that the deformation degree of different positions of the workpiece is different,which ultimately affects the product performance of large forgings.Therefore,it is necessary to study the change rule of internal microstructure and influencing factors of large forgings in the hot forging process,so as to formulate a reasonable production process and obtain the workpiece in line with the actual production requirements.In the article,30Cr2Ni4Mo V low pressure rotor steel as the research material,starting from the original structure of the ingot as cast,using Gleeble-1500D thermal simulation testing machine,under the experimental conditions of 900?1200°C,0.001?1s-1,The dynamic recrystallization behavior of the steel under high temperature deformation was planned.It was found that the strain rate,decrease and temperature all had outcomes on the emergence of dynamic recrystallization.Adding the temperature and decrease were helpful to the course of dynamic recrystallization,and increasing the strain rate in a certain range was beneficial to the grain refinement.The elementary matrix,summit pressure matrix,critical stress model,dynamic recrystallization model and grain size model of the material were constructed by analyzing the data obtained from the thermal simulation test.Based on the DMM theory and the rheological failure criterion established by Prasad and Kumar,the thermal working diagrams of30Cr2Ni4Mo V low-pressure rotor steel in as-cast state with strain variables of0.2,0.4 and 0.6 were established under the conditions of strain rate of0.001?1s-1 and temperature of 900?1200?,and the following rules were obtained:With the increase of strain,the unstable region in the hot working diagram of the steel is gradually concentrated in the region of high temperature and high strain rate.The main instability zone is between 1050?and 1200?,0.1?1s-1.The steel is suitable for hot processing at 1100?1200?with deformation rate of 0.01?0.1s-1.The WHF drawing test was simulated by DEFORM-3D software based on the as-cast 30Cr2Ni4Mo V low-pressure rotor steel.The variation of the material's internal field,recrystallization volume fraction and grain size during the test process was studied.It is inferred that the temperature domain allocation of the forgings and the heterogeneous distortion of the forgings are the core grounds for the heterogeneous microstructure allocation.When the forging ratio increases from 1.5 to 2,the equivalent strain of the center section of the workpiece increases gradually,but the deformation inhomogeneity also increases.When the forging ratio is different,the temperature domain in the focus region of the working parts decreases step by step from the core to the outside,and the temperature diversity decreases step by step with the growth of the forged piece scale.With the growth of the forging scale,the dynamic recrystallization score increases,and the inhomogeneity of internal microstructure is improved to a certain extentThrough the drawing experiment,the microstructure distribution analysis of different forging ratios and different sampling positions shows that the average grain size of forgings decreases with the increase of forging ratios,and the uneven grain size will be improved.In the process of changing forging ratios from 1.5 to 2,the grain size at the core position basically shows a decreasing trend. |
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