Mn18cr18n Retaining Ring Steel Multi-fire Hot Deformation Microstructure Evolution Study

Mn18Cr18N steel is the only material used in large generator retaining ring of more than 300MW. As China's economic development, retaining ring is a basis for power, the market demand and internal quality requirement of which have become more sophisticated. The hot forming process of retaining ring in China is multi-heating and multi-deformation process, which is less controllable, lead to serious forging surface cracks and large grain size .So the hot forging technology is seriously not meet the current requirements of the development.However, the previous study of domestic models is too single. It's specific for the recrystallization behavior and plastic materials of steel retaining ring, which is limited to the study of single-fire. As a result, a comprehensive analysis of microstructure evolution during multi-fire forging of Mn18Cr18N steel retaining ring is of great significance. This paper has established the evolution model by thermal simulation experiments. The recrystallization model and the grain growth model are implemented into the finite element code Deform-3D in order to investigate the microstructure evolution during multi-fire forging of Mn18Cr18N Retaining Ring Steel. The numerical simulation and the verification experiment in the same conditions are presented in this study. Major research findings and conclusions are as follows:(1) The studying of grain growth in different reheating conditions is in the box-type furnace. From the results we can control the reheating conditions to determine the best re-heating temperature. Also the grain growth models are established.(2) The single-pass and double-pass experiment are done in Gleeble-1500D thermal simulation machine to study the impact of flow stress. The hyperbolic constitutive model and recrystallization model are established. The construction of iso-efficiency map is based on the hyperbolic constitutive model and dynamic material model. Combination of iso-efficiency map and microstructure analysis, the result shows the microstructure is small and uniform as a result of a fully dynamic recrystallization when theηJ is greater than 0.2.(3) The one-fire, two-fire and three-fire upsetting test are carried out respectively to study the impact of coupled with heat times and repeated deformation on microstructure at 1050℃of small sample .The deformation of each fire is 20%. The results show that: With the fire increase, the deformation cumulative increases, the region with large grain size reduces. The grain size is majority of 60μm of second fire deformation after solution treatment. The grain size of difficult region of third fire deformation refined to 90μm.(4) The comparison results show that the results of experiment are in good agreement with the simulation results. It validates that the micro-simulation analysis program can be used for prediction of microstructure evolution in multi-fire forging and provides a reference to optimize forging processes in retaining ring production.