Study Of Grain Size Evolution Of 300M High Strength Steel Under Heating

The processing of large forgings needs heating and deforming many times.There are some areas,which haven't been deformed or experienced small deformation will be heated repeatedly,caused the serious grain growth,and finally influence the quality of forgings.This article studied the grain growth evolution of 300 M steel under heat preservation,obtained the relationship between grain sizes and holding temperature,holding time,initial grain size and initial strain,established grain growth model,and finally simulated the heating process by integrating grain growth model into the Deform software.These results provide the basis for studying the method of controlling grain size during the whole process of forging.First,the isothermal preservation experiments of 300 M steel were carried out under different holding temperature and time,to get the series of grain size.It shows that,under the same holding time,the average grain size will increase with the raise of holding temperature,and the grain growth rate also increase with the raise of holding temperature.The relationship between grain size and holding temperature is entirely in the form of exponential function.While under the same holding temperature,the average grain size increased with the prolong of holding time,but the grain growth rate decreased at the same time,and finally grows very little even become stable.This means that,grain growth is limited,and the lower the holding temperature is the earlier the grain size become stable.By a comparative heating preservation experiment under 1150?,we fund that the influence of quenching to grain size between heating preservation is too small to ignore.Based on the result,an experimental data transformation method was proposed.The number of experiments has been largely decreased by using that method,and the data of different initial grain size samples grain grow under different holding parameters was obtained.The grain growth model under isothermal insulation was established by fitting the data all above,and it has a higher precision than other models.The grain will also grow during the heating between two stages of forging.Considered the influence of stain in the pre-stage of forging to grain growth,the established grain growth has been improved.The influence of stain to grain evolution has been found through the heating preservation experiments of different initial stain samples.And the model has also been established by fitting the data.Compared to the experiment results,the model error is 2.49%.At last,the established grain growth model has been taken in the Deform software after changing its form.The grain evolution of outer tube in aircraft front landing gear during blanking has been showed by simulate the process in Deform.It is fund that grain size is nearly the same after a long holding time,but the grain in internal area grow litter if holding time decreased while the surface area still become big.What's more,the grain growth is relative to temperature,whenever heating or cooling.Then two methods of how to control the grain size has been provided based on the grain growth law,one is increase heating rate,the other is decrease holding time,and posed an improved heating curve of blanking.