Research On Accelerating Phase Transformation Of Nano Bainitic Steel With Impacting Rotation Compression After Welding

As a new type of steel, nano bainitic steel not only has high strenghth, but also hasconsiderable elongation, which is ideal material to lighten the weight and improve thesafety of product. At present researches are mainly focused on the optimal design of thematerial but not the weldability. Belonging to high carbon steels, nano bainitic steel isvery easy to turn into hardened martensite after welding. So other researchers putforward the renano bainitic method. But the phase transformation time is too long. Andthis paper is to reduce the phase transformation time.Firstly a research was taken on manufacturing process and mechanical properties ofnano bainitic steel. Nano bainitic steel was transformed by the high carbon steel withheat treatment. At the same heat preservation temperature, different preservation timewould have an influence on the amount of bainitic transformation, microstructure andmechanical properties. Eventually the preparation technique of nano bainitic steel wasgot, which was that high carbon steel was heated up to1200℃to be completelyaustenitized, then it would be cooled down to250℃fast and kept at that temperaturefor5hours. With that preparation technique we got the nano bainitic steel which had thebest mechanical properties and the bainitic transformation was basically completed. Andalso the weldability of nano bainitic steel was studied. Hardened martensite and crackwould not occur in weld with250℃preservation for5hours when nano bainitic steelcooled to250℃after welding. Weld area completed the nano bainitic transformationbasically.Secondly a thermal simulation experiment of nano bainitic steel was carried out toobtain a series of parameters in the process of dynamic recrystallization at different hightemperature. The value of dynamic recrystallization activation energy Qd, expressions ofZ parameter, dynamic recrystallization critical straincand dynamic recrystallizationvolume fraction Xdof the nano bainitic steel were got after the data processing. Thermalsimulation experiment and data analysis would provide material parameters and thedynamic recrystallization refinement model for the following finite element simulationof refining the grains of welding area with the impacting rotation compression.And then in this paper the welding with impacting rotation compression method wasintroduced and adopted to increase plastic deformation and refine the grains to accelerate the bainitic phase transformation of the weld zone. In order to increase theplastic deformation to make the dynamic recrystallization occur to refine the grains ofweld zone with impacting rotation compression, the end face form of impacting rod wasredesigned. With the end face designing of incremental uneven shape, the shear stresson the rotary direction would increase to make plastic deformation occur more easily inweld zone with impacting rotation compression. Subsequently, experiments werecarried out to verify the increment of plastic deformation and refinement of grains ofweld area with impacting rotation compression. By verification experiment weconfirmed that the method of welding with impacting rotation compression couldincrease plastic deformation and refine the grains.Finally finite element simulation of refining weld grains by using the subroutine ofnano bainitic steel with welding with impacting rotation compression was made. Underthe condition of the same process by adjusting the impacting position we changed thetemperature of the impacting weld metal to get the best grain refining effect andimpacting position.The welding with impacting rotation compression experiment was designed andcarried out according to the simulation results of the best impacting position. After theexperiment and the same time of heat preservation we analyzed the weld and the weldwith welding with impacting rotation compression and contrasted their amounts ofbainitic transformation, microstructure morphology and mechanical properties. By theanalysis we found the phenomenon that the speed of bainitic transformation wasobviously accelerated, the arrange of bainitic slices was more homogeneous, length ofbainitic slices was shorter, illustrating that grains were refined after welding withimpacting rotation compression and confirming the acceleration of nano bainitic steelwith welding with impacting rotation compression is feasible and the accuracy of thesimulation calculation results.