变形对18CrNiMo7-6渗碳钢显微组织与残余应力的影响

Effect of deformation on microstructure and residual stress of 18CrNiMo7-6 carburizing steel

为了掌握18CrNiMo7-6渗碳钢受到载荷作用时的变形行为,提高材料性能,确保关键构件的可靠性与服役安全,开展了轴向压缩与拉伸变形试验,对变形试样晶体取向与微观界面的变化进行分析,并测量试样表层残余应力,研究变形过程中微观结构变化对残余应力的影响机制,了解18CrNiMo7-6渗碳钢变形后的显微组织、残余应力大小与分布情况。试验结果表明,在变形过程中,随着外加载荷的增加,试样渗碳层区产生位错运动,导致核平均取向差与位错密度增加,晶粒细化,小角度晶界所占比例增加;马氏体在滑移系{111}〈110〉的取向主要为软取向,容易产生滑移。18CrNiMo7-6渗碳钢在淬火过程中,由于奥氏体向马氏体转变使渗碳层区产生不均匀的残余压应力场。残余压应力沿深度方向的整体分布趋势是先逐渐增加后逐渐降低,在距表面深度约900μm处达到最大值(-224.65 MPa)。轴向变形未对残余应力沿深度方向的整体分布趋势产生影响。当试样变形量为-0.25%、-0.50%和-0.75%时,表面轴向残余应力分别松弛了3.3%、4.9%和18.9%;变形量为0.25%、0.50%和0.75%时,表面轴向残余应力分别松弛了1.3%、14%和27%。研究结果表明,当施加应力与残余应力的矢量和超过材料表面的局部弹性极限时,会产生局部塑性变形,引起位错密度变化,进而导致残余应力松弛。塑性变形量越大,残余应力松弛越明显。

In order to grasp the deformation behavior of 18CrNiMo7-6 carburizing steel under load,improve material performance,and ensure the reliability and service safety of key components.Axial compression and tensile deformation tests are carried out,the crystal orientation and microscopic interface changes of the deformed sample are analyzed,and the surface residual stress of sample is measured to study the influence mechanism of the microstructure change on the residual stress during the deformation process,and to understand the microstructure,residual stress value and distribution of 18CrNiMo7-6 carburizing steel after deformation.The test results show that during the deformation process,with the increase of applied load,dislocation movement occurs in the carburized layer region of sample,resulting in an increase in the Kernel Average Misorientation and dislocation density,grain refinement,and the proportion of low-angle grain boundaries;the orientation of martensite in the slip system{111}〈110〉is mainly soft orientation,which is prone to slip.During the quenching process of 18CrNiMo7-6 carburizing steel,due to the transformation from austenite to martensite,an inhomogeneous residual compressive stress field is generated in the carburized layer area.The overall distribution trend of residual compressive stress along the depth direction is gradually increasing first,then gradually decreasing,reaching the maximum value(-224.65 MPa)at a depth of about 900μm from the surface.The axial deformation has no effect on the overall distribution trend of residual stress along the depth direction.When the sample deformation is-0.25%,-0.50%and-0.75%,the surface axial residual stress relaxes by 3.3%,4.9%and 18.9%respectively.When the deformation is 0.25%,0.50%and 0.75%,the surface axial residual stress relaxes by 1.3%,14%and 27%,respectively.The research results show that when the vector sum of applied stress and residual stress exceeds the local elastic limit of material surface,local plastic deformation will occur,which will cause the change of dislocation density,and then lead to the relaxation of residual stress.The larger the amount of plastic deformation,the more obvious the residual stress relaxation.