18CrNiMo7-6渗碳钢相变塑性系数对残余应力的影响

Effect of Transformation Plasticity Coefficient on Residual Stress of 18CrNiMo7-6 Carburizing Steel

目的研究碳含量对相变塑性系数的影响,并精确仿真18CrNiMo7-6钢渗碳淬火后的残余应力分布。方法制备四种不同碳含量(0.21%、0.49%、0.65%、0.87%)的全渗透试样,分别进行不同应力水平下的膨胀试验,测得不同碳含量试样的相变塑性系数K,并输入DEFORM-HT中进行残余应力仿真,同时对f16.72 mm的圆柱试样进行渗碳淬火试验,以验证残余应力模拟的准确性。结果前三种碳含量下马氏体相变动力学参数α变化不大。基于此,提供了一种便于计算高碳含量试样不完整膨胀曲线的K值的方法,计算四种不同碳含量试样的相变塑性系数分别为7.16×10–5、5.45×10–5、5.53×10–5、6.01×10–5MPa–1。分别采用不考虑相变塑性即K=0MPa–1、取为基体值即K=7.16×10–5MPa–1和试验测得与碳含量相关的相变塑性系数进行残余应力仿真,其中采用试验测得相变塑性系数仿真结果与实测的残余应力吻合度最高,残余压应力从表面到心部呈现先增大后减小的趋势。结论经渗碳淬火后的试样表层存在碳梯度,不同碳含量下影响相变塑性系数数值的主导机制不同,导致相变塑性系数随着碳含量的增加而先降低后升高,且其对18CrNiMo7-6钢渗碳淬火后残余应力分布的影响显著。

The work aims to investigate the effect of carbon contents on the transformation plasticity coefficient and accurately simulate the residual stress distribution of 18 CrNiMo7-6 steel after carburizing and quenching process.The samples with different carbon contents(0.21%,0.49%,0.65%,0.87%)were carburized thoroughly and the expansion tests under different stress levels were performed to determine the transformation plasticity coefficient K of the samples with different carbon contents.The K values were put into DEFORM-HT for residual stress simulation.Meanwhile,a carburizing and quenching test was performed on a f16.72 mm cylindrical sample to verify the accuracy of simulating residual stress.The value of the martensitic transformation kinetic parameters a did not change obviously with the carbon content,and a method was provided for calculating the K value of the incomplete expansion curve of the high carbon content sample based on a.The transformation plasticity coefficients of the samples with different carbon contents were 7.16×10–5 MPa–1,5.45×10–5 MPa–1,5.53×10–5 MPa–1 and 6.01×10–5 MPa–1,respectively.Residual stress simulations were carried out with K=0 MPa–1,K=7.16×10–5 MPa–1,and the K values related to the carbon content measured by experiments.The simulation results of the transformation plasticity coefficients measured by experiments were consistent with the measured residual stresses,and the residual compressive stress increased from the surface to the core firstly and then decreased.There is a carbon gradient in the surface layer of carburized and quenched samples,and the leading mechanisms affecting the transformation plasticity coefficient are different under different carbon contents,which causes the transformation plasticity coefficient to firstly decrease and then increase with the increase of carbon content,and has a significant influence on the residual stress distribution of 18 CrNiMo7-6 steel after carburizing and quenching.