【作者】 刘玉； 秦盛伟； 左训伟；

【Author】 Liu Yu;Qin Shengwei;Zuo Xunwei;School of Materials Science and Engineering,Shanghai Jiao Tong University;

【机构】 上海交通大学材料科学与工程学院；

【摘要】 对于一定尺寸形状的具体钢件,通过盐水淬火降低开裂风险方法早已被热处理工作者发现,但关于盐水冷却与清水冷却淬火工件内部应力的变化过程及其比较目前尚未有报导。本文以直径16mm、长80mm的42CrMo试样为研究对象,对其在清水及10%NaCl水溶液中淬火开裂倾向进行了实验比较。结果表明,在清水淬火的4组试样全部开裂,裂纹特征为纵向裂纹;在10%盐水淬火冷却后的四组试样全未淬裂;对盐水淬火后的试样使用车床剥层至直径6mm时并未发现裂纹存在。由于目前还没有测量淬火冷却过程中过程应力的方法,即使是对淬火冷却后残余应力的测量也是一件十分困难的工作,因此,计算机模拟仿真成了解决淬火冷却过程问题的最有效的方法。本文以ABAQUS有限元软件为模拟平台,通过加入相变动力学方程子程序,建立了淬火时温度、组织及应力三场的耦合的数学模型。因应力对相变动力学的影响较为复杂,在此忽略该影响,因此,为了减少计算时间,温度场和应力场分别单独进行计算。为了描述42CrMo马氏体转变量与温度之间的S型曲线关系,对KM方程进行了修改。通过分别施加不同的换热系数,对工件在盐水和清水中淬火时温度、组织及应力进行了三场耦合对比分析。分析结果表明,盐水冷却开始时试样表面具有较高的马氏体形成速率,并且在其表面形成了较高的压应力,随着冷却的进行,马氏体转变逐步向心部扩展,导致表层压应力逐渐降低;由于清水具有较盐水低的换热系数,工件在清水中淬火时,其表层马氏体形成速率相对较慢,从而在其表面形成较低的压应力,在随后冷却时,因心部马氏体相变膨胀,最终导致表面具有较高的拉应力;两种淬火介质冷却后的试样的环向应力均高于轴向应力及径向应力,这种应力分布状态与纵向开裂实验现象相吻合;盐水淬火后的环向应力最高值处于次表层(0.25R)位置;清水淬火后的环向应力最高值处于试样最表层位置;盐水淬火后的最高值低于清水淬火的环向应力最高值。这从应力角度解释了直径16mm的42CrMo圆棒在盐水淬火开裂倾向较清水淬火低的现象。更多还原

【Abstract】 It has long been found by the heat treatment workers that the tendency of cracking can be lowered by using salt solution for a specific steel part;but the investigation and comparison of the stress evolution of a particular steel part quenched by brine and fresh water has not been reported yet.In this paper,the 42 CrMo cylinders with 16 mm in diameter and 80 mm long were used as research object;the cracking tendency experiment was made by quench those samples in 10%salt solution and fresh water.The test results showed that the 4 samples were all cracked by quenched in fresh water;and the cracks are longitudinal cracks;there was no crack exist,even stripped to 6mm diameter by using lathe for the samples that were quenched by 10%salt solution.Because there has no method exist for the measurement of stress evolution,and even for the determination of residual stress distribution of a steel part is a hard work,the computer simulation becomes the most effective method for optimization of the quenching process.In this paper,the software ABAQUS was used as the simulation tool;by incorporating the phase transformation kinetics equation,the coupled temperature,phase and mechanical model was established.The effect of stress on the phase transformation was neglected because of its complexity.For reducing of computation time,the temperature field and stress field calculation were calculated separately.The KM equation was modified for fitting the S shaped curve relation between martensite volume fraction and temperature.By applying different heat transfer coefficients(HTC),the temperature,phase,and stress evolution of the samples that were quenched by fresh water and salt solution were calculated by the established model.The computation results show that there is a high transform rate of martensite and high level of compression stress at the surface of the sample at the beginning of the quenching process;as the cooling process proceeds,the martensite transformation extends the the core of the sample,which lowered the compression stress of the sample.Because of the relative lower HTC of fresh water compared with salt solution,the transformation rate of martensite is slower,and the relative lower compression stress is formed at the beginning of the quenching process;as the cooling process proceeds,because of the expansion of martensite transformation in the core of the part,the compression stress of the surface is reversed to tensile stress at the end.The higher tangential stress than axial stress corresponds to the experiment result of longitudinal crack.The maximum tensile stress of the salt solution quenched sample is located at the subsurface(0.25R) of it;the maximum tensile stress of the fresh water quenched sample is located at the surface of it;the maximum of tensile stress of the salt solution quenched sample is lower than that of the fresh water quenched sample which explains the relative lower cracking tendency of 16 mm diameter 42 CrMo part quenched in 10%salt solution compared with fresh water.更多还原