渗碳淬硬层中疲劳裂纹的扩展与闭合行为

THE FATIGUE CRACK GROWTH AND ITS CLOSURE BEHAVIORS IN CARBURIZED AND HARDENED CASES

本文采用Ⅴ型缺口四点弯曲疲劳试样,系统研究了20CrNiMo 钢微氮渗碳后不同热处理规范的渗碳层疲劳裂纹扩展与闭合行为,研究表明:渗碳淬硬层中疲劳裂纹扩展速率具有峰谷变化,曲线呈倒"S"型。渗后热处理规范和疲劳力学条件均不改变上述趋势,疲劳裂纹沿渗层的扩展行为受到残余奥氏体转变、初始残余应力和渗层组织分布的共同影响。残余奥氏体相变诱发闭合显著降低高碳区的裂纹扩展速率。初始残余压应力在降低"塑性铰"对闭合的支撑阻碍作用和增大裂纹面残留位移两方面均促进裂纹闭合。渗层组织分布的作用则突出表现在对断裂机制的影响,渗层断裂机制的变化是裂纹扩展速率曲线凹谷形成的主要原因,在此区间急剧增加的初始残余压应力也发挥了积极作用。由于断裂机制,和闭合效应的共同影响,渗层 da/dN-ΔK■曲线在不同区域归于三条平行线,其所对应的断裂机制分别是:准解理、循环解理和循环解理+韧窝。

Fatigue crack growth rates and crack closure behaviors propagating throughcarburized and hardened cases in the 20CrNiMo steel were studied.The varia-tion of the fatigue crack growth rates through the cases exhibited in the re-versed 'S' shape,and the shape is independent on the heat treatment of thecarburized steel and the experimental fatigue parameters used.The fatiguecrack growth behaviors were effected jointly by the strain-induced martensitictransformation of retained austenites,the original residual stress and the mi-crostructure distribution in the case.The retained austenite transformation in-duced crack closure decreases significantly the crack growth rates at high car-bon zones of the case.The original residual compressive stress stimulated ahigher crack closure,which is caused from the larger residual displacementon the crack face.The microstructure distribution affects the fracture mech-anism through the case.The variations of the fracture mechanism and theresidu al compressive stress through the case plays the main roles for theformation of the trough in the fatigue crack growth curves.