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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。
In this paper, Ⅴ-notch four-point bending fatigue specimens were used to systematically study the fatigue crack growth and closure behavior of carburized layers of different heat treatment after micro-nitrogen carburizing 20CrNiMo steel. The results show that the fatigue crack growth rate in carburized hardened layer has Valley changes, the curve was inverted “S ” type. The post-seepage heat treatment norms and fatigue mechanics conditions do not change the above trend, and the propagation of fatigue crack along the diffusion zone is affected by the residual austenite transformation, the initial residual stress and the distribution of the diffusion layer. Retained austenite induced closure significantly reduces the crack propagation rate in the high-carbon region. The initial residual compressive stress promotes crack closure both in terms of reducing the hindered support effect of the “plastic hinge” and increasing the residual displacement of the crack surface. The distribution of the infiltrated layer highlights the effect on the fracture mechanism. The change of the fracture mechanism is the main reason for the formation of the valley of the crack propagation rate curve. The sharp increase of the initial residual compressive stress in this interval also plays an active role. Due to the combined effect of fracture mechanism and closure effect, the da / dN-ΔK ■ curve of the infiltrated layer belongs to three parallel lines in different regions. The corresponding fracture mechanisms are quasi-cleavage, cyclic cleavage and cyclic cleavage + toughness nest.