等离子氮化保温时间对S45C钢疲劳性能的影响

Time Duration Effect of Plasma Nitriding on Fatigue Behavior of S45C Steel

温度500℃,气压400 Pa,N2与H2比例为40%∶60%的混合气体中,采用3组不同的处理时间(8、20、48 h)分别对S45C钢试件表面进行等离子氮化,全面探讨不同保温时间下氮化层及渗透层的微观组织、硬度、氮化层组成、残余应力,分析这些参数对S45C疲劳性能和断裂特征的影响。结果表明,3组工艺分别在S45C钢表面获得6、8、12μm深的氮化层。表面硬度由初始230 HV均提高到620 HV以上。表面残余压应力均提高到约-120 MPa。氮化物为单一γ-Fe4N相。渗氮层深度随保温时间增长而大幅增加,氮元素富集及α-Fe向γ-Fe4N相转变诱导的残余压应力在渗氮层内的扩展,使得等离子氮化保温48 h后S45C钢的疲劳强度提升为未处理试样的约3倍。根据村上公式计算"鱼眼"裂纹应力强度因子,可得其近似为3.6 MPa·m1/2。内部裂纹主要由渗透层内夹杂处萌生,夹杂成分为氧化钙、氧化铝和硫锰化合物等。

A group of S45C medium carbon steel specimen was nitrided by plasma nitriding using a mixture of 40% N2 -60% H2. The processes were carried out at 500 ℃ for three different time durations of 8,20 and 48 hours. The effect of case depth on fatigue perfohn- ance of S45C was investigated. The thickness of compound layer for these three time durations were 6 μm,8 μm, 12 μm,respectively. Plasma nitriding treatment enhanced the surface hardness and the surface compressive residual stress. The compound layer was com- posed of the γ - Fe4N phase. Rotating bending fatigue test s.howed that plasma nitriding treatment increased the fatigue strength of S45C by 190% with respect to the untreated specimen because of the increase of the case depth and the expansion of compressive residual stress in the case depth. The stress intensity factor calculated from the facet area was independent of the number of cycles to failure and was found as at 3.6 MPa m1/2. These fish-eye cracks were originated from inclusions in the case depth, and were calcium oxide, alumi- num oxide and manganese sulfide.