Influences of vacuum ion-nitriding on bending fatigue behaviors of 42CrMo4 steel: Experiment verification, numerical analysis and statistical approach

•The variations of fatigue limit with stress ratio are found.•ANOVA model of loading levels are established.•Resdistributive Model of Residual Stress are established.•The relationships of fatigue life and residual stress redistributive behaviors are found. In this paper, the bending fatigue behaviors of 42CrMo4 nitrided steel and its residual compressive stress redistributive behaviors with different loading levels were investigated. Micro-structure analysis, hardness test and X-ray diffraction test were carried out to evaluate the properties of nitriding layer. By means of the staircase method, the bending fatigue limits of different stress ratios (R = 0.1, 0.3, 0.5, 0.7) were obtained and the polynomial formulas of fatigue limits were fitted based on the distributive characteristics of mean stress and stress amplitude. After the nitriding treatment, the fatigue limits and lives of samples can increase about 8% and 30%-50%, respectively. The redistributive behavior of surface compressive stress during the fatigue process is similar to the distribution characteristics of logarithmic function and has positive correlation with the loading level and the initial residual stress. The analysis of variance (ANOVA) result showed that both of stress amplitude and mean stress have profound influences on fatigue life and their weight ratio is about 4:1.