Effect of surface layer modification and stress concentration on fatigue dispersion of 18CrNiMo7-6 steel

Fatigue test data of metal materials often have large dispersion, which brings uncertainty to the prediction of fatigue life of materials. Various factors contribute to the dispersion of fatigue data, in order to find the influence of surface layer modification and stress concentration on the dispersion of fatigue data, the dispersion of fatigue data of smooth specimens (K_t=1, K_t is stress concentration factor) in three states (quenching tempering, carburizing surface layer modification, mixed jet surface layer modification), carburized specimens and two stress concentration factors (K_t=2, 4) are studied. The dispersity of fatigue life of 18CrNiMo7-6 alloy steel is quantitatively described by means of dispersity index including logarithmic standard deviation and variation coefficient. The fatigue dispersion at different stress levels under quenching tempering, carburizing and mixed jet conditions is analyzed with fatigue test data. Fracture failure analysis is performed on specimens at different stress levels and the correlation between fatigue data dispersion and crack source is discussed. The results show that the fatigue life dispersion of quenched and tempered samples is the highest, followed by carburized samples and the minimum of mixed jet samples. This indicates that surface layer modification process can effectively reduce fatigue life dispersion and improve service reliability of components.The fatigue life dispersion of carburized sample is the highest when K_t is 4, followed by K_t equals 2 and K_t equals 1 is the lowest. This indicates that with the increase of stress concentration, the fatigue dispersion increases, and also confirms the general law that stress concentration is the main source of fatigue failure. Cracks in samples with short fatigue life mainly originate from the surface, while cracks in samples with long fatigue life originate from inclusions in the sub-surface of the sample.