Influence of the C Content on the Fatigue Crack Initiation and Short Crack Behavior of Cu Alloyed Steels

The mechanical properties of Cu alloyed steels are influenced significantly by the Cu content and the respective state of Cu precipitations as well as the C content. In this context, the effect of an increased C content on the fatigue crack initiation and growth of differently aged Cu alloyed steels with 0.005 (X0.5CuNi2-2: X0.5) and 0.21 wt.-% C (X21CuNi2-2: X21) was investigated in this study. Notched specimens were examined via SEM in interrupted fatigue tests to detect the location of crack initiation and growth. The results showed that fatigue crack initiation and growth occurred for both steels at grain boundaries, and within ferrite grains. However, a higher C content increased the incidence of crack initiation and growth at grain boundaries. This is caused by the smaller grains of X21 and especially by the presence of cementite on the grain boundaries. This explains why, in contrast to X0.5, no influence of the Cu precipitation state on the defect-based failure was observed for X21, as the precipitates are located within the ferrite grains and, thus, only have a minor impact on the fatigue failure mechanisms of X21.