Ratchetting-fatigue behavior of a 42CrMo steel under near-yield mean stress

42CrMo high-strength structural steel is widely used to manufacture the bolt fasteners of engineering machinery, which are subjected to cyclic loading with a near-yield pre-stress in service. In this contribution, we focused on the intrinsic correlation between macroscopic ratchetting-fatigue behavior and microscopic mechanisms in a quenching & tempering 42CrMo HSS steel under near-yield mean stress (σm) conditions. The results indicate that fatigue strength obviously decreases as σm increases from low level (51.9% of yield strength, σy) to near-yield range (80–95% of σy), which is followed by a slight decrease with σm increasing in near-yield σm range. Two distinct fracture patterns were found: (i) high-cycle fatigue failure under low σm and (ii) ratchetting-fatigue failure with plastic strain due to obvious ratchetting effect under near-yield σm. Compared to low σm, near-yield σm leads to the disappearance of fatigue striations, but emerging of some plastic micro-strains, sub-grains and high-density dislocation cells. Fatigue crack-path analysis demonstrates that the decreases in secondary crack size and crack deflection degree are favorable to fatigue crack propagation at a higher rate. Furthermore, fatigue crack tends to propagate along the high Schmid factor regions with the specific slip system of {110}<111> under near-yield σm, corresponding to quicken fatigue cracking behavior. However, it did not occur under low σm. Therefore, it can be concluded that near-yield σm and the resulting ratchetting effect play significant roles in fatigue cracking behavior of 42CrMo steel, which should be carefully taken into account for predicting fatigue lifetime of actual bolt fasteners.