Crack Propagation Behavior in Rotational Bending Fatigue Test of Nitrocarburized JIS SCM420 Steel

The crack propagation behavior of nitrocarburized JIS SCM420 steel was investigated in a rotating bending fatigue test, focusing on crack stagnation behavior. The crack had clearly stagnated at a length of approximately 200 µm at the fatigue limit of 400 MPa, indicating that crack stagnation could control fatigue strength. The crack stagnation cannot be explained only by the change of the stress intensity factor, since the calculated value in this process increases with the depth from the notch. A large amount of plastic strain was observed around the tip of the crack by EBSD analysis. Because the stagnated position corresponds to the critical depth between the hardened and unhardened regions formed by nitrocarburizing, it can be easily deformed. Therefore, it is inferred that the crack stagnation nitrocarburized JIS SCM420 steel can be explained by a plastic-induced closure mechanism.