Fatigue behavior of plasma radical nitrided SCM435 steel in super-long life regime

Fatigue behavior in a super-long life regime was investigated for plasma radical nitrided JIS SCM435 steels at room temperature in air. Specimens were nitrided at the 723 K and 823 K for 3 h and at 773 K for 1 h, 3 h, and 5 h. The fatigue strengths of the nitrided specimens were greater than that of the un-nitrided specimen. However, the fatigue strength and fracture mechanism of the nitrided specimens were markedly influenced by nitriding temperature and time. Both surface cracking and subsurface cracking, e.g. fish-eye fractures, were observed in the specimens subjected to the nitriding conditions. Hardened layers and compressive residual stress were formed on the specimen surfaces by nitriding, which led to an improvement of the fatigue strength and determined the initiation type of the fatigue cracking. Stress intensity factor at crack initiation was evaluated in fish-eye type fractures. As a result, the stress intensity factor range in the facet area indicated a constant value, approximately 3.5 MPa·m1/2, regardless of the fatigue life.