Fatigue Properties of Nitrided Ultrafine Ferrite-Cementite Steels under Rotating Bending Fatigue Testing

Fatigue tests under rotating bending were conducted on nitrided ultrafine ferrite-cementite steels. The ultrafine ferrite-cementite steels included carbon-increased and phosphorus-added versions to investigate the effects of grain growth suppression during nitriding. In the carbon-increased versions, grain growth was successfully suppressed both in the nitrided layer and in the core region. On the other hand, the low-carbon versions showed grain growth in the core region, even in the phosphorus-added types, although grain growth was suppressed in the nitrided layer. In the fatigue tests, many of the nitrided specimens revealed fish-eye fractures originating from inclusions located in or beneath the nitrided layer. In spite of the occurrence of fish-eye fractures, the fatigue strength of the carbon-increased versions was markedly improved due to nitriding, whereas it was a little improved in the low-carbon versions. The fatigue strength of the nitrided specimens was closely related to hardness at the fracture origin, even when fish-eye fractures occurred. This was why nitriding markedly improved the fatigue strength of the carbon-increased versions in which grain growth was successfully suppressed and high hardness was maintained beneath the nitrided layer.