Rotating Bending Fatigue Properties of Case Carburized Steel with Different Fractions of Retained Austenite

Rotating bending fatigue properties of the gear steel 16CrMnH have been investigated after carburized and re-austenitized at different temperatures with 18-30% of retained austenite in the carburized case. It was found that the fatigue life tended to increase from 10(5) to 10(7) cycles with the decrease of the stress amplitude in the range of 1200-1000 MPa. The fatigue limits were higher than 1000 MPa, independent of the fraction of retained austenite. The retained austenite in the carburized case did not transform into martensite after fatigue tests, but the hardness in the carburized case was increased presumably as a result of cyclic hardening of retained austenite. The fatigue limits of the carburized specimens were in good agreement with the calculated values based on the case hardness measured after fatigue tests. Observations of the fracture surfaces of the fatigue specimens showed that the fatigue crack initiated at the non-metallic inclusions or the matrix in the carburized case with a depth of no more 0.6 mm from the surface. Thus, a good fitting of S-N curves could be obtained by taking into consideration of the case hardness after cyclic hardening as well as the non-inclusion sizes.