Observation of Fatigue Fracture Surface of Carburized Steel

Abstract

Rotating bending fatigue tests were conducted on the gas carburized specimens with internal oxides and non-martensitic microstructure beneath the surface. The fracture surface and the axial section at the fatigue fracture origin were observed using a scanning electron microscope. The following results were obtained.
(1) The S-N diagram of gas carburized steel with internal oxides and non-martensitic microstructure revealed two knees on its curve. The test results appeared to be divided into two groups: a group with the number of cycles to failure N_f under 10⁵, and that with N_f over 10⁶.
(2) The fractographic feature of the group with N_f<10⁵ was intergranular fracture from the surface to the depth of 25μ, followed by transgranular fracture reflecting martensitic microstructure.
(3) For the group with N_f>10⁶, fracture surface inclined about 45° to the specimen axis was observed. This surface was smoothened by rubbing each other with another surface during the fatigue test. The depth of the inclined smooth fracture surface reached to the depth of 100μ from the surface. It was concluded that the inclined smooth fracture surface consisted of the Stage 1 cracks and Stage 2a cracks.