The effect of high temperature gas carburizing on bending fatigue strength of SAE 8620 steel

The bending fatigue performance of high temperature gas carburized SAE 8620 steel was evaluated and compared to conventional temperature gas carburization. To vary the case depth, different carburizing times were applied. The typical times were: 3 and 5 h at a temperature of 940 °C and 1 and 2 h at a temperature of 980 °C. To cause failure in the rotating bending fatigue specimens, the applied load was chosen as the equivalent load at 106 cycles when the material is subjected to bending fatigue. The characterization of residual stress and percent retained austenite in the specimens was performed using X-ray diffraction techniques; optical metallographic examination was also carried out. The fracture surfaces were cleaned in an ultrasonic bath and examined using a scanning electron microscope (SEM) equipped with EDX to evaluate the crack initiation and growth characteristics of the materials in the core and carburized case regions of the specimens. The results indicate that the fatigue performance of the high temperature gas carburized specimens was relatively poor compared to the conventionally gas carburized specimens.