Effects of Testing Temperature on the Fatigue Behavior of Carburized Steel

The effects of elevated testing temperature on the fatigue behavior of carburized steel were evaluated by testing modified Brugger bending fatigue specimens at room temperature, 90 °C and 150 °C. SAE 4023, SAE 4320, and SAE 9310 steel were studied to assess the influence of alloy content and stability of retained austenite. Fatigue samples were gas-carburized and tested in air at 30 Hz with a stress ratio of 0.1. An infrared spot lamp was used to heat samples to 90 °C (150 °F) or 150 °C (302 °F) during testing. S-N curves were developed for the room temperature baseline tests as well as elevated temperature tests. The endurance limits determined are as follows: SAE 4023-RT (1170 MPa), SAE 4023-90°C (1140 MPa), SAE 4320-RT (1210 MPa), SAE 4320-90°C (1280 MPa), SAE 9310-RT (1380 MPa), SAE 9310-90°C (1240 MPa). At 150 °C all samples exhibited permanent plastic deformation under the test conditions examined; therefore, it was not possible to obtain meaningful endurance limits at this temperature. The results showed that for all steels, the high stress/low cycle fatigue performance decreased with increased testing temperature. The endurance limit of SAE 9310 steel was most affected by the elevated testing temperature, resulting in a 140 MPa decrease in endurance limit. The endurance limits of the lower alloyed materials, SAE 4023 and 4320 steels, were essentially unaffected by the elevated testing temperature.