Abstract
SAE 8620 and EX 24 steels have been partial pressure carburized at 930 °C (1710 °F) and 1050 °C (1920 °F), and tested in bending fatigue in both as-quenched and reheated conditions. Endurance limits at or above 827 MPa (120 Ksi) were measured in carburized specimens not subzero cooled. No surface oxidation was noted, but a high carbon content retained at the fatigue specimen corners resulted in proeutectoid carbide formation and large amounts of retained austenite in as-carburized specimens. Scanning electron microscopy showed that fatigue cracks initiated at the high carbon corners. Reheating caused refinement of the martensitic structure, reduction in retained austenite, and spheroidization of the proeutectoid carbides, a combination of microstructural changes that resulted in improved fatigue resistance.
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