The Effect of Retained Austenite on the Impact Fracture of High-Carbon 4300 Series Steel

摘要

Instrumented Charpy impact testing was used to evaluate the effects of retained austenite on the dynamic fracture behavior of heat treated SAE-4320H ingot cast steel carburized and homogenized to a uniform carbon content of 0.76 weight percent. Variations in retained austenite content were achieved by altering the final quench temperature. Total energy absorbed during fracture indicated the existence of a critical minimum of retained austenite for the improvement of impact fracture toughness. Scanning electron microscopy performed on impact fracture surfaces indicated an intergranular fracture mode associated with samples containing amounts of retained austenite below the critical minimum, while samples above the minimum showed a quasi-cleavage fracture mode. The intergranular fracture mode is attributed to microcracks formed along the prior austenite grain boundaries from impinging martensite plates formed during heat treatment.