Bending Fatigue Crack Characterization and Fracture Toughness of Gas Carburized SAE 4320 Steel

Crack initiation and propagation in an SAE 4320 steel gas carburized to a 1.0 mm case depth was examined in specimens subjected to bending fatigue. Cellulose acetate replicas of incrementally loaded specimens showed that small, intergranular cracks were initiated during static loading to stress levels just above the endurance limit. The intergranular cracks arrest and serve as initiation sites for semi-elliptical, transgranular fatigue crack propagation. The maximum depth of stable crack propagation was between 0.17 and 0.23 mm, a depth which corresponds to the maximum hardness of the carburized case. Three equations which provide approximations to the stress distribution in the fatigue specimens were used to calculate K_IC for the carburized case with values of maximum applied stress and measured stable crack geometry. Although all three equations yielded values in good agreement with published values of K_IC for high-carbon, hardened steels, the equation for a semi-elliptical, surface crack under a bending stress state was judged superior. Variations in K_IC are discussed relative to crack initiation geometry, strain-induced austenite transformation, and residual stresses.