Comparison of the fractographic features of a carburized steel fractured under monotonic or cyclic loading

In failure analysis, in order to determine whether a component has failed under monotonic or cyclic loading, it is sometimes necessary to develop standards for each type of failure, especially when the fracture surface of a part that has failed in service contains features which may be ambiguous in their interpretation. Such is the case in the present instance, where in order to categorize the fractographic features of an AISI 9310 carburized steel, failures under monotonic and cyclic loading had to be obtained. The specimens were round bars which contained a circumferential semicircular notch, and were loaded in four-point bending. The effective case depth of the carburized layer was 1 mm, with a hardness of 60 HRC. The core hardness was 40 HRC (“RC”). Under monotonic loading, a circumferential crack developed in the case near the case-core interface, and striationlike markings were observed at low magnification on the fracture surface of the core which might be interpreted as fatigue markings. On the other hand, under cyclic loading, no circumferential cracks were observed in the case near the fracture origin, but they did develop in the region of final separation. In addition, the fracture surface in the core was markedly different in appearance from that obtained under monotonic conditions. These differences were further established by scanning electron microscopy analysis. The factors giving rise to the various fractographic features are discussed.