CONDITIONS PREVAILING IN THE CARBURISING PROCESS AND THEIR EFFECT ON THE FATIGUE PROPERTIES OF CARBURIZED GEARS

Gears used in power transmission in mining equipment are subject to se vere operating conditions. The performance limits of the gears are inf luenced by material selection, choice of forming processes, heat treat ment, surface treatment and surface finish. Carburised steels are used widely for gears which undergo cyclic loading (fatigue). In this pape r, three steels, viz., EN39B steel, steel X and steel Y, were treated differently (heat and surface treatments) and evaluated in a single-to oth bending test rig. Fractographic observation, using optical and sca nning electron microscopes, showed that inclusion-initiated fatigue cr acks initially grew in transgranular mode, reaching a critical size be fore rapid fracture occurred through the carburised case in a brittle manner, resulting in mixed intergranular/transgranular cracking. In EN 39B and steel X, fatigue cracks were observed to have initiated from s ub-surface MnS inclusions whereas cracks in steel Y were sub-surface i nitiated. Subsequent failure of the core material occurred in a ductil e mode. An important outcome of this study is that a cheaper steel (st eel Y) with a combination of treatments (carburising and glass-bead pe ening) has out-performed the other two steels. The results are present ed and discussed.