Conditions prevailing in the carburising process and their effect on the fatigue properties of carburised gears

Gears used in power transmission in mining equipment are subject to severe operating conditions. The performance limits of the gears are influenced by material selection, choice of forming processes, heat treatment, surface treatment and surface finish. Carburised steels are used widely for gears which undergo cyclic loading (fatigue). In this paper, three steels, viz., EN39B steel, steel X and steel Y, were treated differently (heat and surface treatments) and evaluated in a single-tooth bending test rig. Fractographic observation, using optical and scanning electron microscopes, showed that inclusion-initiated fatigue cracks initially grew in transgranular mode, reaching a critical size before rapid fracture occurred through the carburised case in a brittle manner, resulting in mixed intergranular/transgranular cracking. In EN39B and steel X, fatigue cracks were observed to have initiated from sub-surface MnS inclusions whereas cracks in steel Y were sub-surface initiated. Subsequent failure of the core material occurred in a ductile mode. An important outcome of this study is that a cheaper steel (steel Y) with a combination of treatments (carburising and glass-bead peening) has out-performed the other two steels. The results are presented and discussed.