No abstract

Metallographic examination performed on a number of micropitted gears has revealed microstructural features similar to those reported in the literature in fatigued bearings, namely, dark etching regions and white etching bands. In addition, a new type of fatigueinduced microstructural constituent was observed close to the surface below asperities referred to as the plastic deformation region. Finally, it is shown that the initiation and propagation of cracks leading to the formation of micropits are related to the phase transformations mentioned previously.

Nitriding is a heat treatment process that is commonly used to enhance the surface properties of ferrous components. Traditional quality control uses sacrificial pieces that are destructively evaluated. However, efficient production requires quality control where the case depths produced are non-destructively evaluated. In this study, four different low alloy steel materials were studied. Nitriding times for the samples were varied to produce varying case depths. Traditional Barkhausen noise and Barkhausen noise sweep measurements were carried out for non-destructive case depth evaluation. A prediction model between traditional Barkhausen noise measurements and diffusion layer hardness was identified. The diffusion layer hardness was predicted and sweep measurement data was used to predict case depths. Modelling was carried out for non-ground and ground samples with good results.

No abstract

Recent studies have shown that micropitting initiated pitting appears to be the dominant metal fatigue mode in modern bearings and gears. If the formation of micropits can be controlled, the fatigue life of the bearings and gears can be readily lengthened, so the useful life of the engine or transmission can be radically extended. The lack of in-depth understanding of micropitting initiation mechanism hinders progress to control the micropitting-initiated pitting failure mode. In this study, we explore the initial stages of micropitting in relation to material differences that impact the crack propagation process. To investigate these mechanisms, we study micropitting and subsequent pitting using pinion and wheel pairs made from two different carburized steels, SAE 8620 and SAE 4027, in a 91.5mm helical back-to-back gear test rig using a typical transmission fluid. For comparison, a similar study is being carried out with the pinion and wheel pairs made from the same SAE 8620 steel. The preliminary results show that steel material differences may change the ultimate pitting fatigue life but may not significantly influence the micropit formation process and subsequent pitting failure mode.