Micropitting and martensite decay in gears

Abstract

Micropitting is a type of surface contact fatigue often observed in gears and rolling element bearings operating under mixed or elastohydrodynamic lubrication (EHL). Once initiated, micropitting will lead to the catastrophic failure of the affected components which then requires unplanned industrial shutdowns to allow for their replacement. Micropitting in carburised gears has become a major concern in the wind power industry and other sectors where gears operate at relatively high loads and relatively slow speeds. It occurs most often in parallel axis gears (spur and helical) but it was also reported in other types of gears such as spiral bevel gears. An important feature that has been observed in bearings damaged by micropitting was the transformation of the initial microstructure. The change in microstructure, known as martensite decay, consists in the development of a new phase known as Dark Etching Region (DER) due to its appearance in reflected light microscopy. This microstructural change which has also been observed in gears leads to changes in the mechanical properties in the affected regions with implications on the initiation and propagation of the cracks leading to the formation of micropits. The fatigue life of gears can be extended by controlling the formation of micropits but this requires an in-depth understanding of the micropitting mechanism. The aim of this project was to identify and characterise the microstructural changes accompanying micropitting in gears and to develop a micropitting mechanism which describes the formation of the micropits. The microstructure has been investigated by electron microscopy techniques such as Scanning Electron Microscopy (SEM), Electron Backscattered Diffraction (EBSD), and Transmission Electron Microscopy (TEM). Nanoindentation was used to determine the mechanical properties of the affected regions. The proposed micropitting mechanism is based on the results from the above investigations as well as Density Functional Theory (DFT) calculations of material properties combined with Finite Element Analysis (FEA) of the contact region.