Microstructural characterization of failed bevel gear of aero-engine

Abstract

The present paper reports the microstructural characterization of bevel gear of Aero-engine. The chemical compositions of the bevel gear teeth obtained by Electron Probe Micro Analyzer (EPMA) for both the normal and worn out gear teeth are approximately close to the standard composition. The cut pieces after mounting and polishing clearly display the presence of carburised layer. In addition, a white layer adjacent to the carburised layer on outer side of teeth is also observed. This indicates that the decarburisation has occurred during the heat treatment. The microstructures exhibit the presence of tempered martensite along with fine precipitates. The sizes and shape of precipitates are varying. There is lot of variations in micro-hardness values measured across the cross sections of the normal and worn out gear teeth. In addition, there is also a variation in the hardness values in the carburized layer. This can be attributed to the presence of decarburized and carburized layers. The decarburized layer in all the teeth might have formed during the heat treatment after carburizing. Some of the hardness values in the carburized layer are falling below the 700 HV. The typical values of hardness in the carburized layer should be higher than the 700 HV (≥HRC 60). This reflects that the surface of gear teeth has softened due to depletion of C in the matrix and this has resulted in worn out of the material. The presence of compressive residual stresses on the gear teeth also indicates that these are not responsible for the worn out of material. The material has worn out from the surfaces of bevel gear teeth due to formation of decarburized layer at the surfaces.