Failure analysis of AMAGB lube pump gears

Lube pump gears of an Aircraft Mounted Accessory Gearbox (AMAGB) were found to have cracks on the face and gear root areas after completion of 290 h of flight trials and 1000 h of endurance test respectively. After a preliminary investigation, the subject gears were forwarded to this laboratory for analyzing the cause(s) of failure. The gears were provided in cut-pieces. Dye penetrant inspection (DPI) carried out showed presence of cracks in gears AGB 02 1008 and AGB 02 1009. The DPI signatures were very feeble indicating that the cracks were very tight. Subsequent examination of the gears in scanning electron microscope revealed presence of one crack in gear AGB 02 1008 and six cracks in gear AGB 02 1009. However, no cracks could be identified on the available portion of gear AGB 02 1013. A few representative cracks observed in gear AGB 02 1009 were pulled open for further study. All these cracks were found to be very shallow and they were found to be confined only to the carburized layer. Macroscopic examination of the crack surface gave no evidence of corrosion, fatigue marks or rapid ductile failure. Based on these, the crack generation either by fatigue mechanism or stress corrosion cracking (SCC) was ruled out. Electron fractographic study, however, showed typical brittle, intergranular mode of crack propagation, suggestive of hydrogen embrittlement (HE) failure. Investigation revealed that the gears were given phosphating treatment. In view of the hydrogen embrittlement as the cause of crack generation, the question arises whether or not the phosphating treatment had been performed as per standard practice. It is reported that the phosphating treatment involved electrolytic processes. Therefore, the possibility of hydrogen embrittlement is inherent in this case. In the event of not performing baking operation after phosphating treatment, failure by hydrogen embrittlement is inevitable. Metallurgical examination showed that neither the chemical composition of the steel nor the hardness profile of the gears conformed to the specification. The gear material was found to contain high amount of MnS inclusions. The effective case depth and the core hardness were found to be more than the specifications in all gears. In addition, the surface of the gears was found to have asperities resulting during the phosphating treatment. In summary, the gears in question have failed by HE. The steel picked up hydrogen during phosphating treatment. The susceptibility to HE was further increased due to high hardness in the material, stress concentrations in the hard carburized layer, and more importantly, presence of MnS in the microstructure