Metallurgical Examination of a Failed Spiral Bevel Gear

摘要

A failed spiral bevel transmission gear from an Army cargo helicopter was firstsent to the primary contractor for analysis and then the Army Research Laboratory. The failure occurred during a training flight at Ft. Meade, Maryland, where a reported 'loud bang and shutter' forced an immediate landing. Subsequent inspection of the Number 2 engine revealed a 15-20 degree section of the gear had fractured and penetrated the transmission housing. The gear was fabricated from X-2M steel, a high hot hardness alloy. Light optical microscopy of the failed gear section revealed characteristics consistent with a fatigue failure, including a relatively flat fracture surface and beach marks. The fracture origin was located at a defect characterized by a darkened half-moon shaped region. The darkened topography was considered evidence that a pre-existing crack was present during the manufacturing process of the gear. Metallography confirmed that a pre-existing crack was a grinding bum, since evidence of rehardening and retempering were observed. Also a possible contributory factor to crack propagation was the presence of a carbide network in the carburized case of the X-2M steel part. Metallographic examination of the damping ring groove area (fracture origin location) showed a deeper than acceptable carburized case. It was later learned that this region was not masked, and was subject to a double carburization by mistake. Further visual examination of the failed component revealed small grinding cracks at the comer of the tapered surface.