Brittle Cracking of a Cylindrical Spiral Gear of Axle Gearbox

A cylindrical spiral gear of locomotive axle gearbox failed after standing in site for 10 days. The failed gear had been assembled with the spiral gear-shaft. Inspection in site found that cracking occurred on the spoke plate between the shaft bore and the oil-pressed hole. The gear teeth are required to be carburized, but the surfaces of the shaft bore and the oil-pressed hole of gear are surface-coated to prevent carburizing. The crack initiated from the surface of oil-pressed hole and propagated along the radial of the oil-pressed hole. Metallurgical examination reveals that anti-carburizing measure was not conducted on the surface of oil-pressed hole as the specification. A simplified evaluation of the maximum hoop tensile on the wall of oil-pressed hole subjected to internal pressure for swelling bore is far insufficient to lead to brittle fracture. Delayed fracture of gear, the predominance of intergranular fracture micro-mechanism, and the presence of several secondary micro-cracks in the depth of case layer on the oil-pressed hole surface are a clear indication of hydrogen embrittlement. Hydrogen absorption occurred during gas carburizing process.