Metallurgical Analysis of Premature Failure of a Phosphor Bronze Worm Wheel

摘要

Worm drives are commonly used where large reduction in speed and greater transfer of torque are required within a small space. Phosphor bronze is normally used material for this application owing to its lower coefficient of friction and good wear and fatigue properties. This work presents a failure analysis of a worm wheel. Analysis revealed that around 40% of the teeth of worm wheel were broken. Failure mode was identified to be intergranular brittle fracture using scanning electron microscopy. A number of casting voids could be observed. In addition, network of intermetallic phases were present along the grain boundaries. These phases were identified to be Cu~(3)Sn and Ni~(3)P using elemental mapping through wavelength dispersive spectroscopy technique. These phases were found to have significantly higher hardness compared to the matrix and their precipitation along grain boundaries made the alloy susceptible to intergranular fracture even under small increase in service stress than nominal level.