Mechanical and metallurgical characterization of contact fatigue mechanisms in ADI spur gears

Abstract

Surface pitting of gear teeth flanks is one of the most common causes of gear failure. The increase of the contact fatigue strength of gears is usually achieved through the development of new combinations of materials and heat-treatment processes. Austempered Ductile Iron (ADI) is one of the results of this research field that leaded to a material which is replacing conventional steel in many mechanical applications. This paper shows the experimental results obtained from tests performed on ADI spur gears in order to characterize the contact fatigue crack nucleation and growth and understand the possible interaction with the graphite spheroids and casting defects. Using a four-square test rig, it was carried out a systematic monitoring of the evolution of the damage, collecting several images concerning crack development at gear surfaces. At the end of the tests several metallurgical analyses of surface and sub-surface areas were performed to understand the fracture mechanism at the base of the pitting failure in ADI. All experimental analyses were complemented by theoretical calculations, useful for a better understanding of the phenomenon.