Austempered Ductile Iron (ADI) for gears: Contact and bending fatigue behavior

Austempered Ductile Iron (ADI) represents an alternative solution for the manufacturing of the housing of small planetary gearboxes, with the gear teeth obtained directly on the housing itself: such solution combines a cost-effective process with the possibility of obtaining complex geometry of the case. With respect to most traditional solutions, by means of ADI the requirements of strength and accuracy of the gear teeth can be satisfied without an additional finishing step after the heat treatment: the teeth can be obtained by broaching and, thanks to the low distortion which can be granted by the austempering process, a subsequent finishing operation is not needed. For these reasons, ADI has been selected for the application to a family of small gearboxes for automation. Due to the limited experience and data available for such material, to improve the design and rating processes, a testing campaign has been performed. The aim was to obtain strength data for bending and contact fatigue, considering the specific manufacturing and heat treatment processes. The paper describes the test procedures adopted and the test results, which have been obtained on gears specimens by means of Single Tooth Fatigue (STF) and pitting tests on a FZG type bench respectively. The tests are supported by metallurgical investigations on the failed teeth, to describe and understand the failure mechanisms. The results are then compared with the data and the shape curves provided by the international standards.