Austempered Ductile Iron (ADI) for gears: manufacturing, performances and tests

Austempered Ductile Iron (ADI) represents an alternative solution for producing the housing of small planetary gearboxes with teeth of the internal gear manufactured directly on the housing itself: such a solution combines a cost-effective process with the possibility of obtaining complex geometries of the case. With respect to traditional solutions, the requirements of strength and accuracy of gear teeth can be satisfied, by means of ADI, without an additional finishing step after the heat treatment. In this case, the teeth can be obtained by broaching and, thanks to the low distortion which can be granted by the austempering process, a subsequent finishing process is not needed. For these reasons, ADI has been select for the application to a family of small gearboxes for automation. Due to the limited experience and data available for such a material, a testing campaign has been performed in order to obtain strength data for bending and contact fatigue, taking into account the specific manufacturing and heat treatment processes. The paper describes the adopted test procedures and the test results obtained on gear specimens by means of Single Tooth Bending Fatigue (STBF) and pitting tests. The results are compared with the data and the shape curves provided by international standards. Metallurgical investigations on the failed teeth are presented in order to describe and understand the failure mechanisms.