Grades for power transmission parts with improved tempering and fatigue resistance

Picot Thomas - Arcelormittal Maizieres Research SA (France)

The coming changes in vehicle motorization from combustion engines to hybrid and electric motor has a great impact on the transmission system. In that context, ArcelorMittal aims to develop new steel solutions for transmission in electric vehicles by anticipating the future needs of automotive manufacturers. Automotive transmission gears are usually produced by forging (cold, warm, hot) followed by a surface hardening process (carburizing, carbonitriding) with several machining operation along the process depending on the type of part. In this study it was chosen to work on the most usual process route i.e., hot forging and carburizing.

Higher rotation speed and higher torque undergone by gears in electric vehicles lead to new constrains for the material, namely higher risk of superficial heating and increased contact fatigue stresses. The superficial temperature increase on the teeth flank of the gears can lead to local softening of the material that can cause surface pitting or eventually crack initiation. For these reasons tempering resistance was identified as a key property to optimize the life of the parts in electric vehicles transmission. Since carburizing at higher temperatures can be a solution for optimizing energy consumption of the surface treatment, grain size control during carburizing treatment is also studied for usual and higher temperatures. Grain size is also one of the major parameters to optimize fatigue life of the surface hardened parts by improving tooth root bending fatigue resistance in that case.

The last years of work on modified tempering resistant transmission grades, will be presented in this paper. Two modified grades were analyzed, in a first time all the usual requirement of the automotive manufacturers were controlled. In a second time mean grain size, abnormal grain growth and tempering resistance were measured and compared to a reference. This study is a first step for ArcelorMittal in.