Tempering behavior of low alloy case hardening steels with bainitic and martensitic microstructures

Increasing power density and rotational speed pose significant challenges for transmission design, especially in the aerospace and electromobility sectors. Due to increased energy input and reduced heat dissipation, higher operating temperatures occur in high performance gears. At higher temperatures, the hardness and microstructure of conventional bearing and gear materials are affected by annealing effects, which can reduce the load capacity of these components. Therefore, increased operating temperatures can only be considered if the components are made of special heat-resistant, high-performance material systems.

Heat treatment is essential to achieve the required performance. Today, high performance gears are typically case hardened to achieve the best performance in service. Due to the meta-stable properties of martensite and retained austenite, especially for low alloy case hardening steels, the microstructure can degrade in service if the temperature equals or exceeds the previous tempering. As a result, the hardness and performance of the components will decrease. Alternative steel grades with increased alloy content can mitigate but are in most cases more expensive. Therefore, an increase in temperature resistance through heat treatment of the low-alloy steels would be of increased interest. To achieve a more stable microstructure state, new heat treatments and alternative microstructures must be considered.

This presentation will address the tempering behavior of martensitic and bainitic microstructures under long-term thermal stress above typical tempering conditions at 210 °C for up to 200 hours. The microstructure degradation and hardness change as well as the effect on gear performance will be shown.