Low-Distortion Heat Treatment of Transmission Components

摘要

In many applications the high demands regarding service life of transmission components can be reached only by the application of a customized case hardening. This case hardening process results in a wear-resistant surface layer in combination with a tough core of the component. However, as a side effect, the components get distorted during heat treatment. This distortion has a significant cost impact because distorted components often need to be hard-machined after heat treatment. Therefore the proper control of distortion is an important measure to minimize production costs. By applying the technology of low-pressure carburizing (LPC) and high-pressure gas quenching (HPGQ), heat treat distortion can be significantly reduced. HPGQ provides a very uniform heat transfer coefficient. The predictability of movement during quenching is more certain and uniform throughout the load. Further improvements can be achieved by the "dynamic quenching" processes where the quenching severity is varied during the quench sequence by step control of the gas velocity. Proper fixturing is another factor for distortion control. Modern CFC (carbon-reinforced carbon) materials are well suited as fixture material for gas quenching. This paper presents how LPC and HPGQ processes are successfully applied on internal ring gears for a six-speed automatic transmission. The specific challenge in the heat treat process was to reduce distortion in such a way that subsequent machining operations are entirely eliminated. As a result of extensive development in the quenching process and the use of specialized CFC fixtures, it was possible to meet the design metrological requirements. The internal ring gears addressed in this report have been in continuous production since 2006. Subsequent testing and monitoring over a two-year period progressively demonstrated that consistent metrology was achieved and quality inspection was reduced accordingly.