Robust and cost efficient hard part turning of transmission components (ROCOST)

Purpose and goal

Relations between steel grade and carburizing, and relations between tool wear and process dynamics were investigated and related to the material microstructures. The goal was to provide know-how on how to optimize the carburized case of typical transmission components to provide a robust and cost-efficient hard turning process, aiming for a 50% productivity increase. Productivity is here defined as the sum of processes influencing a controlled and predictable tool wear robustness.

Expected results and effects

A test matrix of lab components relevant to production showed that: Rough machining is affected by the outer shell and distortion through increased cutting wear and spread. Selection of steel grade affects the outcome in machining more than RA and HV. Tool wear can be controlled; Tool wear increases in the 159S steel within limited RA content and at high RA and high cutting speed. Small sensitivity to RA and HV in the 236F steel. Phase transformation on component surface occur during machining. It is larger in the corresponding limited RA content in the 159S, such as the tool wear.

Planned approach and implementation

Lab-components were produced from two steel grades: Ovako 236F (20MnCr5) and Ovako 159S (18CrNiMo7-6). They were case hardened targeting 0.64, 0.72 and 0.80 mass% C and tempered at 160 C and 180 C to differentiate hardness. Hard turning was performed in both a roughing-like and a finishing-like process at 150-275 m/min. Tool wear was numerically measured (crater depth and flank wear). Retained austenite was also measured after machining. A methodology to evaluate the robustness was developed from vibration, sound, and machine current data.