Steel design and high speed machining aspects in the transition from case hardening to induction hardening of automotive transmissions (MAC D) : final report

The MAC D project has shown a realistic potential to replace today's carburizing route in the manufacturing of a helical gear with an induction hardening route. The transition includes evaluation and choice of a new suitable steel and as delivered microstructure from the forging, data of machinability in the vital machining processes, a comparison of fatigue strength and a complete comparison of manufacturing costs. A comparison of direct costs of heat treatment was made including investment, maintenance, energy requirement and staff. The costs are as follows: Carburizing: 2.41 € per gear; Induction hardening: 0.64 € per gear. The total costs of the machining processes were obtained from machinability data of the project and production data of Fiat Powertrain. The cost of the carburizing route using 18CrMo4 was 9 € per gear. The cost of the induction hardening route using 50CrMo4 was 12 € per gear. The fatigue strength of an induction hardened gear has a full potential to rival the strength of a carburized gear. More detailed studies are recommended before the industrial introduction of an induction hardened gear application. Green machining of a 50CrMo4 steel at 345 HB in both rough turning and a simulated hob milling test showed roughly a 50% reduction in performance as compared with the reference 18CrMo4 steel at 160 HB. That extra cost is to some extent compensated by reduced costs in hard part turning and tooth grinding. The primary cost associated with lower performance is the extended machine time and more staff costs.