Investigation on deformation of gear for automotive application caused by case hardening

G. Casalino, L. Cautero, L. A. C. De Filippis, A. D. Ludovico, D. Longo, C. Maselli:

Proceedings of the 8th Seminar of the International Federation for Heat Treatment and Surface Engineering , IFHTSE 2001, Dubrovnik- Cavtat, Croatia, 12 – 14 September 2001.

The case hardening is a topical step of gear manufacturing for automotive application. It is usually divided into two parts. Firstly the gear is vacuum carburized then is quenched with high pressure gas. Since the functionality of the gear box is highly influenced by the dimensional precision of gears, the design of the process mainly consists of the control of the deformation caused by the thermal cycle. For a fixed furnace, the main process parameters are the temperature, the pressure, the carburizing medium, the quenching cooling rate and medium.
In the Getrag plant of Bari, heat deformations during case hardening have eventually been under control after a deep investigation. On the basis of preliminary experiments, a plan of experiments was designed and the most important parameters varied at the same time. In this way it was possible to evaluate the effect of their contemporary variation on the deformation. After setting the rough ranges for the studied parameters, the deformation of gear was within the range proposed by the DIN standard. Nevertheless, it was still necessary to deal with a manufacturing drawback entailed with grinding the teeth. Due to the planarity error caused mostly by the case hardening which altered the correct position on the machine, the grinding of teeth was often inadequate. The consequences were an increase of grinding time. Therefore a deeper investigation was carried out. It was supported by a new plan of experiments that took into account the hardenability of the material and numerical simulation using the Systus FEM software. Even if, the chamber geometry of the furnace has limited the efficiency of the proposed receipts for case hardening, the results of investigation produced finer range for process parameters.

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