Thèse soutenue

Etude expérimentale et numérique du faïençage thermique en rectification du 32CrMoV13 nitruré
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Auteur / Autrice : Alexis Van Robaeys
Direction : Hédi Hamdi
Type : Thèse de doctorat
Discipline(s) : Mécanique et ingénierie
Date : Soutenance en 2015
Etablissement(s) : Ecole nationale d'ingénieurs (Saint-Etienne ; 1961-....)

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Résumé

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Helicopter power transmission parts are manufactured in high-tensile steel 32CrMoV13, which is case-hardened by nitriding. After nitriding, functional surfaces such as bearing races are grinded in order to give them adequate tolerances, roughness and surface topology. When not properly controlled, the grinding operation can create micro-crack networks on the finished surface. The aim of the study was to gain an understanding of the phenomenon by determining the local conditions at the surface generating and propagating the cracks. A literature study highlighted a strong link between micro-cracks and cyclic thermal loads. This load was identified for grinding operations by measuring the temperature and forces at the tool-workpiece interface. The results have shown a strong link between specific removal rate, efforts, and thermal load. The analysis of residual stresses after grinding showed the existence of a transition domain in which the surface is not cracked, but is in a tensile state. A 3D thermo-mechanical numerical model for grinding of nitrided 32CrMoV13 steel was developed using SYSWELD®, in order to predict the residual stress distribution in the workpiece after grinding. It uses a pure thermal load to represent the action of the tool on the surface. Its magnitude and distribution were studied in order to accurately represent the material removal induced by the grinding wheel. Thanks to the simulations, the degradation mechanism was exposed: under the thermal load, the matter heats up locally and undergoes a high compression, surpassing its tensile stress. During cooling, the damaged matter then passes to a tensile state, prone to cracking.