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Etude de l'influence de la microstructure et des paramètres de coupe sur le comportement en tournage dur de l'acier à roulement 100Cr6

Abstract : Due to environmental and economic constraints, machining is currently a field under development. Hard turning is a recently developed process which makes it possible to machine hardened steels (i.e. with hardnesses higher than 45 HRc). It is possible to obtain, without lubrication, a part with a precision and surface quality near to those resulting from grinding. The control of the machining process requires the comprehension of the mechanisms intervening in the cutting zone. These depend both on the nature of the tool used and on the material. This study is principally concerned with the effect of the microstructure on the machinability of bearing steel and the resulting surface integrity. The bearing steel (100Cr6) was studied in various metallurgical states characterized by the presence or absence of carbides in a bainitic or martensitic matrix. The two metallurgical states were studied for identical hardnesses ranging between 45 HRc and 59 HRc. Several mechanical and thermal analyses techniques were used to better understand the driving mechanisms during the formation of chips. In particular, a thermotribometer was used to describe the influence of the microstructure and hardness on the tribology of the interface 100Cr6-cBN. Indeed, it is shown that the presence of carbides increases the coefficient of friction. Hopkinson bar tests made it possible to identify, by a reverse approach, the Johnson-Cook material behaviour laws for different metallurgical states. The tests were carried out on hat-shaped specimens. With this type of test it is possible to reproduce the dynamic shearing in the primary shear zone generated in the cut. The influence of hardness on the yield values was highlighted. Conventional straight turning tests (3D turning) were then carried out. The results showed that the cutting force is relatively insensitive to the metallurgical state of the material. Three different types of behaviour were observed with the cutting speed. The evolution of the surface integrity with respect to the cutting parameters was quantified by the resulting surface quality and residual stress measurements. The surface quality is improved for slow advances, a hardness of 55 HRc and with the presence of carbides. High hardness and the absence of carbides tend to generate more compressive surface residual stresses. Complementary orthogonal cutting tests (2D turning) made it possible to examine the thermal aspects of the cutting zone, by using a CCD-Near Infra-red camera. In addition, the analysis of the chips and white layers were carried out. It is shown that the presence of carbides increase the temperature of the secondary shear zone, whereas they decrease the temperature at the emergence of the primary shear zone. The existence of a white layer on the reverse faces of the chips and on the machined surfaces was established. It is shown that the white layer is more intense for high speeds and more homogeneous for the metallurgical state without carbide. Very localised shearing and a rise in the temperature support its appearance. Lastly, we will explain these influences through a comparison of the results obtained with those of the material behaviour study.
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Submitted on : Thursday, September 9, 2010 - 3:41:27 PM
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  • HAL Id : pastel-00002142, version 1



Malek Habak. Etude de l'influence de la microstructure et des paramètres de coupe sur le comportement en tournage dur de l'acier à roulement 100Cr6. Sciences de l'ingénieur [physics]. Arts et Métiers ParisTech, 2006. Français. ⟨NNT : 2006ENAM0057⟩. ⟨pastel-00002142⟩



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