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Analyse et modélisation des transformations tribologiques de surface lors de la mise en forme à froid des aciers martensitiques

Abstract : There are several types of suspensions in the automotive industry. Those that are cold formed allow variability and flexibility in the design. However, this flexibility leads to the creation of defects. During the production of coil springs, the step of winding the SAE 9254 steel wire sometimes causes the appearance of a surface defect commonly called "white layer" because of its white appearance under the optical microscope. This hardened phase is the result of a tribological surface transformation, i.e., a modification of the surface properties of the material due to severe friction. In the automotive industry, this defect is called "hardened martensite" because it refers to the hardening of a martensitic steel under the effect of severe mechanical and tribological stresses. Several tribological test campaigns were carried out in order to understand the role of the key parameters of the winding process in the creation of the white layer. The first series of tests, on a pin-disc tribometer, made it possible to evaluate the role of the pollution of lubricants by oxide on the coefficient of friction over long distances. These tests showed a stabilizing role of the polluting oxide on friction, when it is used with an appropriate lubricant. The second series of socalled conventional tests was carried out on the Upsetting Sliding Test (UST) experimental platform. It aimed to understand the role of sliding speed, contact pressure, and lubrication on the coefficient of friction and the creation of the white layer. These tests have shown the nondependence of the friction coefficient on the sliding speed. On the other hand, they showed that by increasing the contact pressure and the viscosity of the lubricant, the third body created by this mixture acts as a thick film layer, improving the friction conditions and reducing the thickness of the white layer. The last test campaign reproduced the conditions of the industrial process, notably by measuring the influence of the origin of the wire, the influence of lubrication as well as the influence of the wear rate. These tests have shown the dependence of the coefficient of friction on tool wear, lubrication and the origin of the thread. Finally, a finite element simulation of the cold coiling process, taking into account Lemaitre and modified Lemaitre damage models (LEL), was developed. Results showed that the modified Lemaitre model is able to predict the moment and the location of appearance of the damage of the spring to the winding.
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Submitted on : Wednesday, September 21, 2022 - 4:59:42 PM
Last modification on : Thursday, September 22, 2022 - 5:11:11 AM

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  • HAL Id : tel-03782995, version 1

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Hans Curtis Boungomba. Analyse et modélisation des transformations tribologiques de surface lors de la mise en forme à froid des aciers martensitiques. Mécanique [physics.med-ph]. Université Polytechnique Hauts-de-France; Institut national des sciences appliquées Hauts-de-France, 2022. Français. ⟨NNT : 2022UPHF0020⟩. ⟨tel-03782995⟩

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