Nasrine BOUALEM's thesis defense

During the hot forging process, tools are subjected to severe cyclic mechanical and thermal loading. These extreme forming conditions can damage tools through the development of abrasive wear, cracking, thermal and mechanical fatigue. This damage compromises the mechanical strength of tools and reduces their service life. To optimize tool life, several factors need to be taken into account, such as tool material properties, tool geometry, oxidation, lubrication and forging parameters (temperature, pressure and sliding speed).

The aim of this thesis is to understand how the number of repeated forging cycles impacts hot forging tools, in terms of changes in surface finish, changes in mechanical properties and microstructure, geometric modifications, and changes in tribological behavior.

The aim is to propose a tribological study that considers the state of die wear, with the possibility of integrating it into numerical simulations of the forging process.

Firstly, a determination of the rheological law of grade 38MnSiV5 (part material) is carried out by compression tests with the aim of feeding numerical simulations of the forging processes. Then, to study the influence of the forging cycle on the surface condition of the tools, a topographic analysis is carried out on tools with different geometries and different life cycles (new tool, during life and end of life).

The study reveals that surface roughness increases throughout the tool life cycle due to complex phenomena such as wear. Mechanical properties, such as tool hardness and Young's modulus, are also influenced by the forging life cycle and by changes in contact conditions. Nanoindentation tests show a decrease in hardness and a decrease or stabilization of Young's modulus, depending on the zone and life cycle. The test results were then correlated with the results of microstructural analysis obtained by SEM/EDS. They show the formation of cracks in the oxide layer and a transition from martensite to globular cementite. In addition, the formation of a decarburization layer is observed in certain zones, which seems to be linked to the formation of a non-adherent oxide layer. A tribological study of the forging process is carried out using the WHUST test bench, which is used to reproduce the contact conditions of the industrial process. Different geometries and configurations of contact conditions are analyzed as a function of the life cycle.

This analysis revealed a stabilization or decrease in the friction indicator at high temperatures. The presence of oxides and changes in tool geometry significantly influence tribological and wear behavior: oxide layers, whose thickness increases with the number of forging cycles, act as thermal barriers, influencing tool friction and wear behavior. However, they also act as crack initiation sites, contributing to surface damage over time. Wear-induced geometric changes affect contact conditions and the overall performance of forging tools. Numerical simulations highlight that changes in tool geometry lead to variations in contact pressure and temperature.

Mr LAURENT DUBAR, Université Polytechnique Hauts de France, Thesis co-director
. Ms Marion RISBET, Université de Technologie de Compiègne - Roberval CNRS FR 3272, Rapporteur
Mr. M. Régis BIGOT, ENSAM METZ, Rapporteur
Ms Mirentxu DUBAR, Université Polytechnique Hauts-de-France, Valenciennes (UPHF), Co-supervisor
. Mr Philippe MOREAU, Université Polytechnique Hauts-de-France, Valenciennes (UPHF), Examiner
Mr Philippe BRISTIEL, Université Polytechnique Hauts-de-France, Valenciennes (UPHF) M. Philippe BRISTIEL, STELLANTIS, Examiner
Mrs Emmanuelle VIDAL-SALLE, INSA de Lyon, Examiner
. M. José Grégorio LA BARBERA SOSA, UPHF, LAMIH UMR CNRS 8201, Invited
M. Tarik SADAT, UPHF, LAMIH UMR CNRS 8201, Invited

Wear, tooling, hot forging.