The Arts et Métiers campus Arts et Métiers is pleased to invite you to the thesis defense of Yanxue Zhang, a doctoral student at the MSMP laboratory, in collaboration with LISI Automotive, on December 8, 2022.
Thesis title: Gas nitriding of M2 high-speed steel to improve the surface mechanical properties of screw-rolling dies
Summary of work:
M2 high-speed steel rollers used in the manufacture of electric parking brake screws often suffer premature failure in service. Damage and crack initiation occur several micrometers below the surface of the roller teeth due to high contact loads, with a sliding effect, and the presence of large segregated carbides. An industrial case study using Dang Van's fatigue criterion shows that nitriding increases the fatigue resistance of the material near the surface.
The kinetics of gas nitriding of M2 steel are studied using several sets of parameters. Increasing the nitriding potential and treatment temperature accelerates the nitriding process due to changes in boundary conditions and thermochemical reaction kinetics, respectively. Gas nitriding treatment is sensitive to the cleanliness and mechanical finishing method of the treated surface.
The hardness gradient in the nitriding layer is generally consistent with the nitrogen concentration gradient, which peaks at the surface. The evolution of residual stresses depends heavily on the volume change induced by the phase transformations caused by nitriding. They reach their maximum after carbon accumulation (at a shallower depth, before nitrogen decreases significantly) and decrease near the surface.
The nitriding limit potential for the formation of the combination layer is identified to achieve the highest hardness on the surface without the formation of a combination layer. Its high brittleness and specific morphology (such as porosity) are problems for the application of the material.
The nitriding parameters are ultimately defined by considering a high residual compressive stress on the surface and a low effect on the hardness at the core of the material. Wear resistance tests are then defined and carried out.
Wear, particularly adhesion wear, is significant on the non-nitrided M2 sample. The selected nitriding parameters effectively improve the wear resistance of M2 steel.
Keywords: Gas nitriding, M2 steel, Residual stresses, Hardness, Wear resistance, Surface fatigue
Jury:
- Mathieu RENOUF, LMGC, University of Montpellier: rapporteur
- Thomas CHRISTIANSEN, Technical University of Denmark: rapporteur
- Philippe JACQUET, ECAM Lyon: examiner
- Laurent BARRALLIER, MSMP, Arts et Métiers examiner
- Sébastien JÉGOU, MSMP, Arts et Métiers examiner
- David MERCS, LISI Automotive: examiner
Practical information:
Location: Arts et Métiers Campus Arts et Métiers (room H015)
Time: 1:30 p.m.
The defense can also be followed via videoconference using a Teams link.
