Sébastien Jégou

Sébastien Jégou
Ecole Nationale Supérieure d'Arts et Métiers | ENSAM · Laboratoire Mécanique, Surfaces, Matériaux et Procédés (MSMP)

Associate Professor

About

30
Publications
5,907
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157
Citations
Additional affiliations
September 2011 - May 2016
Ecole Nationale Supérieure d'Arts et Métiers
Position
  • Professor (Associate)
June 2010 - July 2011
Technical University of Denmark
Position
  • PostDoc Position

Publications

Publications (30)
Article
Nitriding of low-alloy steels is widely used for gears and bearings in aerospace industry. Some highly stressed surfaces require high nitrided depth, which involves long time/high temperature nitriding treatment. This study focused on identifying process parameter effects on hardness and residual stress profiles in a large range of nitriding time a...
Article
Gaseous nitriding of steels is a well-established thermochemical surface treatments that increases the fatigue resistance of treated parts. The present work proposes a modelling of the fatigue life as a function of the applied stress. It takes into account the hardness and compressive residual stresses that developed during nitriding. The model is...
Article
The gaseous nitriding behaviour of 33CrMoV12-9 steel was investigated. The co-diffusion of nitrogen and carbon, precipitation at grain boundaries and mechanical properties evolutions (microhardness, residual stresses) are compared for several nitriding potentials, nitriding time and temperatures. While hardening unequivocally depends on the precipi...
Article
Anomalous small angle scattering (ASAXS) is a powerful non-destructive technique that can provide characteristic features of nanoscale precipitates such as the volume fraction, and chemical composition. In this paper, the technique is used for the first time to explore nanoscale MN nitrides (M = Cr,Fe) after nitriding of a model iron alloy (Fe-0.35...
Conference Paper
The aim of this study is to adapt and use a procedure to investigate the influence of the microstructure on the local mechanical behavior of a tempered and nitrided low-alloyed steel. A well-known technique is employed, consisting in Digital Images Correlation (DIC) of SEM field images acquired during In-situ loads. To avoid heterogeneous macroscop...
Presentation
Uniaxial cyclic loadings of homogeneously nitrided steel plates were carried out to charac-terize the crack initiation mechanisms with respect to the microstructure. S-N (Stress - Num-ber of cycles to failure) curves corresponding to the nitrided layer of a 33CrMoV12-9 steel grade were created in order to estimate the number of cycles for interrupt...
Conference Paper
Full-text available
Le traitement thermochimique de nitruration est un traitement de surfaces permettant d'améliorer la durée de vie en fatigue des pièces traitées. Les dernières avancées dans ce domaine permettent de relier les paramètres technologiques du procédé (temps, température, potentiel azote, composition de l'acier) aux caractéristiques mécaniques des couche...
Article
The present work focuses on the industrial inhibition of gaseous nitrided parts and on the industrial in-situ pre-treatment that can be used to avoid these inhibitions. The first objective is to obtain a repeatable defective nitriding process on carbon iron-based alloys, while the second is to determine the ability of determined in-situ pre-treatme...
Conference Paper
Full-text available
The objective of this work is to propose a model abled to optimize the mechanical properties from gaseous nitriding of steels (hardness, residual stresses) for a given loading and fatigue lifetime. Multiaxial fatigue criteria are used and the present study focuses on the influence of residual stresses. Starting from a surface loading profile, the t...
Article
The present study aims to develop an algorithm able to predict the fatigue lifetime of nitrided steels. Linear multi-axial fatigue criteria are used to take into account the gradients of mechanical properties provided by the nitriding process. Simulations on rotating bending fatigue specimens are made in order to test the nitrided surfaces. The fat...
Article
Full-text available
Le but de cette étude est de mettre en place un modèle de calcul en fatigue prenant en compte les caractéristiques mécaniques d’un matériau nitruré. Le travail est réalisé sur acier de nuance 32CrMoV13 utilisé dans la conception de pignon de transmission de puissance par Hispano-Suiza.
Article
The GFAC (French Association for residual stress analysis) decided in 2007 to work on external reference samples for residual stress analysis by X-ray diffraction as defined in the XPA 09-285 and EN 15305-2009 standards. Seven materials are studied: ferritic steel, martensitic steel, aluminium alloy, titanium alloy, 2 types of Nickel-Chromium alloy...
Article
Full-text available
The evolution of residual stresses during short time gas nitriding of 33CrMoV12-9 steel grade is studied. It aims understanding the influence of nitriding parameters (temperature T and nitriding potential Kn) on the generation and evolution of residual stresses in the very first stage of nitriding. The samples are gas nitrided using a thermobalance...
Article
A methodology is proposed combining the scattering vector method with energy dispersive diffraction for the non-destructive determination of stress- and composition-depth profiles. The advantage of the present method is a relatively short measurement time and avoidance of tedious sublayer removal; the disadvantage as compared to destructive methods...
Conference Paper
Full-text available
In a previous study, the capability of X-ray diffraction technique to qualify damage occurring during friction had been proven [1]. The purpose of this study is to complete this research in order to develop a method to quantify the damage induced by the friction loading.
Article
Full-text available
Development and evolution of compressive residual stress during nitriding treatment are studied. A model carbon iron-based alloy Fe-3wt.%Cr-0.35wt.%C was nitrided in gas at 550 °C for different times. Microstructural investigation indicated an influence of the transformation and the associated carbon diffusion on the in-situ relaxation of residual...
Article
Full-text available
Nitriding is a well-established thermochemical surface treatment of carbon micro-alloyed steels aiming enhancing surface properties such as fatigue, wear and corrosion resistances. The idea is taking benefits from the high hardening level due to a fine nitride precipitation and also the compressive residual stress state. Due to some complex interac...
Article
Full-text available
Cette étude présente les capacités d’une modélisation de la nitruration, procédé thermochimique destiné à améliorer la tenue en service des pièces mécaniques. La formation d’un gradient de microstructure accompagné d’une variation de volume génère un gradient de contraintes résiduelles. Le modèle se base sur une étape de diffusion / précipitation d...
Article
Phase transformations during nitriding of a ternary carbon iron-based alloy Fe–3%Cr–0.345%C were studied, aiming for a better understanding of residual stresses generation and evolution. The relationship between the precipitation of Cr7C3 carbides and CrN nitrides, the induced volume change and the mechanical properties were investigated at three d...
Article
Full-text available
This work deals with the development of residual stresses during nitriding of steels. The main features of a chemico-thermo-mechanical model of nitriding are presented. A micro-macro approach is applied based on volume change computation in agreements with thermochemical modifications. Results are correlated with the characterization of a ternary F...
Conference Paper
Nitriding is a well-established thermo-chemical surface treatment of carbon micro-alloyed steels aiming better surface properties such as fatigue, wear and corrosion resistances. The idea is taking benefits from the high hardening level due to the fine nitride precipitation and also the compressive residual stress state. Relaxation of residual stre...
Article
Residual stresses of nitrided steels find origins through volumetric misfit accompanying the precipitation during the treatment. A residual stress gradient is generated from the primary gradient of chemical composition and the resulting gradient of microstructure. It is also the consequence of macroscopic strain incompatibilities. Volumetric strain...
Article
Les contraintes résiduelles de nitruration ont pour origine les incompatibilités de déformations volumiques issues de la précipitation au cours du traitement. Le gradient de contraintes résiduelles résulte des gradients de composition chimique et de microstructure, ainsi que des incompatibilités de déformations macroscopiques. L'évolution du gradie...

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Projects

Projects (2)
Archived project
Design of high-performance and lighter mechanical systems needs a perpetual work on materials used. In the power transmission field, gaseous nitriding thermochemical treatment enhances significantly the life-cycle of the parts which are higher stressed on the surface. While the contribution of the mechanical property gradients due to the treatment on the fatigue life is well-known, the microstructure effects on the (micro-)plasticity mechanisms remains undetermined. This work concerns exclusively the nitrided 33CrMoV12-9 graded steel, used for gears and bearings in the naval and aeronautical industry. It aims particularly at defining mechanical properties linked to crack initiation mechanisms during high-cycle fatigue. Properties such as Young modulus, hardness, yield stress and fatigue limits are looked at several scales, from microstructural ones to hardened case one. In-situ and post-mortem investigation on loaded and damaged nitrided layers leads to estimating the microstructural influence on local deformations, microplasticity and damage but no particular mechanisms were found. Special fully-nitrided parts were designed to perform the experiments. This tool helps to carry out simple loadings and measure the strain-hardening curve and fatigue limits. Finally, it as been attested that measured properties depend on scales, means and methodology.
Project
Nitriding of carbon steels from technological parameters to fatigue life of treated parts, simulation and experimental approaches