Determination of residual stress in nitrided steels with transmission angle-dispersive diffraction

Residual stress analysis with transmission angle-dispersive diffraction is carried out on nitrided steels. It aims evaluating residual stresses within the ferritic matrix but also within secondary phases such as nano-scale nitrides and polycrystalline cementite. Samples of various alloying elements contents are investigated. Results are compared to laboratory X-ray diffraction and micromechanical modeling of gas nitriding of steels

Optimisation de la nitruration gazeuse des aciers par une modélisation multiphysique

La nitruration gazeuse est un traitement thermochimique contrôlée par la température, le temps et le potentiel azote superficiel qui influencent de façon notable les propriétés méca- niques (dureté, contraintes résiduelles) et la durée de vie des pièces mécaniques traitées. Cet article propose une méthodologie permettant l’optimisation des paramètres de nitruration en utilisant un modèle multiphysique décrivant complètement le traitement : microstructure, mécanisme de diffusion/précipitation, dureté et génération des contraintes résiduelles.Thèse CIFRE Nit+

Evolution of residual stresses during short time nitriding of 33CrMoV12-9 steel grade

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 during 2h30 and 5 hours for various temperatures and nitriding potentials. Residual stress analyses are carried out by laboratory X-ray diffraction.Projet Nit+

Phase transformations and induced volume changes in a nitrided ternary Fe–3%Cr–0.345%C alloy

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 distinct depths of the diffusion zone. The relaxation of residual stresses arose through phase transformations according to the diffusion of nitrogen but also of carbon.The authors acknowledge Aubert & Duval (Eramet Group) and the Safran Group for their support, as well as CP2M laboratory, Paul Cezanne University, Marseille, for TEM investigations

Residual stress development during nitriding of steels

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 interactions of phenomena during nitriding, the last has not been completely explained yet. When interest is focused on stress depth gradient and time evolution, difficulties find origins in pronounced heterogeneities whether it is the chemical gradient due to nitrogen diffusion, the resulting gradient of microstructure or the gradient of volumetric misfits. Relaxation of residual stresses is so usually described using a thermally controlled creep phenomenon due to the couple of diffusion and stress, but depend on phenomenological descriptions. A key point is also disregard that is the diffusion of carbon and its redistribution across the nitrided surface during the treatment. Based on experimental characterizations of model carbon iron-based alloys, the role of phase transformations, especially carbides, is explored in order to give better understandings of the residual stress development during nitriding

On Residual Stresses Development During Nitriding of Steel: Thermochemical and Time Dependence

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 Fe-C-3w.%Cr alloy nitrided at 550°C for various time. Residual stress-depth analyses are carried out by X-rays diffraction. Residual stress generation is deeply dependant on chemical and thermodynamical evolutions during the treatment, taking advantage on microstructural effects

Optimization of gaseous nitriding of steels by multi-physics modelling

Gaseous nitriding is controlled by temperature, time and surface nitrogen potential. Tuning these parameters lead to the expected mechanical properties (hardness, residual stresses) and durability of treated mechanical parts. In this paper, a methodology is proposed to optimize nitriding parameters by using a multi-physics modelling of the nitriding treatment (microstructure, diffusion/precipitation mechanisms, hardness and residual stress generation). This model is particularly suited to the diffusion layer and does not apply to the compound layer. The scope concerns low-alloyed carbon steels and gaseous nitriding. The role of carbon is established and taken into account in the proposed methodology.Thèse Nit+

Traitement de nitruration et durabilité des pièces mécaniques

Le traitement thermochimique de nitruration est un traitement de surface permettant d'améliorer la durée de vie en fatigue des pièces traitées. Les dernières avancées dans ce domaine permettent de définir les paramètres technologiques du procédés en fonction des caractéristiques mécaniques en fatigue recherchées. Ce papier permet de parcourir l'ensemble des phé- nomènes physicochimiques (diffusion, précipitation, mécanique des matériaux) mis en jeux dans le traitement de nitruration, leurs modélisations multiéchelles et leurs limitations

Phase transormations involving residual stresses during gaseous nitriding of steel

Nitriding of steels is an important treatment in duplex hardening methods. This treatment is known in the art but some advances still need to be done in terms of residual stress understanding. Although residual stresses originate from volume micro-loading accompanying the precipitation of nitrides, questions about their in-depth distribution of a nitrided layer during the treatment are still a challenge. A chemico-thermo-micromechanical model has been developed on the volume change computation of secondary phase transformation. Supported by some experimental observations (TEM, X-ray analysis, Electron Probe Micro Analysis (EPMA), ...), this model gives some better understanding about nitriding. Residual stresses are mainly due to the precipitation of semi-coherent MN nitrides (M=Cr,V,Mo,. . . ). Moreover carbon, often not enough considered in the literature, is shown to be of importance in the process as it involves a second type of precipitation that is the transformation of carbides M23C6 and M7C3 into incoherent nitrides. The definition of the volume change of this transformation is a critical entry data of the mechanical modelling.We would like to thank Aubert & Duval and SAFRAN Group for supporting this study

Traitement de nitruration et durabilité des pièces mécaniques

Le traitement thermochimique de nitruration est un traitement de surface permettant d'améliorer la durée de vie en fatigue des pièces traitées. Les dernières avancées dans ce domaine permettent de définir les paramètres technologiques du procédés en fonction des caractéristiques mécaniques en fatigue recherchées. Ce papier permet de parcourir l'ensemble des phé- nomènes physicochimiques (diffusion, précipitation, mécanique des matériaux) mis en jeux dans le traitement de nitruration, leurs modélisations multiéchelles et leurs limitations