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Genèse des contraintes résiduelles de nitruration. Étude expérimentale et modélisation

Laurent Barrallier 
Abstract : Nitriding of steel parts leads to an increase in their service lifetime as a result of the improvement in the fatigue strength of the material due to surface hardening in combination with the generation of residual compressive stresses in the near-surface layers. A new method for calculating the local changes in volume is proposed.This method is based on a thermodynamical evaluation of the volume fractions of the various types of precipitates existing at equilibrium in the nitrided layer. A theoretical approach using micromechanical concepts has put into evidence that the residual stress fields built up in each phase are essentially different and hence different from the average stress field. Experimental checking has been performed using the X-ray diffraction technique to establish the residual stress depth profiles in the ferrite phase and the hole-drilling method to determine the mean residual stress field as a function of depth in the nitrided layer. The analysis of the diffraction peak profiles has shown that the residual plastic strains are negligible in nitrided layers. The integral breadth of the diffraction peaks is almost exclusively indicative of the number-average crystallite size in the ferrite phase. In addition, direct observations in the TEM have confirmed that plastic deformation is localized in the vicinity of the precipitates. Residual stress relaxation has been investigated as a function of time and temperature. It appeared clearly that the relaxation phenomena are closely related to the diffusion of nitrogen during the process and that the residual fields generated nitriding show a high degree of thermal stability. Application of the proposed method to the prediction of the mechanical behaviour of a thin plate nitrided on one side confirms the previous results. Such a specimen could be used as a means of evaluating the intensity of a particular nitriding operation in common industrial practice (patent pending)
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Submitted on : Saturday, April 10, 2010 - 8:00:00 AM
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Laurent Barrallier. Genèse des contraintes résiduelles de nitruration. Étude expérimentale et modélisation. Engineering Sciences [physics]. Arts et Métiers ParisTech, 1992. English. ⟨pastel-00005978⟩



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