Nouvelle loi phénoménologique de l'adoucissement d'un acier à outil au cours du revenu, application en fatigue thermique

Sylvain Jean , Brice Miquel , Farhad Rezai-Aria (1) , Christophe Levaillant

1 CROMeP - Centre de Recherche Outillages, Matériaux et Procédés

Among other loadings, hot-work tool steels are subjected to fa s t (few seconds) and large range (o ver 600 °C) tem perature cycles. In addition to oxidation, this implies that mechanical evolu tions are due to both thermomechanical fatigue and thermal softening. Among mechanical properties, hardness is widely used, as it is related to metallurgical characterisation o f tool steels. For example, estima tion o f maximum temperature sustained by critical parts o f tools is often done through measured hardness o f a die associated with tempe ring resistance properties o f the steel. This assumes that hardness evolution does not depend on mechanical straining. Furthermore, hardness is never expressed as an explicit function o f time and tempe rature, thought it is supposed to be so. In this paper, we suggest a new model which is such an explicit func tion. This model is based on the observation o f evolutions o f hardness during tempering. It is written thanks to fou r boundary conditions o f the evolution o f hardness with time and temperature, and the mathema tical principles on which time-temperature equivalence is based. Direct experimental verifications lead then to a complete validation o f the model in that particular case, that is the mathematical form o f the func tion and values o f the constants. The general shape o f the model is : It is verified in the case o f the tempering o f a 5 % chromium steel with initial hardness 485 Hv : Is it shown that constants have physical related values. Furthermore, the model we suggest leads to a formulation o f the well known heat treat ments parameters as functions o f the desired hardness and the constants related to the thermoactivated processes like Jaffe's one. For example, R é s u m é : L'équivalence temps-température est un concept général des cinétiques des processus thermiquement activés, qui s'appuie sur les formulations mathématiques maintenant bien connues des paramètres de traitem ent therm ique, de type H ollomon et Jaffe ou de type Maynier. Mais, ces paramètres ne sont pas des fonctions explicites de la dureté, c 'est-à-dire que nous ne connaissons pas de loi générale means that all the couples o f values (t,T) which implies the same value o f give the same heat treatment result, which is the hardness in that case. Thanks to our model, we can show that with Hv1 the desired evolution o f hardness. Maynier param eter and Murry parameter are also shown to be such functions, but with parti cular shape. So, though the shape o f this model has been suggested only by a phenomenological method, it can be seen that it is in agree ment with existent parameters. In fact, it gives them a physical inter pretation which tends to unify them all into a single model in any case where the model is proved to be valid. Using this model allows the evaluation o f the maximum tem perature sustained by the m aterial assuming that only time and temperature are involved in the evolution o f hardness. If one knows the thermal history (number o f cycles N, exposure time p e r cycle t) and the measured hardness H v1 then the temperature at which it is necessary to heat the material is Using this model makes it also possible to show that thermomechani cal straining has a strong influence on the evolution o f hardness. In the case o f our thermal fatigue experiment, we show that a modifica tion o f the activation energy is not enough to explain the influence o f thermal straining. If one would search fo r the maximum temperature sustained by the thermal fatigue specimens using only time-temperatu re process kinetic, the error made may be as great as 200 °C, though the experimental maximum temperature has been set to 650 °C. K e y w o r d s : Thermal fatigue, hardness, tool steel. d'évolution de la dureté au cours du temps par revenu isotherme. De plus, cette équivalence établie en statique (sans contrainte) est parfois appliquée pour rechercher la température subie par un outil soumis à des sollicitations thermomécaniques à partir d'une évaluation du cycle thermique et d'une courbe maîtresse de revenu. Nous proposons et démontrons dans un cas particulier une nouvelle loi phénoménologique

Domaines: Sciences de l'ingénieur [physics]

Sylvain Jean, Brice Miquel, Farhad Rezai-Aria, Christophe Levaillant. Nouvelle loi phénoménologique de l'adoucissement d'un acier à outil au cours du revenu, application en fatigue thermique. Bulletin du cercle d'études des métaux, 2000, 17 (1), p.V1-V8. ⟨hal-01847829⟩