Durcissement superficiel d’aciers inoxydables austénitiques par jet d’azote cryogénique à hautes pressions

Abstract : This work focuses on the study of an original surface treatment technique that uses supercritical cryogenic nitrogen jet. This process was initially designed for environmentally friendly surface cleaning, where indeed such gas recycles in the air after operation. In the present work, this technique is implemented for surface hardening use without damage of the surface to be treated. Two types of operation cases are studied: static jet tool impingement, cinematic using jet tool scanning on the top surface. In fact, these two static and cinematic treatment cases can be used in industrial operations. In the first stage, the treatment was performed under static conditions in order to map the domains of use of the process. Variation of the experimental parameters (standoff distance and dwell time - treatment time-) made possible to define several uses of the nitrogen jet. In particular the hardening without any damage of the surface of the material to be treated such as AISI 316L stainless steel. Thus, the influence of the standoff distance and the dwell time on the evolution of surface microstructure and damage and hardening was studied. To quantify the effects of nitrogen jet on the microstructure, SEM (Scaning Electrons Microscope) observations and micro hardness measurements were carried out on the treated surfaces. As a result, for different conditions of treatment, the relationship between hardness and martensite rate during surface transformation process, is shown and plotted. Secondly, we focus on hardening without surface damage. The treatments were essentially carried out on both AISI 316L and AISI 304L metastable stainless steels. The influence of both torch velocity and jet static pressure on the variation of microstructure, martensite fractions and hardening level, was also studied and discussed. Thanks to both SEM/EBSD analysis and micro hardness measurements, the relationship between martensite rate and increase of hardness, is highlighted. It is also established that the treatment using several passes allows to increase the surface micro hardness without damage. Finally, it is found that, for some particular working parameters, the nitrogen jet process can also be used for surface hardening without martensitic transformation
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https://hal.univ-lorraine.fr/tel-01925056
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Submitted on : Friday, November 16, 2018 - 2:26:25 PM
Last modification on : Tuesday, May 28, 2019 - 3:40:16 PM

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  • HAL Id : tel-01925056, version 1

Citation

Mustapha Yahiaoui. Durcissement superficiel d’aciers inoxydables austénitiques par jet d’azote cryogénique à hautes pressions. Science des matériaux [cond-mat.mtrl-sci]. Université de Lorraine, 2017. Français. ⟨NNT : 2017LORR0373⟩. ⟨tel-01925056⟩

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