Papers by Author: Jacques Merlin

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Authors: Aurélie Hug-Amalric, Xavier Kleber, Jacques Merlin, Hélène Petitgand, Philip Meilland
Abstract: The potentialities of using the magnetic Barkhausen noise measurement in characterization of metallurgical transformations have been highlighted in multi-phase High Strength (HS) steels. This kind of steels are composed of different metallurgical constituents, such as ferrite, bainite, martensite or residual austenite. Recently, we found that it was possible to assess the proportion of phases in ferrite-martensite steels and in industrial Dual-Phase steels too. In this work, we show that the Barkhausen noise measurements can be also suitable to follow bainitic transformation in a TRIP steel.
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Authors: Véronique Massardier-Jourdan, E. Le Patézour, F. Thiery, N. Lavaire, Jacques Merlin
Abstract: In Fe-C-Mn steels, the carbon atoms in solution can be either completely free in the iron matrix or in interaction with manganese atoms. In this context, a methodology based on the combined use of thermoelectric power and internal friction measurements was developed in order to evaluate quantitatively these two populations of carbon atoms. This methodology was used to determine the binding energy of the C-Mn dipoles and to follow the precipitation kinetics of the two populations of carbon atoms and/or their segregation kinetics to the dislocations during an isothermal treatment. Lastly, the influence of each population of carbon atoms on the strain aging of extra-mild steels was discussed.
623
Authors: Xavier Kleber, Aurélie Hug-Amalric, Jacques Merlin
Abstract: In this work, we show that the measurement of the Barkhausen noise allows the residual stresses in each of the two phases of ferrite-martensite steels to be characterized. We have first studied the effect of a tensile and a compressive stress on the Barkhausen noise signature. We observed that for a ferrite-martensite steel, the application of a tensile stress increases the Barkhausen activity of the martensite and ferrite phases, whereas a compressive one reduces it. In a second time, we induced residual stresses by applying a plastic deformation to ferrite-martensite steels. After a tensile plastic deformation, we observed that (i) compressive residual stresses appear in ferrite, and (ii) tensile residual stresses appear in martensite. An opposite behavior is observed after a compressive plastic deformation. These results show that the Barkhausen noise measurement makes it possible to highlight in a nondestructive way the distribution of the stresses in each of the two phases of a ferrite-martensite steel. This result could be used to characterize industrial Dual- Phases steels that are plastically deformed during mechanical processes.
655
Authors: Aurélie Hug-Amalric, Xavier Kleber, Jacques Merlin, Hélène Petitgand
Abstract: Magnetic Barkhausen noise measurements have been carried out to characterize ferritemartensite duplex microstructures and industrial Dual-Phase steels. We have first studied ferritemartensite duplex steels, for which the volume fraction and the carbon content of martensite were higher than for industrial Dual-Phase steels. We found linear evolutions between ferrite peak parameters and its proportion. We applied these results to industrial Dual-Phase steels and show that Barkhausen noise measurement can be successfully used for Dual-Phase steels characterization, and in particular for assessment of ferrite proportion.
639
Authors: Sylvain Dancette, Véronique Massardier-Jourdan, Jacques Merlin, Damien Fabrègue, Thomas Dupuy
Abstract: Advanced High Strength Steels (AHSS) are key materials in the conception of car body structures, permitting to reduce their weight while increasing their behavior in crash conditions. Nevertheless, the weldability of AHSS presents some particular aspects, in that complex failure types involving partial or full interfacial failure can be encountered more often than with conventional mild steels during destructive testing, despite high spot weld strength levels. This paper aims at characterizing the behavior of different AHSS spot welds under two quasi-static loading conditions, tensile shear and cross tension, often used in the automotive industry for the determination of their weldability. Interrupted cross tension and tensile shear tests were performed and spot welds failure was investigated with optical micrographs, SEM fractography and 3D-tomography in order to follow the three-dimensional crack paths due to the complex loading modes. A limited number of failure zones and damage mechanisms could be distinguished for all steel grades investigated. Moreover, numerical simulation of the tests was used to better understand the stress state in the weld and the influence of geometrical features such as weld size on the occurrence of the different failure types.
130
Authors: Véronique Massardier-Jourdan, David Colas, Jacques Merlin
Abstract: The thermoelectric power (or TEP) technique was used to determine the segregation kinetics of the interstitial atoms (C or N) to the dislocations in various extra-mild steels submitted to a heavy deformation by cold-rolling when substitutional atoms (Mn or Cr) are simultaneously in solid solution. It was shown that the substitutional atoms (Mn or Cr) have almost no influence on the segregation kinetics of carbon and on the activation energy associated with the segregation of this element. In contrast, these elements tend to delay the segregation kinetics of nitrogen to the dislocations all the more so as their content in solution is high. In the mean time, the activation energy associated with the segregation of nitrogen is increased.
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