Jacques Merlin

Kinetic and microstructural study of aluminium nitride precipitation in a low carbon aluminium-killed steel

The precipitation kinetics of free nitrogen present in a low carbon aluminium-killed steel was quantitatively followed at temperatures ranging from 600 to 700 °C using a methodology based on a succession of thermoelectric power (TEP) measurements. These were compared with the precipitation kinetics of aluminium nitrides determined by conventional TEP measurements. This comparison showed a difference between the two types of kinetics. In particular, it was observed that after the complete elimination of nitrogen from the solid solution (SS), the microstructure of the steel keeps evolving. Transmission electron microscopy (TEM) observations, combined with chemical X-ray energy dispersive spectroscopy (EDS), allowed to attribute this result to a phase transition between metastable (Al, Cr)N precipitates of cubic structure and equilibrium AlN nitrides of hexagonal structure. The presence of chromium in the initial precipitates was interpreted by a preferential segregation of this element on the former grain or sub-grain boundaries of austenite during or after hot-rolling. Then, during the precipitation of the nitrides, clusters of segregated chromium atoms could serve as nucleation sites for the (Al, Cr)N nitrides.

Investigations on the Mechanical Behavior of Advanced High Strength Steels Resistance Spot Welds in Cross Tension and Tensile Shear

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.

Theoretical and experimental study of a device for energy repartition transformation of a laser beam by the recombination of 2 sub-beams

Presents a study of the superposition of two sub-beams obtained from a primary Gaussian beam. The authors have realized a device and demonstrated it. They utilized an analysis method from observed degradations on paper and in this way the energy beam repartitions before and after beam transformation were precisely described. Software for the energy repartition simulation was developed in the scope of the theoretical study and theory and real behaviour were compared. The fit is satisfactory.

Microstructure and Mechanical Properties of Low Carbon Al-Killed Steels after Ultra-Rapid Annealing Cycles

Ultra rapid annealing cycles were conducted on two low carbon Al-killed steel sheets differing mainly by their coiling temperatures (600°C or 700°C). For the lowest coiling temperature, the mean grain size of the steel was found to gradually decrease with an increase of the annealing temperature from 700°C to 920°C. A more complex grain size evolution was detected in the case of the steel coiled at high temperature. This led us to the conclusion that the size and the distribution of the iron carbides present before annealing, which is mainly governed by the coiling temperature, plays a very important role on the mechanisms involved in the grain refinement of extra-mild steels during ultra-rapid annealing cycles.

Theoretical and experimental studies of an optical device for transformation of laser beam in rectangular spot

In order to increase the quality of laser surface treatments of materials, the authors propose an optical device that permits the recombination of a gaussian laser beam in a square spot of variable dimensions with an homogeneous energy repartition. They calculate, neglecting interference phenomenon, the repartition of energy issued from this optical device. Taking into an account optical phenomena due to the principle of a such device (as diffraction and interference), they show precisely the theoretical studies of the energy repartition issued from a similar device leading to a rectangular spot. These studies are compared with experimental results and for different observation planes. Practical utilizations of this device during thermal treatments of metallic materials are shown.

Characterization of Metallurgical Transformations in Multi-Phase High Strength Steels by Barkhausen Noise Measurement

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.

Dual-Phase Steels Characterization Using Magnetic Barkhausen Noise Measurements

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.

Characterization of the Residual Stresses in Plastically Deformed Ferrite-Martensite Steels Using Barkhausen Noise Measurements

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.

Characterization of the Interaction of the Interstitial Atoms with the Substitutional Atoms and with the Dislocations in Extra-Mild Steels through Coupled Use of Thermoelectric Power and Internal Friction Measurements

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.

Use of diffraction fringes induced by a knife-edge to characterize a high power laser beam

In the present study, from Fresnel approximation in diffraction, we established a relation which is able to evaluate the distances between diffraction fringes induced by a knife-edge. Then, we have suggested a method to use the relation to evaluate the parameters which characterize the high power laser beam, on any point of its optical path before or after crossing an optical device.