Véronique Massardier

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.

An Investigation of Failure Types in High-Strength Steel Resistance Spot Welds

On the shop floor, as in laboratories, destructive testing remains the main means of quality control of spot welds. After hand or mechanized weld destruction, the so-called “plug diameter” is measured and is usually considered a good indicator of the weld quality. However, it turns out that the “plug” failure of spot welds is far from being the rule. Moreover, fracture may occur in different zones of the weld, leading to very different meanings of the plug diameter. Therefore, the plug diameter is most of the time not well-correlated with the weld strength, which is the main value of the spot weld. Partial or full interfacial failures (through the weld nugget) exhibit equivalent mechanical strengths and therefore should not be rejected. Through this work, it is shown that the failure type depends on various parameters (nugget diameter, sheet thickness, loading mode, …), and consequently, it is not an intrinsic property of the steel grade. In this way, recommended quality criteria are based on weld strength, weld diameter (including the interfacial fracture area if any), or absorbed energy independently of failure occurrence. In addition, non-destructive techniques might be key investigation methods and have to be developed further.

Modeling of the Recrystallization and Austenite Formation Overlapping in Cold-Rolled Dual-Phase Steels During Intercritical Treatments

Austenite formation kinetics of a DP1000 steel was investigated from a ferrite–pearlite microstructure (either fully recrystallized or cold-rolled) during typical industrial annealing cycles by means of dilatometry and optical microscopy after interrupted heat treatments. A marked acceleration of the kinetics was found when deformed ferrite grains were present in the microstructure just before austenite formation. After having described the austenite formation kinetics without recrystallization and the recrystallization kinetics of the steel without austenite formation by simple JMAK laws, a mixture law was used to analyze the kinetics of the cold-rolled steel for which austenite formation and recrystallization may occur simultaneously. In the case where the interaction between these two phenomena is strong, three main points were highlighted: (i) the heating rate greatly influences the austenite formation kinetics, as it affects the degree of recrystallization at the austenite start temperature; (ii) recrystallization inhibition above a critical austenite fraction accelerates the austenite formation kinetics; (iii) the austenite fractions obtained after a 1 hour holding deviate from the local equilibrium fractions given by Thermo-Calc, contrary to the case of the recrystallized steel. This latter result could be due to the fact that the dislocations of the deformed ferrite matrix could promote the diffusion of the alloying elements of the steel and accelerate austenite formation.