Hydrogen distribution in 3rd generation high strength steels and its effect on mechanical properties
Résumé
Third generation steels are multiphase steels that contain high amounts of residual austenite that can transform into martensite during deformation and provides the steel a better ductility/formability. During their final elaboration steps, they are generally heat-treated in reducing atmospheres and potentially submitted to zinc electroplating, processes during which they may absorb a significant amount of hydrogen. Hydrogen embrittlement susceptibility of these steels is therefore to be quantified. Two 3rd generation steels have been studied an austenitic-ferritic MnAl steel and an austenitic-martensitic quenched and partionned (QetP) steel. Both were elaborated and finally heat treated under controlled reducing atmosphere (high temperature HT), using deuterium as isotopic tracer for hydrogen. Hydrogen introduction in the material during zinc plating was simulated by cathodic charging (low temperature LT). Thermal desorption mass spectrometry (TDMS) was used to study diffusion and trapping evolution in such complex materials; the localization of deuterium in the different phases was studied by nanoSIMS. The effect of hydrogen/deuterium on mechanical properties was investigated by crossed tensile tests and TDMS. Aging after deuterium charging followed by TDMS evidenced for MnAl and QetP alloys two contributions (570 K and 630 K) assigned to low energy trap sites. Besides, the QetP alloy exhibits an extra contribution around 730 K, whose assignment will be discussed with respect to nanoSIMS analysis. Modelling of the TDMS spectra, taking into account the microstructure of the materials and their history in terms of hydrogen exposure and aging, will give elements for the discussion.Finally, based on tensile tests and fractography analysis, the effect of deuterium or hydrogen on the mechanical behavior of these materials will be discussed in terms of hydrogen origin (HT or LT), concentration and localization.