Study of the hydrogen introduction in bare and Al-Si coated steels during the hot stamping process

The development of new ultra high strength steels is driven by the economical and ecological challenges of the automotive industry coupled to safety considerations. The hot stamping process of Al-Si coated steels ensures these strength levels and, moreover, allows an easy forming of the material without the need of a protective atmosphere. At high temperatures, the coating is hydrogen-permeable, while at low temperatures (below 120°C), it becomes tight to hydrogen. In this context, the aim of this work is to understand how hydrogen enters in the material during the hot stamping process with respect to different atmospheric sources. An important issue is the continuous evolution of the material during the process: as temperature increases, the initially solid coating first melts and then forms intermetallic compounds Al-Fe-Si. The influence of different hydrogen sources has been studied by the use of dihydrogen, water vapour and deuterated water vapour. The diffusive hydrogen content in coated samples as a function of the annealing conditions (time and atmosphere humidity) has been quantified owing to thermal desorption analyses (TDA) and compared to bare steel, considered as a reference for this work. Moreover, the interactions between oxidation, decarburization and hydrogen uptake in bare steel are considered.