On the interaction mechanisms between atmospheric sources of hydrogen and Al-Si coated or bare high strength steels during the hot stamping process

The economic and ecological challenges of the automotive industry coupled to safety considerations involve lightweight design based on the development of new ultra high strength materials. The hot stamping process of Al-Si coated steels ensures these strength levels and, moreover, allows an easy forming of the material without the necessity of a controlled 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 aspect of the problem is the continuous evolution of the material during the hot stamping process. At ambient temperature, the coating is indeed a solid Al-Si alloy, which has weakly reacted with iron during deposition. As temperature increases, this coating first melts and then forms ternary intermetallic compounds Al-Fe-Si. As a first step in the study of the different sub-systems, bare steel has been considered. The influence of the different hydrogen sources has been studied: water vapour, which is probably the active source in the industrial process and dihydrogen, a more fundamental study case. Heavy water vapour (D2O) is also used to ensure a controlled atmosphere in the furnace, while it allows to avoid misinterpretation that could arise with the dissociation of water into hydrogen. In the case of water and heavy water vapours, another important parameter to characterize the humidity of the atmosphere is the dew point, which has been varied between two temperatures, -10°C and +15°C. Thermal desorption analyses (TDA) have permitted to quantify the diffusive hydrogen present in the material with respect to those different atmospheres. Metallographic observations of the surfaces of the samples and glow discharge optical emission spectroscopy (GDOES) analyses have also been correlated with the atmosphere parameters to understand how the equilibrium between atmospheric and diffusive hydrogen can be established