Abstract
In the automotive industry, current economic and ecological challenges drive towards a mass reduction of the vehicles and, definitely, towards the development of high strength steels. An interesting shaping solution to combine high strength and formability is the hot stamping process, which consists of austenitizing followed by a simultaneous quench and shaping in a press. 22MnB5 steel, commonly coated with Al-Si alloy, is a well-known candidate for such process. During this process, steel is exposed to non-controlled atmospheres involving hydrogen-containing sources (such as water vapour). Such atmosphere can lead to a hydrogen uptake, which could result in hydrogen embrittlement (HE) of the material in case of severe load. Furthermore, the sensitivity to hydrogen embrittlement of these high strength steels constitutes a brake for their future development.
In this context, a major issue consists in preventing hydrogen uptake. If further heat treatments exist to promote hydrogen outgassing, they are not necessarily affordable.
Deep trapping is also a well-known solution but requires an alteration of the steel grade. As an alternative, the deposition of thin coatings to prevent the hydrogen absorption and/or diffusion appears as a third promising approach.
In this regard, this work aims at studying the effects of several coatings on the hydrogen uptake during austenitization in atmospheres containing water vapour. Coatings are deposited by physical vapour deposition (PVD) before austenitization of the material and the subsequent hydrogen content of the material is measured by thermal desorption analysis and compared to an uncoated reference material (i.e. standard Al-Si coated steel). A special focus is made on the understanding of the barrier mechanisms for the most promising solutions, enabled by further SEM, GDOES and XPS characterizations.
Based on the investigation of various materials, an optimum has been found in our context with a layer composed of Ni and Cr. Since the coating covers all the surface, an optimum thickness has been found at 200nm reaching better than 50% reduction of H uptake