Characterisation of microstructure evolution and hydrogen permeability of Alusi coating on hot stamped steel

In the automotive industry, one of today’s most important challenges is the weight reduction of the vehicle. In this respect, car manufacturers have developed high-strength steels and new forming techniques allowing to produce thinner components for the car body with equivalent crash resistance. The problematic of hydrogen embrittlement has become a problem for the development of these high-strength steels. This phenomenon has led to take more consideration to the delayed fractures induced by diffusible hydrogen particles inside the steel. A practical exemple is found in hot stamped Usibor steel. This boron steel have an anti-corrosive coating made of aluminium and silicone, named AluSi. Before hot stamping, the steel sheets need to undergo an austenitisation process during which the AluSi coating goes through several phase transformations. The results obtained in this master thesis thanks to these different techniques allowed to draw some conclusions regarding the evolution of the coating structure during heat treatments but also regarding the interaction between this structure and hydrogen diffusion. Specimens have been heat treated at 650C, 800C, 830C and 900C. Then, various techniques of characterisation have been used for the phase identification: Scanning Electron Microscopy (SEM), Electron-dispersive X-ray spectroscopy (EDX), Glow Discharge -- Optical Emission Spectroscopy (GD--OES) and nanoindentation. As well as techniques for analyses linked to hydrogen: Thermal Desorption Analysis (TDA) and hot gaseous permeation. A special attention has also been given to the exploration and improvement of preparation techniques. Different alternatives for the mounting using Epoxy resin have been investigated as well as a substitute in the form of a metallic clamp.