Abstract
Press hardening applications are continuously increasing in the automotive industry for reinforcement parts and Aluminium-silicon (Al-Si) coated TS2000MPa grades are on serial cars since 2020 ensuring anti-intrusion crash performance and providing up to 10% weight reduction compared to the 1st generation of press-hardening steels TS1500MPa. As stresses are induced by part assembly operations, under extreme conditions, hydrogen induced delayed fracture risks could be triggered. Particularly for Al-Si coated press hardened parts the sensitivity is enhanced locally at the substrate surface with a hardness peak developed by recarburization phenomena. This is consequence of the growth of Al-Fe interdiffusional layer (IDL) during the hot stamping high temperature treatment. Moreover, diffusible hydrogen introduced into the steel during this treatment becomes unable to be released after die quenching because of the low temperature barrier effect of the alloyed Al-Si coating.
To minimize hydrogen induced delayed fracture risks, in this work, investigations were directed towards a tailored surface design adopting testing methodologies closely representative of the application conditions. Results are presented on the impact of surface softening induced by processing conditions prior to press hardening favoring surface decarburization. Testing methodologies were focused on properly assessing the behavior of the surface layer under conditions simulating applied assembly strains. Plane strain at the surface was simulated by Four Point Bending tests and the results clearly revealed both the high sensitivity of the embrittlement resistance to the substrate surface and the efficiency of the proposed solution. These are reported along with a comparison with three point bending tests with dominant expansion strains at the surface.