On the hydrogen embrittlement mechanism of 2 GPa-grade press-hardened steel at various strain rates: Experiments and modeling

Abstract

Hydrogen embrittlement (HE) in 2 GPa-grade press-hardened steel (PHS) has posed a great risk to its lightweighting application in automotive crash-resistant components. While conventional slow strain rate tensile tests show that the precharged hydrogen concentration of 3.5 wppm induces a severe loss in strength and ductility, the high strain rate tests conducted at 1–10³ s⁻¹ that simulate the crash condition demonstrate no loss in strength and a minimal loss in ductility. Such strain rate dependency cannot be exclusively explained via hydrogen diffusion and redistribution to susceptible prior austenite grain boundaries, as the tensile testing of precharged samples with jumping strain rates offers a sufficient redistribution period at slow-strain-rate loading, but does not necessarily lead to a high level of HE afterwards. Detailed fractography analysis acknowledges that hydrogen-induced microcracks nucleated within early deformation stages are directly responsible for the high HE susceptibility of all test conditions. A phase-field simulation comprising 2 GPa-grade PHS's microstructure features and the hydrogen diffusion under tested loading conditions is applied. The calculation reveals that the hydrogen redistribution behavior is spatially confined to the crack tip areas but to a much greater extent. It thus facilitates continuous crack growth following the main crack with minimal plastic deformation and avoids branching to form secondary cracks. The combined experiments and modeling highlight the vital role of microcracks in the HE performance of 2 GPa-grade PHS, upon which the safety factor of HE in high-strength martensitic steels shall be established.