Evaluation of the hydrogen embrittlement susceptibility in DP steel under static and dynamic tensile conditions

Hydrogen (H) induced mechanical degradation is studied in DP steel by performing tensile tests under static and dynamic conditions. Tensile specimens were electrochemically H charged and tensile tests were done ex-situ after charging. Different H contents were charged into the samples by modifying the current density. The strain rate is increased from static (1.67*10−2 and 1.67 s−1) to dynamic (450 and 900 s−1) conditions to verify the effect of H diffusivity during the tensile tests on the hydrogen embrittlement (HE) susceptibility. Therefore, a reproducible methodology was established by using a standardized tensile machine for static testing and split Hopkinson bar experiments for dynamic conditions. The HE degree increased with current density due to higher amount of H, as confirmed by melt extraction. The HE% also increased with slower strain rates since H was able to diffuse to a crack tip, hence accelerating failure. Even at the highest strain rate (900 s−1), the material lost about 10% of its ductility due to H present in the sample and not because of H diffusion during the test. This was concluded since H induced brittle failure initiated at the edges of the samples at slow strain rates. Though at a strain rate of 1.67 s−1, fracture initiated in a ductile way from the center similarly as for tests performed without charging. Fractographic visualization of the fracture surfaces revealed an embrittled central line when charged with H, which evolved into a major crack. MnS inclusions were found in this central line accounting for the H induced crack initiation.