Fractographic analysis of the role of hydrogen diffusion on the hydrogen embrittlement susceptibility of DP steel

The present work investigates the phenomenon of hydrogen embrittlement in dual phase steel by evaluating tensile tests performed on both hydrogen saturated and non-saturated samples together with a thorough investigation of the role of hydrogen diffusion. First, a hot/melt extraction was done to study the evolution of the hydrogen level up to saturation as a function of the electrochemical charging time. The results were correlated with in-situ hydrogen charged tensile tests performed after different pre-charging times. An increasing ductility loss was observed with longer pre-charging times up to a maximal 50% ductility loss for hydrogen saturated samples. Furthermore, changing the cross-head deformation speed allowed evaluating the effect of hydrogen diffusion. It was clearly demonstrated that the hydrogen sensitivity increased when lowering the test speed, since hydrogen could diffuse to critical regions ahead of the crack tip. Lastly, different cross-head deformation speeds were also applied on non-pre-charged samples. Here, hydrogen charging just started at the same moment as the actual tensile test. Fractography allowed to correlate the features of the fracture surface with the hydrogen diffusion distance during the test.