Effect of hydrogen on the formability and fracture behaviour of high strength multiphase steels under multiaxial loading

Advanced High Strength Steels (AHSS) are widely used in automotive applications because of their lightweight and crash potential. Nevertheless, concerning the com-ponent design their significant sensitivity to hydrogen embrittlement has to be consi-dered. Hydrogen can be introduced into the material during production, coating pro-cesses, forming due to lubricants or by contact with hydrogen within the application. In this work the influence of hydrogen on the formability and fracture behaviour of a CP1000 steel sheet under uniaxial and multiaxial tensile loading is investigated. Therefore Nakajima experiments were carried out according to DIN EN ISO 12004-2 [1] for non-charged reference specimens as well as for hydrogen-precharged spe-cimens. The precharging was performed by electrochemical charging with a hydro-gen concentration of about 3 weight ppm. For all investigated stress states the H2-precharged specimens show a significant reduction in local failure strain in compa-rison to the non-charged reference specimens. For the uniaxial and the plane strain specimens this hydrogen embrittlement did not lead to a reduction in the forming limit. Though for the equibiaxial stress state the H2-precharged specimens show an early change of the strain path in direction of plane strain deformation with a de-crease of formability compared to the non-charged reference specimens. These in-vestigations provide more insights into the hydrogen susceptibility of AHSS and should be considered in the development of lightweight structures especially re-garding the manufacturing process.