Comparative evaluation on hydrogen damage behavior of two martensitic high strength press hardening steels.

High strength press hardening steels have a broad perspective of application for automobile light-weight technique. However, they also face a huge challenge of hydrogen related mechanical degradation as atomic hydrogen intrudes into products during their production or in service. Two press hardening steels with different strengths were studied comparatively under the conditions of high hydrogen fugacity without external loading to evaluate their susceptibility to hydrogen embrittlement (HE). In combination with the microstructure characterization through TEM and EBSD, the results showed that these two steels display different surface hydrogen damage modes, e.g., hydrogen blistering for the steel with strength of 1000 MPa and hydrogen-assisted cracking for the steel with strength of 2000 MPa, although their damage sensitivity ratio largely obeys the tradeoff relation between the strength and HE susceptibility. Internal hydrogen pressure is a necessity for formation of hydrogen blisters because the pressure should provide a driving force for plastic deformation of blister surface, which indicates that the hydrogen blistering has some difference in HE mechanism compared to the hydrogen-assisted cracking. The main crack propagation mode is transgranular cracking for both steels, and the crack path concentrates on the {110} slip plane, especially along the block/lath boundaries, which is indicative of the inter-lath decohesion cracking. Therefore, it can be concluded that in the absence of applied loading the intra-variant boundaries have a higher HE susceptibility compared to the prior austenite grain boundaries. • Different surface damage modes (e.g., hydrogen blistering or cracking) occurred on their surface of two PHSs. • Inter-lath decohesion cracking along {110} slip plane is a common mode for crack propagation. • Plastic deformation of hydrogen blistering is controlled by internal hydrogen pressure. • Hydrogen cracking for the PHS2000 forms by a coupled mechanism of HESIV and HEDE. [ABSTRACT FROM AUTHOR]