Hydrogen Embrittlement of Commercially Produced Advanced High Strength Sheet Steels

The susceptibility of Advanced High Strength Steels (AHSS) to hydrogen embrittlement (HE) was evaluated on selected high strength sheet steels (DP 600, TRIP 780, TRIP 980, TWIP-Al, TWIP, and Martensitic M220) and the results were compared to data on a lower strength (300 MPa tensile strength) low carbon steel. Tensile samples were cathodically charged and then immediately tensile tested to failure to analyze the mechanical properties of the as-charged steel. The effects of hydrogen on deformation and fracture behavior were evaluated through analysis of tensile properties, necking geometry, and SEM images of fracture surfaces and metallographic samples of deformed tensile specimens. The two fully austenitic TWIP steels were resistant to hydrogen effects in the laboratory charged tensile samples. In contrast, with an increase in hydrogen content, all of the ferritic steels exhibited a decrease in tensile ductility and a transition in failure geometry from normal ductile fracture associated with localized through-thickness necking in flat tensile samples, to a more brittle fracture along a plane perpendicular to the maximum tensile stress. The dominant fracture surface feature observed for the steels susceptible to HE was a transition from ductile void coalescence when uncharged to transgranular cleavage in the charged condition; additionally, the fraction of the fracture surface that exhibited cleavage fracture increased with an increase in diffusible hydrogen content. Metallographic analysis showed the presence of internal voids and cracks that were primarily associated with second phase particles and increased in number with an increase in hydrogen content. Martensite, in particular untempered martensite in some deformed TRIP steel samples, appeared to be the constituent most susceptible to hydrogen content.