Processing of industrially produced high strength fasteners has been investigated with respect to hydrogen embrittlement. Fasteners were made of bainitic carbon steels Ck67 (UNS G10700) and CK67 modified with 0.45 % Mo (Ck67Mo, no UNS no.), respectively, with a hardness of 58 HRC each. Investigated industrial scale process steps are heat treatment including austenitizing and subsequent tempering in a salt bath as well as zinc-galvanizing. Heat treated specimens with different hydrogen contents charged electrolytically in laboratory were also tested.
Three-point-bending-tests of fasteners were done in a salt spray chamber. Various bending stresses up to yield strength were applied. Cracking times of 20 simultaneously tested specimens for each studied parameter were statistically evaluated and time to failure cumulative distributions were determined. Hydrogen content of specimens was
measured before and after testing by hot extraction method after freezing samples in liquid nitrogen. Fracture surfaces were investigated by SEM and microstructure was characterized by optical microscopy. Results are that cracking starts only above a critical hydrogen content around 1 ppm
at yield strength. At stresses below yield strength levels no cracking did occur. The influence of initial hydrogen content on cracking is discussed as well as that of bending stress level. Molybdenum alloyed steel has a significantly higher susceptibility to hydrogen embrittlement.
Keywords: hydrogen embrittlement, HIC, hydrogen induced cracking, high strength bainitic steels, Ck67, Ck67Mo, diffusion