Less appreciated is the long term effect of hydrogen with cracking of high strength steels and ferrous alloys under atmospheric exposure conditions. Whether absorbed hydrogen causes cracking or not is a complex interaction of material strength, external stresses and temperature. We have recently analyzed fasteners where failure was attributed to hydrogen embrittlement cracking. A common denominator in the failures is that the fasteners had been in service many years before failure. Macroscopic evidence of hydrogen embrittlement normally includes a rough, brittle-appearing fracture surface, usually with a single origin, although multiple fracture origins can be present.
Fractures typically occur in corroded thread roots.
Another characteristic of hydrogen embrittlement cracking is that fractures do not necessarily occur in the last engaged thread as do fatigue fractures. In fact, fracture can occur inside the nut. SEM examination of fracture surfaces typically show intergranular crack origins, with the final fracture areas exhibiting any combination of intergranular, cleavage, or ductile overload features. Metallographic examination of failed fasteners will normally not show any secondary hydrogen cracks, although subsurface cracking at inclusions has been observed. These standards specify greater maximum allowable hardnesses than other common fastener specifications, i.e., A325, A193, A307 and A449.