Development of quasi-cleavage crack leading to fracture of hydrogen charged high strength steel

Nature of the critical crack, which triggers hydrogen assisted fracture of the high strength bolt steel ASTM A490 has been studied with an emphasis on the metallurgical observation of the subsurface quasi-cleavage crack. The macroscopic fracture facet, appeared on the SEM photograph of the fractured surface, is formed by coalescence of a number of neighbouring cracks, which have different initiation sites. Detailed examination with TEM and optical microscope revealed that the initiation and early stage of propagation prefer transgranular mode. The preferencial crack path in this type of high-strength steel traces mostly along the lath or packet boundary of the prior austenite grains.