Hydrogen-promoted fracture in transportable gas-container steels

Abstract

Table 1 Typical analysis of hydrogen from filling plant, vpm* and 95 MN/m3/2• The specimens were suspended in a standard 1·98m long, 230 mm dia. hydrogen gas container filled to 22·7 MN/m2 and left for two weeks. A typical analysis of hydrogen from the filling plant is given in Table 1. Loading the pre-cracked specimens outside the hydrogen environment has the disadvantage that the oxide layer which protects the surface from direct contact with hydrogen will remain unbroken. If loading in hydrogen were performed, much more rapid hydrogen crack initiation could be expected, as hydrogen will gain access to clean metal slip steps at the tip formed during loading.3 In the interests of simplicity, however, the former course was adopted on the assumption that hydrogen diffusion through the oxide layer would result in eventual initiation. It is also possible that stress relaxation in parts of the loading bolts caused by logarithmic creep could cause sufficient reverse plasticity at the crack tip to initiate a hydrogen crack. Upon removal from the container, it was found that crack extension had occurred in the three specimens preloaded to the highest levels of stress intensity. The specimens were broken open by fatigue cycling in air to extend the crack and then overloading. The portions of fracture face formed in hydrogen could clearly be seen. No crack growth in hydrogen could be detected in the specimens loaded to 43 and 57 MN/m3/2, and the cracks in specimens preloaded to 71 and 95 MNfm3!2 had deviated out of a plane perpendicular to the loading axis; the stress intensities subsequent to the initiation of hydrogen crack growth could not be determined. Crack-length measurement and determination of the load in the remaining specimen immediately after removal from the container enabled the stress intensity at the tip of the extended crack to be determined as 50 MN/m3/2• Clearly, this is not the minimum value for crack extension, as there was no means of knowing if the crack had stopped growing. In addition, the estimate of the load was probably high, since some plastic deformation would have occurred at the crack tip during the initial loading and would make a contribution to the opening at the crack mouth that would be retained after the crack extended in hydrogen; this would produce a larger measured value for the opening at the crack mouth than would be appropriate for the actual load. The absence of crack growth in the two specimens tested at stress intensity values of 43 and 57 MN/m3/2 suggests that there is an incubation period before hydrogen-assisted crack growth can occur, and that the period was longer