Influence of anisotropy of mechanical properties on hydrogen induced cracking

Hydrogen induced cracking is a form of wet H2S cracking. Blistering or crack propagation is a result of the mechanical tearing of material by high-pressure hydrogen gas forming on internal material discontinuities, like non-metallic inclusions. This failure mechanism is typically associated with low and medium-strength pipeline steels, however, it does also occur in high-strength rolled wire. This evaluation aims to elucidate the mechanism of this susceptibility. The characteristic failure pattern where cracking occurs near the wire centreline and propagates perpendicular to the rolling direction leads to believe that the wire anisotropy, developed during cold rolling, plays a critical role. A mechanical property - flow resistance in principal directions - was measured using the Wheeler and Ireland technique. It was found that the "weak" direction is perpendicular to the crack propagation direction. The failure rate does not correspond to the flow resistance, but rather to the flow resistance ratios. It is proposed that those ratios are not only a measure of anisotropy but also a measure of microstructural damage inflicted by the cold rolling process. This microstructural damage is partially reversible by heat treatment processes.