High temperature hydrogen attack (HTHA) is a material degradation and cracking mechanism caused by reduction of carbides within steel, resulting in the substitution of carbides by bubbles of methane gas. The strengthening effect of carbides is lost, and the material is further weakened by the presence of gas filled voids, which may link up to form fissures or cracks.
HTHA is seen in steels which are operating in hydrogen rich atmospheres at temperatures which are high enough for molecular hydrogen to dissociate and dissolve into the steel, and to react with the carbides. The risk of HTHA is managed in industry by guidelines presented in APR RP 941. In this document, plots of temperature vs. partial pressure of hydrogen are presented, which are populated with data on occurrence of HTHA and also successful operation without HTHA, which have been collected principally from refinery operation since the 1940s. The plots were originally produced by G. A. Nelson and the boundaries between regions with and without HTHA are known as ‘Nelson Curves’.
Recent incidents of HTHA under conditions which were expected to be safe according to the Nelson Curves have been attributed to the effects of residual stress. There are other possible contributory factors, however, which have received insufficient attention. In particular, the composition, stability and distribution of carbides in weld heat affected zones (HAZs), and the possible contribution of dissociation of gasses other than H2 (such as H2S) to hydrogen within the steel.
In an initial project, the effects of H2S on HTHA will be explored by comparing the effects of different gas mixtures on steel samples in a simple furnace. In future work the effects of simulated and/or real HAZs could be explored. The use of simulated HAZs would allow the possible effect of residual stress to be eliminated, so that microstructural effects could be investigated. The effect of stress can be explored, but would require development of equipment capable of applying a load to a specimen within a high temperature, pressurised hydrogen atmosphere.