A constitutive equation for the kinetics of high temperature hydrogen attack and its use for structural life prediction

The damage due to high temperature hydrogen attack (HTHA) accumulates over time through the formation, growth, and coalescence of methane-filled cavities, followed by microcrack and macrocrack formation and presenting a threat to structural integrity. Early attempts at the prevention of this damage mechanism from occurring in steels in hydrogen service at high temperatures were based on the first publication of the Nelson Curves in 1949 but have not always been successful. The consequence of HTHA failures have often been disastrous and much work is currently being done with a view to better prediction of the course of the HTHA damage process and assessment of structural integrity in the face of HTHA. Current work coordinated by the WRC is based on the calculation of the dynamic methane pressure in grain boundary cavities, since the methane pressure provides the driving force for cavity growth and damage evolution. The WRC work makes use of a Methane Factor, α, which is material and microstructure specific. It also utilizes a concept of an "additional stress", Δσ, due to the internal methane pressure. In this paper, high-temperature strain gauging was used on samples exposed to accelerated HTHA conditions in an autoclave. The strain gauges were applied to various zones of a C-0.5Mo welded joint in order to track the damage evolution in the base metal, HAZ and weld metal. Over time, the process of HTHA damage gave rise to steadily increasing strain due to the formation and growth of voids and the subsequent HTHA damage processes, up to a saturation point, beyond which the strain remained static. These curves were subsequently converted to Damage Fraction - Time curves, with the Damage Fraction ranging from 0 to 1, similar to the MPC-Omega creep curves used in API 579. It was found that the kinetics of HTHA damage evolution could be described by a single sigmoidal equation with different constants for the various weld zones and for different steels. The results of published strain-time curves for a carbon steel were similarly analyzed and similar findings were made. Analysis of the HTHA damage evolution rate vs time curves, indicate that the damage rate steadily increases with increasing time up to a maximum value, where after it decreases steadily up to the point where the damage rate is zero, at the end of the HTHA damage evolution process. The time corresponding to maximum damage evolution was generally found to be midway through the HTHA damage process. This behavior was attributed to the depletion of carbides with increasing damage evolution, resulting in declining damage rates at times beyond the time corresponding with the maximum damage rate. It is shown how the HTHA damage curves can be compared to experimentally determined or calculated creep damage curves in order to determine the "additional stress" and Methane Factor terms, used in the WRC HTHA structural integrity assessments. It was furthermore discussed how the kinetic equations formulated can be used in conjunction with the in-situ application of high-temperature strain gauges to various structural elements in HTHA service, can be used as an aid in HTHA life assessment.