Criteria for the onset of structural integrity degradation due to high-temperature hydrogen attack of a carbon- manganese steel

Samples of a pressure vessel C-Mn steel were exposed to 46 bar pure hydrogen at 550°C in an autoclave for various exposure periods. Some samples were instrumented with a high-temperature strain gauge to track the accelerated expansion strain in order to evaluate the extent of embrittlement. Samples of all three orientations relative to the plate rolling direction were thereafter subjected to tensile testing. The results demonstrated that severe degradation of the tensile ductility occurred after experiencing expansion strains much smaller than that anticipated earlier, especially for samples in the plate through-thickness orientation. For these samples, the critical HTHA strain, as far as through-thickness tensile ductility is concerned, could be identified as being in the order of 9 με or a strain rate of 0.4 με/ h. After approximately 28 με expansion or 0.82 με/ h, through-thickness ductility degradation approached 100 %. Two other criteria for rapid HTHA degradation were also evaluated. A critical Pw-value, derived from the McKimpson and Shewmon work for low pH2 conditions of PWcr = 8.15 was determined experimentally. It was also shown that the critical ductility degradation index of ΦR = 15 %, when applied to the through-thickness sample orientation, would serve as an early indicator for rapid degradation in the bi-axial plate loading directions. was used. Comparison of the current critical values for rapid damage with those published earlier, indicate that the current values are more realistic and accurate as fair as critical strain and Pw – values are concerned. The critical strain values published earlier were found to be non-conservative as far as through-thickness property degradation is concerned.