Hydrogen Embrittlement Evaluation of Different Heats of AISI 8630 Steel in Subsea Applications

Abstract

Abstract AISI 8630 steel used in subsea applications is subject to hydrogen embrittlement from externally applied cathodic protection. In this work, susceptibility variation on three grades of 8630 steel of different yield strength (YS), 100 ksi, 110 ksi and 120 ksi, were evaluated in terms of fracture toughness. Two different testing methods, slow rising displacement and incremental step loading testing, were used to generate J-R curves and determine the fracture toughness. The tests were performed at 40°F in air and simulated sea water (3.5 wt.% NaCl) with applied CP of −1050 mV SCE. The normalized fracture toughness values are discussed in this work. In all three grades, regardless of the test methods, the environmental toughness under CP conditions were significantly lower than the in-air values indicating that they are susceptible to hydrogen embrittlement. Effect of YS on the hydrogen embrittlement was also observed. The incremental step loading technique showed that Jth and the J-R curve slope decreased as the YS increases. The rising displacement method also clearly showed the lowest toughness and the flattest J-R curve slope in the highest strength grade (120ksi) despite showing similar performance between the 100ksi and 110ksi grades. The true stress-true strain analysis showed different strain hardening behavior among the three grades that may suggest the strain hardening plays a role in influencing the hydrogen embrittlement behavior among the grades. Overall, slight variation could be observed in the results from the two test methods which, however, did not appear to be biased against each other in determining lower bounds of the results.