Environmentally Assisted Cracking of UNS N07718 in Salt Water with Cathodic Protection—Effect of Strain Rate and Processing on Hydrogen Embrittlement

Abstract Alloy 718 is commonly used in the upstream oil and gas industry for multiple applications that are exposed to wellbore fluids and seawater with cathodic protection (SWCP). The alloy is prone to a hydrogen-induced stress cracking (HISC) risk if the influencing factors are not carefully selected and designed. The influence of strain rate on the threshold stress intensity factor, Kth was evaluated using two heats obtained from slow rising displacement (SRD) and constant load K tests. The results exhibited a wide variation in Kth values. The inferior heat exhibited more intergranular cracking compared with the more resistant heat, which exhibited slip band cracking. For both heats, the SRD tests resulted in Kth values that were lower than those from the constant K tests. Strain rate effect was also evident on the measured crack growth rate (CGR), with the CGR in the SRD tests being significantly greater than in the constant K tests for both heats. The sensitivity to the strain rate was evident in the fatigue crack growth rate (FCGR) response, which increased with decreasing frequency and did not exhibit any evidence of a plateau even at low frequencies. Further work is necessary to understand the differences between the processing and microstructural features of the heats that might be responsible for the widely different HISC responses.