Assessing Hydrogen Assisted Cracking Modes In High Strength Steel Welds PDF Download

Approximately ten years ago, carbon steel was replaced by duplex stainless steel (DSS) to fabricate the reactor effluent air cooler (REAC) of hydrocracker units in order to improve the performance and service lifetime of these units. Unfortunately, several catastrophic failures from around the world have been reported in REAC units constructed of DSS, most within five years of service. Based on failure analysis reports, the failures were generally associated with welded joints and were caused by crevice/pitting corrosion and stress corrosion cracking. Given the condition of hydrogen-rich environment, high-pressure process fluid, and service temperature, this type of cracking is most likely a form of hydrogen assisted cracking (HAC). It is highly influenced by phase balance (ferrite/austenite) after welding and welding procedures, with high levels of ferrite in the weld metal or HAZ increasing the susceptibility to HAC. In this study, different weld metal phase balances were prepared by autogenous gas tungsten arc welding (GTAW) for using different welding parameters and shielding gases. The delayed hydrogen cracking test (DHCT) was used to evaluate the effects of the weld phase balance on the susceptibility to HAC in DSS 2205 welds. Using this approach, weld metal ferrite levels on the order of 90 vol% ferrite led to very rapid failure, while reducing the ferrite level to approximately 50-60 vol% greatly increased resistance to HAC. Fractography was performed using a scanning electron microscope (SEM) and showed that brittle fracture morphologies occurred in the higher ferrite pass of overlapping two pass welds for each DHCT sample. A mixture of quasi-cleavage and intergranular fracture modes occurred during the crack nucleation and propagation process, and final sample failure was caused by overload exhibiting a microvoid coalescence fracture mode. The failure mechanism closely reproduced the actual service failures in REAC welds. For different phase balances in HAZ, the HAZ samples were simulated over a range of cooling rates by Gleeble® 3800 system. It was found that the microstructure had significantly higher ferrite content with faster cooling rate controlled by different free span distances. The same approaches of DHCT and fractography to evaluate the susceptibility to HAC will be used for the simulated HAZ samples. The additional testing will be needed to examine the reproducibility of DHCT and to establish guidelines for the maximum ferrite content in 2205 weld metal and HAZ that will prevent service failures. In summary, the DHCT exhibited high sensitivity and good reproducibility in determining the effect of weld metal ferrite content on HAC susceptibility in autogenous GTA welds of DSS 2205, and it can be an effective method to evaluate the effect of ferrite/austenite balance on the susceptibility to HAC in both the weld metal and HAZ.