We were asked to perform a failure analysis on numerous ASME Section VIII Code Pressure Swing Absorber Vessels were found to contain about 1300 chevron cracks.
The vessel materials were studied in addition to reviewing the detailed requirements of the ASME Boiler and Pressure Vessel Code, Section VIII, Div. 1 and 2, British Code 5500, as well as technical literature on hydrogen induced cracking (HIC), and radiographic & ultrasonic inspection technologies.
A leak was found in the nozzle weld of a pressure swing absorber (PSA) vessel at a hydrogen plant. The leak was in a SA 350 LF2 carbon steel forged outlet nozzle in the bottom head of a vessel. A full penetration butt weld joined the nozzle to the SA 516 Grade 70 carbon steel head 2.4 inches thick. The materials of construction and design were entirely appropriate for the intended service and there were no exceptionally difficult fabrication problems.
SA 516 Grade 70 steel is commonly used in safety related welded pressure vessels because of its mechanical properties including strength, ductility and toughness. SA 350 LF2 forgings are normalized and have high toughness at low temperature. The Submerged Arc Welds were made using an electrode/flux combination with high notch toughness and low hydrogen. SMAW repairs were made with low hydrogen E7018 electrodes. Rejectable radiographic indications in the Submerged Arc Welded (SAW) groove weld had been gouged out and repaired by Shielded Metal Arc Welding (SMAW) using an E7018 electrode.
In order to assure that other PSA vessels could safely continue to be used, a major inspection, evaluation, and repair program was initiated. This effort included shear wave ultrasonic examination of welds in 95 PSA vessels. About 1300 hydrogen-induced chevron cracks were found in the deposited weld metal of 78 PSA vessels. The vessels were evaluated for Fitness-for-service, taken out of service, repaired or continued in service with periodic inspection, depending on the results of the API-579 Fitness for Service fracture mechanics evaluation.
Extensive investigation of the cracking was conducted, including UT, RT, and Magnetic Particle (MP) examinations; a joint test protocol for destructive testing on boat samples, chemical composition analyses, metallographic evaluations, and fractographic analyses. Numerous light micrographs and scanning secondary-electron and back-scattered electron images (SEI and BSEI) of good quality were obtained at various magnifications. These images and photographs were quite definitive in identifying the root cause of the cracks. Additionally, similar cracks were found in vessels which had seen no service.
Multiple microcracks exhibiting a staircase pattern unique to hydrogen-induced chevron cracks were found. These transverse weld cracks were inclined at 45 degrees to the weld surface in the direction of welding or against the direction of welding. The secondary cracking and multiple quasi-cleavage features connected by ductile shear fractures are also characteristic of hydrogen-induced chevron cracks.