Development of Fitness-for-Service Rules for the Assessment of HIC and SOHIC Damage in API 579-1/ASME FFS-1

Hydrogen induced cracking (HIC) a form of wet H2S cracking that has a stepwise (through-thickness crack linkage) nature that is created from laminar (in-plane) cracking. HIC damage is initiated when atomic hydrogen generated by a surface corrosion reaction permeates into the steel and combines at non-metallic inclusions, discontinuities or other imperfections forming hydrogen molecules that are too large to diffuse out of steel. Lower strength carbon steel plate is most susceptible to hydrogen blistering and HIC damage, especially when exposed to an aqueous hydrogen environment, such as sulfide, cyanides, and hydrofluoric acid. This mechanism typically occurs below 300°F and is common in the oil and gas and petroleum refining industries, particularly with older high sulfur or phosphorous steels. HIC damage typically has not lead to breaches of containment in pressurized equipment. Stress oriented HIC (SOHIC), much rarer than HIC, is the arrangement of small HIC cracking perpendicular to the stress in a stacked array in stee and is typically located in the residual stress fields of welds. SOHIC has lead to breaches in containment. This paper focuses on the FFS assessment of HIC and SOHIC damage and describes some of the background to the new rules in Part 7 of API 579-1/ASME FFS-1 issued in 2007. It discusses laboratory tests undertaken on steel removed from a vessel that suffered severe HIC damage in service, but did not fail or leak. The results of those tests indicated that HIC damaged steel has considerable remaining strength and this lead to the new rules in API 579 Part 7. Because of the difficulty of ascertaining the presence of SOHIC when HIC damage is present, an additional crack-like flaw check is required to ensure that the HIC damaged area is acceptable as a crack-like flaw (Part 9) in addition to being acceptable as a locally thin area (Part 5) with a slight credit for remaining strength.