scope:

The codes covering fabrication of virtually all steel structures and pressure vessels require that weldmetal

strength equal or exceed base-metal strength. Until recently, however, it was rare that consideration would be given to how much weld-metal strength in excess of basemetal strength was acceptable. There are certain instances, such as when machining of the weld is to be done, or when the weld is to see service in a corrosive environment, that an overly strong or hard weld causes problems. The particular situation of hard welds in corrosive service has caused concern in the petroleum-refining industry and has provided the motivation for the study to be described herein, which was sponsored by the American Petroleum Institute's Division of Refining, Subcommittee on Corrosion, under the direction of the Task Group on Corrosion Research. A number of failures have occurred recently in plain carbon-steel (70 ksi ultimate tensile strength or less) vessels containing among other substances, water and hydrogen sulfide, as might occur near the end of a petroleumrefining operation. Frequently, failure has consisted of transverse weld cracking in submerged-arc welds. The cracked welds are usually substantially harder than the base metal. Some instances of weld heat-affected-zone cracking are also known to occur. The cracking mechanism is variously referred to as "sulfide-corrosion cracking" "hydrogen-stress cracking," "stress-corrosion cracking," "wet sulfide cracking," and the like. A combination of high-strength metal, stress and a source of nascent hydrogen (usually a corrosion reaction) seems to be required to produce the cracking.