L51935 Limitations of Cellulosic-Coated Electrodes

Cellulosic-coated electrodes (primarily AWS EXX10-type) are traditionally used for stovepipe welding of pipelines because they are well suited for deposition of one-sided welds and are capable of high deposition rates when welding downhill. Despite advances in mechanized welding technology and the development of low-hydrogen consumables suitable for pipeline girth welding, manual welding, using cellulosic-coated electrodes is still widely utilized for pipeline construction throughout the world. Several incidents involving significant hydrogen-assisted cracking in the weld metal of pipeline girth welds made using cellulosic-coated electrodes have been reported recently. Two of these cases required removal of many welds at an expense of over $4 million dollars, in spite of established procedures having been used. The objectives of this project were to identify the primary mechanisms contributing to transverse cracking of field welds and to provide recommendations regarding safe preheat/interpass temperatures that should be utilized when welding heavy-wall pipe and fittings using cellulosic-coated electrodes. Two cases of severe weld metal hydrogen-assisted cracking were investigated. In each case, the composition of the weld metal was substantially richer that would typically be expected for E8010-G electrodes. Subsequent investigation into factors influencing the composition of weld metal from cellulosic-coated electrodes revealed that arc length has a pronounced effect on carbon, manganese, and silicon recovery. The increase in composition observed with variation in arc length could not explain the extremely rich compositions observed in the cracked girth welds, however. Subsequent investigation demonstrated that it was possible to effectively double the manganese concentration and triple the silicon concentration when using cellulosic coated electrodes that have low coating moisture content.