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Evaluation of Hydrogen Cracking in Weld Metal Deposited Using Cellulosic-Coated Electrodes
  • Published Date:
    2009-06-16
  • Language:
    English
Filetype[PDF-1.56 MB]


Details:
  • Corporate Creators:
  • Publication/ Report Number:
    PR-185-04508 ; 46354CAP/47964GTH ;
  • Resource Type:
  • Geographical Coverage:
  • TRIS Online Accession Number:
    01648529
  • Edition:
    Final Report, Rev. 1
  • Corporate Publisher:
  • Format:
  • Abstract:
    Cellulosic-coated electrodes (primarily AWS EXX10-type) are traditionally used for "stovepipe" welding of pipelines because they are well suited for deposition of pipeline girth welds and are capable of high deposition rates when welding downhill. Despite advances in mechanized welding technology, development of low-hydrogen self-shielded flux-cored arc welding consumables, and substantial improvement of basic-coated low-hydrogen vertical-down shielded metal arc welding electrodes, manual pipeline welding using cellulosic-coated electrodes is still widely utilized throughout the world. Cellulosic-coated electrodes are also used for critical applications in offshore pipeline construction such as tie-in welds and repair welds. Several incidents involving significant hydrogen-assisted cracking in the weld metal of pipeline girth welds made using cellulosic-coated electrodes have occurred in recent years. Two of these cases required removal of many welds at an expense of over 4 million dollars, in spite of the fact that established welding procedures were used. In previous work at EWI for PRCI, PR- 185-9909 – Limitations for Cellulosic Electrodes,(1) two cases of severe weld metal hydrogen cracking were investigated. In each case, the composition of the weld metal was substantially richer than would typically be expected for the type of electrode in question. 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 a low coating moisture content. Under the current program, an extensive study involving a variety of manufacturers’ cellulosiccoated electrodes in strength levels ranging from 60 to 90 ksi has allowed the conditions that can lead to hydrogen cracking in weld metal to be further defined. Specifically operator preference (arc length), electrode properties, power supply selection, and materials handling were examined. The effect of re-hydrating the electrodes was also studied. The results of the project have been used to develop safe handling guidelines. It was determined that while different manufacturers’ electrodes may respond somewhat differently to drying, they all exhibited some change in chemistry and strength level. Exposure to bright sunlight in warm, dry areas may be just as damaging as low-temperature baking, so it is best to keep cellulosic-coated electrodes in cool, shaded regions throughout the work day. Rehydrating the electrodes did not restore them to the as-received condition, so the best practice is to discard electrodes that have been exposed to particularly hot or dry conditions, and under no circumstances should cellulosic electrodes ever be placed in electrode drying ovens. While proper handling may help to reduce the risk of hydrogen cracking, past experience indicates that extreme caution should be exercised when applying cellulosic electrodes to low-alloy steels. Many manufacturers, including Lincoln, ESAB, and Hobart, produce cellulosic electrodes for welding steels of grade X70 and above, and improvements have been made in these products in recent years. However, the most conservative approach is to use low-hydrogen electrodes, gas metal arc welding or flux-cored arc welding when welding these higher-strength grades. If cellulosic electrodes must be used for these steels, the lowest grade which will produce adequate strength for the intended application should be chosen. Minimum preheat and interpass temperature recommendations should be strictly adhered to, and if possible, a post-weld hydrogen bake-out should be utilized. Electrodes should be stored in cool, dry locations and not exposed to direct sunlight. Any electrodes which have been exposed to hot, dry conditions, or very humid conditions should be discarded.

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