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Weld Quality and Process-Specific Influences

Some welding processes are more sensitive to the generation of certain types of weld discontinuities, and some weld discontinuities are associated with only a few types of welding processes. Conversely, some welding processes are nearly immune from certain types of weld discontinuities. Contained below are the popular welding processes and their variations, along with a description of their associated sensitivity relative to weld quality.

SMAW — The unique limitations of shielded metal arc welding fall into three categories: arc length related discontinuities, start-stop related discontinuities, and coating moisture related problems. In SMAW, the operator controls arc length. Excessively short arc lengths can lead to arc outages, where the electrode becomes stuck to the work. When the electrode is mechanically broken off the joint, the area where the short has occurred needs to be carefully cleaned, usually ground, to ensure conditions that will be conducive to good fusion by subsequent welding. The electrode is usually discarded since a portion of the coating typically breaks off of the electrode when it is removed from the work. Excessively long arc lengths will generate porosity, undercut, and excessive spatter. Because of the finite length of the SMAW electrodes, an increased number of starts and stops is necessitated. During arc initiation with SMAW, starting porosity may result during the short time after the arc is initiated and before adequate shielding is established.

Where the arc is terminated, under-filled weld craters can lead to crater cracking. The coatings of SMAW electrodes are sensitive to moisture pick-up. While newer developments in electrodes have extended the period for which electrodes may be exposed to the atmosphere, it is still necessary to ensure that the electrodes remain dry in order to be assured of low hydrogen welding conditions. Improper care of low hydrogen SMAW electrodes can lead to hydrogen assisted cracking, i.e., underbead cracking or transverse cracking. See 2.1 on care and storage of low hydrogen electrodes.

FCAW-g — In FCAW-g, as with all gas shielded processes, it is important to protect the gas shielding around the weld deposit. If FCAW-g gas shields are disturbed by winds, fans, or smoke exhaust equipment, porosity can result. The deep penetrating characteristics of FCAW-g are generally advantageous, but excessive penetration can lead to centerline cracking because of a poor width-to-depth ratio in the weld bead cross section.

FCAW-ss — Excessively high arc voltages, or inappropriately short electrode extension dimensions can lead to porosity with FCAW-ss. When excessive voltages are used, the demand for shielding increases, but since the amount of shielding available is relatively fixed, porosity can result. When the electrical stickout distance is too short, there may be inadequate time for the various ingredients contained within the electrode core to chemically perform their function before they are introduced into the arc. This too can lead to porosity. Because of the extremely high deposition rate capability of some of the FCAW-ss electrodes, it is possible to deposit quantities of weld metal that may result in excessively large weld beads, leading to a decrease in fusion, if not balanced with a corresponding increase in travel speed.

SAW — Submerged arc welding is sensitive to alignment of the electrode with respect to the joint. Misplaced beads can result from improper bead placement. The deep penetration of the SAW process can lead to centerline cracking due to improper width-to-depth ratios in the bead cross section.

GMAW — When solid electrodes are used, and particularly when welding out-of-position, the short arc transfer mode is frequently used. This can directly lead to coldlap, a condition where complete fusion is not obtained between the weld metal and base material. This is a major shortcoming of the GMAW process and is one of the reasons its application is restricted by the D1.1 code with respect to its prequalified status. As with all gas shielded processes, GMAW is sensitive to the loss of gas shielding.

Weld Quality and Process-Specific Influences

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