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Weld Discontinuities - Part 2 - Unacceptable Weld Profiles
The profile of a completed weld may have considerable affect on the performance of that weld in service. It is the welding inspector’s function to identify these discontinuities through visual inspection and often to evaluate their acceptance, or rejection, through the use of the applicable welding code or standard acceptance criteria. Unacceptable weld profiles can cause problems associated with a reduction in base material thickness, a reduction in the affective weld size, or provide stress concentrations on the weld or plate surface. These types of weld discontinuities can often seriously detract from the overall performance of a welded component in service. We will consider some of the discontinuities associated with weld profiles: Undercut, Overlap, Insufficient Throat, and Excessive Convexity.
Undercut – This discontinuity is defined as a groove melted into the base metal adjacent to the weld toe, or weld root, and left unfilled by weld metal. The term undercut is used to describe either of two conditions. The first is the melting away of the base material at the side wall of a groove weld at the edge of a bead, thereby producing a sharp recess in the side wall in the area where the next bead is to be deposited. This type of undercut can facilitate the entrapment of inclusions that the recess which may be covered by a subsequent weld bead. This condition, if necessary, can be corrected, usually by grinding the recess away prior to depositing the next bead. If the undercut is slight, however, an experienced welder, who knows how deep the arc will penetrate, may not need to remove the undercut. Undercut of the side wall of a groove weld will in no way affect the completed weld if the condition is corrected before the next bead is deposited. The second condition is the reduction in thickness of the base metal at the line where the weld bead on the final layer of weld metal ties into the surface of the base metal. This position is known as the toe of the weld. This condition can occur on a fillet weld or a butt joint. The amount of undercut permitted at the surface of the completed weld is usually specified within the welding code or standard being used. The maximum permissible undercut requirements for completed welds should be followed stringently because excessive undercut can seriously affect the performance of a weld, particularly in services subjected to fatigue loading. Both types of undercut are usually caused by the welding technique used during welding, incorrect electrode positioning and/or incorrect travel speed. High currents and a long arc length can increase the probability for undercut.
Overlap – This discontinuity is defined as the protrusion of weld metal beyond the weld toe or weld root. This condition can occur in fillet welds and butt joints and can produce notches at the toe of the weld that are undesirable due to their resultant stress concentration under load. This discontinuity can be caused by incorrect welding techniques or insufficient current settings.
Insufficient Throat – This condition describes a weld profile that is usually concave in shape, and due to its concavity, provides an inadequate throat thickness. Excess concavity, that can produce an unintentional reduction in throat thickness, can occur in fillet welds and butt joints. The problem associated with this discontinuity is its ability to considerably reduce that part of the weld that controls the weld’s strength, namely the throat thickness. This condition is usually caused by excessive welding current or arc lengths.
Excessive Convexity –This discontinuity can produce a notch effect in the welded area and, consequently, stress concentration under load. For this reason, some codes and standards will specify the maximum convexity of a weld profile. This condition is usually caused by insufficient current or incorrect welding techniques.
Conclusion – A number of discontinuities are associated with the exterior profile or shape of the completed weld. These discontinuities are generally detected through visual inspection of the weld, however, some are detectable through other inspection methods such as radiography, liquid penetrant, and magnetic particle inspection. The maximum acceptable limitations associated with these discontinuities is dependent on the performance requirements of the welded component and is usually specified in the appropriate welding code, standard or specification. The welding inspector is often required to determine the extent of these discontinuities and to establish their acceptance, or rejection, based on the relevant acceptance criteria.