Overlap occurs when molten metal flows over the surface of the base material and then cools without fusing with the base material. A typical cause of overlap is the supply of too much weld metal due to low welding speed. Overlap in fillet welds is caused by the droop of excessive molten metal due to gravity.
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.
To avoid overlap, it's advisable to use small electrodes. This is because small electrodes reduce the gap due to the application of less weld at each point. Small electrodes lessen the chance of excess materials and overlaps.
Undercutting is a groove or crater that occurs near the toe of the weld. When this weld flaw occurs, the weld metal fails to fill in that grooved area, resulting in a weak weld that is prone to cracking along the toes.
An undercut is usually produced from overheating and poor welding technique. Undercutting can be caused by maintaining to long of an arc length. Another common reason for undercutting is the improper selection of a gas shield. Also, an undercut can happen if a welder tries to weld quickly with a high welding current.
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.
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