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Weld Quality in Steels Construction #2

A variety of types of discontinuities can exist in welds. Characteristics, causes, and cures of common examples may be summarized as follows:

Undercut is a small cavity that is melted into the base metal adjacent to the toe of a weld that is not subsequently filled by weld metal. Improper electrode placement, extremely high arc voltages, and the use of improper welding consumables may result in undercut. Changes to the welding consumable and welding procedures may alleviate undercut.

Excess concavity or excess convexity are weld surface profile irregularities. These may be operator- and/or procedure-related.

Overlap (or cold-lap) is the protrusion of weld metal beyond the toe of the weld where the weld metal is not bonded to the base material. Overlap usually is associated with slow travel speeds.

Incomplete penetration is associated with weld joint details that rely on melting of base metal to obtain the required weld strength. A typical example would be a square-edged butt joint. Incomplete penetration occurs when the degree of penetration is inadequate, and is generally attributable to insufficient current density, improper electrode placement, or excessively slow travel speeds.

Lack of fusion, or incomplete fusion, is the result of the failure of the weld metal and the base metal to form the metallurgical bonds necessary for fusion. Lack of fusion can range from small, isolated planes, or, in extreme cases, may consist of a complete plane between the weld metal and the base metal where fusion does not exist. Improper filler metal selection, improper welding procedures, and poor surface preparation are common causes of this condition. Improper use of GMAW short-circuit-ing transfer is a common cause of lack of fusion.

Arc strikes consist of small, localized regions of metal that have been melted by the inadvertent arcing between electrically charged elements of the welding circuit and the base metal. Welding arcs that are not initiated in the joint leave behind these arc strikes. Arcing of work clamps to the base metal can cause arc strikes, as can welding cables with improper insulation. SMAW is particularly susceptible to creating arc strikes since the elec- trode holder is electrically ‘hot’ when not welding. The use of properly insulated welding equipment and proper welding practices minimize arc strikes. Grinding away the affected (melted) metal is an effective way of eliminating any potential harm from arc strikes.

Slag inclusions describe non-metallic material entrapped in the weld metal, or between the weld metal and base metal. Slag inclusions are generally attributed to slag from previous weld passes that was not completely removed before subsequent passes were applied. Slag may be trapped in small cavities or notches, making removal by even conscientious welders difficult. Proper joint designs, welding procedures, and welder technique can minimize slag inclusions.

Spatter is the term used to describe the roughly spherical particles of molten weld metal that solidify on the base metal outside the weld joint. Spatter is generally not considered to be harmful to the performance of welded connections, although excessive spatter may inhibit proper ultrasonic inspection, and may be aesthetically unacceptable for exposed steel applications. Excessive spatter is indicative of less than optimum welding conditions, and suggests that the welding consumables and/or welding procedure may need to be adjusted.

Porosity consists of spherical or cylindrical cavities that are formed as gases entrapped in the liquid weld metal escape while the metal solidifies. The D1.1-96, Table 6.1 code defines acceptable limits for porosity as a function of its type, size, and distribution. Porosity occurs as the result of inadequate shielding of the weld metal, or excessive contamination of the weld joint, or both. The products used for shielding weld deposits (gases, slags) must be of appropriate quality, properly stored, and delivered at a rate to provide adequate shielding. Excessive surface contamination such as oil, moisturerust, or scale increases the demand for shielding. Porosity can be minimized by providing proper shielding, and ensuring joint cleanliness.

Cracking is the most serious type of weld discontinuity. Weld cracking is extensively discussed in others articles. (please refer to cracking tag)

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