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Removing Tack and Temporary Welds

When tack welds are required to be removed, and when temporary welds are removed, it is important that that the weld be fully removed without damaging the base metal. A typical approach is to thermally cut the weld or attachment off (using air arc gouging, oxy fuel cutting, or plasma cutting), and follow up with grinding. When cutting is performed too close to the final surface, one may inadvertently gouge the base metal.

The procedure described above assumes, however, that the tack weld, or the temporary weld, was properly made in the first place. Consider the improper procedure wherein a tack weld is placed outside the joint, but the weld is made without preheat, or the needed minimum heat input, or with an improper electrode. Such a procedure could result in an underbead crack, an excessively hard HAZ, or other weld defects. Simple removal of weld metal from the surface of the steel will not automatically remove the defect that may reside in the base metal. This can result in performance problems for the weldment, particularly when subject to cyclic loading.

In the case of Fracture Critical Members (FCMs), D1.5 requires that, when weld removal is required, the weld plus 1/8 in. [3mm] of adjacent metal be removed. The surfaces are faired in at a slope not steeper than 1 in 10 on the surface (see D1.5: 2002, 12.13.3). This conservative provision ensures that the whole weld, plus any affected base metal, is completely removed, along with any unacceptably hard or cracked material.

Summary

Neither tack welds nor temporary welds should be viewed as inconsequential, secondary welds, particularly when applied to cyclically loaded weldments. Whether the tack weld is to be made in the joint or not will affect the overall approach to the weld. If it is made in the joint, whether the tack weld is to be remelted or incorporated will determine the ideal configuration for that tack weld. For tack welds and temporary welds that will be removed, care must be taken to protect the base metal. There are plenty of opportunities to make tack and temporary welds improperly. Fortunately, it is not difficult to make them correctly

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