In Part 1, we examined tack welds in general and discussed tack welds made in the weld joint. This month, we'll consider in detail some toughness-related issues with tack welds in the joint, and also look at tack welds outside the joint.

When notch toughness is a requirement

When a weld is required to have a specific minimum level of notch toughness, and when different filler metals are used for making the root pass than for the fill passes, the two materials should be investigated to be sure they are compatible. It is not uncommon, for example, for tack welds to be made with one process, and then for the joint to be filled using another.

The composite (or intermixed) weld metal must be capable of delivering the required Charpy V-Notch (CVN) toughness. It is possible that either the tack-welding material or the fill-pass materials may, by themselves, have adequate toughness, and yet the mixed-weld metal may not.

For steel applications, the concern involves the use of self-shielded flux-cored arc welding (FCAW-S), followed by other processes such as submerged arc welding (SAW). The same type of problem can exist when self-shielded FCAW is followed by gas-shielded FCAW. Or, perhaps the tack welding is done with SMAW and the fill passes made with self-shielded FCAW. When one of the two filler metals involved is self-shielded FCAW, there is the potential for toughness degradation. Such a decrease does not always occur, however, and thus case-by-case investigations are required. Self-shielded electrodes can be intermixed with each other without such concerns. (See Figures 1 and 2.)

AWS D1.8:2005 Seismic Welding Supplement has a specific test to evaluate such conditions. The test is fully described in Annex B of that standard. A helpful commentary provides more insight into this phenomenon. AWS D1.1:2008 Structural Welding Code - Steel has incorporated new provisions to address this condition in clause 5.18.4(3).

As a reminder, the concern exists only when one (but not both) of the processes is self-shielded FCAW, and when notch tough weld metal is required. It is always safe to use the same electrode for tacking and for filling (see Figure 3). When intermixing with self-shielded FCAW is required, tests can be performed to be sure the materials are compatible. Some filler metal manufacturers have test data on acceptable combinations.

Tack welds outside the weld joint

When tack welds are placed outside the weld joint, other factors must be considered. Simply put, these welds should be treated as any final weld. They should be made with materials, procedures, techniques and quality levels that would be acceptable for final welds. Tack welds outside the weld joint fit into two categories: permanent, and those that will be removed.

Tack welds outside the weld joint must be evaluated to determine if they can remain in place without causing unintended consequences. D1.1 addresses this issue in clause 5.18.2, which provides the following helpful information:

Clause 5.18.2(1) states that for cyclically loaded members (that is, those subject to fatigue), tack welds outside the weld joint are prohibited in tension regions, except for the condition of tack welds to backing, which will be addressed in detail below.

Clause 5.18.2(2) states that when the steel involved is quenched and tempered and has a minimum specified yield strength greater than 70,000 psi, tack welds outside the weld joint are prohibited, unless approved by the engineer. These steels are, of course, the type that would be most negatively affected should they be tack welded with inadequate preheat, or inappropriate heat input.

Tack welds and backing

As was discussed, for cyclically loaded members, tack welds in the tension zone are prohibited, according to AWS D1.1. The one exception involves tack welds that attach steel backing that will be left in place. For longitudinal groove welds (that is, for welds parallel to the stress field), tack welds that are made outside the joint are required to be continuous (see D1.1:2008, clause 2.16.2.4). Figure 4 illustrates this condition.

The fatigue category for continuous CJP groove welds would be Category B with the highest allowable stress range. Intermittent fillet welds attaching the backing make this joint into a Category E connection with nearly the lowest allowable stress range. For this reason, the detail is prohibited in D1.1.

If the intermittent tack welds are removed by grinding, the connection becomes Category B again. Alternately, continuous welds on the outside of the backing would also make this a Category B detail.

It is good to remember this adage — “There are no secondary members in welded design” — when evaluating the suitability of leaving in place tack welds that are made outside the weld joint.

Removing tack welds

When tack welds are required to be removed, it is important 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 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. Simply removing 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 it is subjected to cyclic loading.

For improperly made tack welds, it is advisable not only to remove the tack weld itself, but also to remove by grinding all of the weld metal and any hardened heat affected zone (see Figure 5.)

Summary

Tack welds should not 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 that will be removed, you must take care to protect the base metal. There are plenty of opportunities to make tack welds improperly. Fortunately, it is not difficult to make them correctly.

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Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the “Top 125 People of the Past 125 Years” by Engineering News Record. Blodgett may be reached at (216) 383-2225.