Remelted Tack Welds

The basic concept behind remelted tack welds is that the subsequent weld passes will effectively eliminate the previous tack weld (see Figure 3). Accordingly, it is reasonable that quality criteria associated with tack welds that will be remelted would be more relaxed than for tack welds that become part of the completed weld. This is reflected in D1.1 clause 5.18.5(1), which eliminates the requirement for preheat for tack welds under certain conditions, as follows:

“Preheat is not required for single pass tack welds remelted by continuous SAW. This is an exception to the qualification requirements of 5.18.1.”

To help ensure remelting of the tack weld by the subsequent SAW pass, 5.18.5(2) restricts fillet weld tacks to 3/8 in.

It should not be assumed that remelting will occur automatically when SAW is used. Nor is it appropriate to assume that other welding processes cannot remelt tack welds, even though D1.1 extends this option to only SAW.

Heavy sections of steel, and higher strength steels with their corresponding higher carbon and/or alloy levels, typically require preheat. Even though a small tack weld on non-preheated thicker sections may result in a hard, crack sensitive heat affected zone around the tack weld, the high heat input levels of SAW passes that remelt the tack welds also will reheat the HAZ. Likewise, discontinuities in tack welds that will be remelted are not a concern, as the remelting process eliminates the discontinuities as well.

Incorporated Tack Welds

When tack welds are placed within the joint and not remelted, they must be incorporated into the subsequent final weld as shown in Figure 4. Everything associated with an incorporated tack weld (preheat, filler metal requirements, WPS parameters, minimum weld size, heat input and quality of deposit) should be the same as would apply to the weld root pass. Quality concerns would include undercut levels, porosity limits, bead shape criteria, and the absence of cracks. Remember: these tack welds will be part of the final weld.

A major shift in thinking is required when tack welds are to be incorporated, as compared to the remelted alternative. For example, incorporated tack welds should be made of a size, and with a heat input level, that will ensure good fusion. These welds should meet the minimum size requirements that would be imposed on any final weld. Thus, for a required joint strength, incorporated tack welds will be larger in size, but perhaps shorter in length, as compared to the remelted option.

Large, intermittent tack welds may require that the gaps between the tack welds be completely welded before the subsequent layers are made. Welding over large tack welds may disrupt the arc, or may affect the appearance of the subsequent final weld. The acceptable geometry of the tack weld is dependent upon the ability of the final weld procedure to properly incorporate the tack weld into the final weld. This is the reason, for example, that 5.18.4(2) of D1.1 requires that multipass tack welds have cascaded ends as shown in Figure 5.

Special Caution

Cyclically loaded weldments (those subject to fatigue) require special attention to details, and tack welds are no different. As is illustrated in Figure 5, the transition from the root pass to the tack weld may leave behind a slag pocket, or point of incomplete fusion, that can create a notch-like condition. This stress raiser may be the point where fatigue cracks initiate. To ensure proper fusion at this point, it may be necessary to grind the ends of tack welds to ensure sound transitions.

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.