As we have done in recent months, we will continue to focus on the little details that can cause problems, this time looking at tack welds. Let's start with a definition: AWS A3.0 Standard Terms and Definitions defines a tack weld as: “A weld made to hold the parts of a weldment in proper alignment until the final welds are made.”

Note that a tack weld is not defined in terms of its size or length, but rather in terms of its purpose. When parts can be placed into fixtures, tack welds may not be required. However, for many one-of-a- kind or limited production weldments, the various parts of the assembly may be tack welded in place. The tack weld must be sufficiently strong to keep the parts in place until the final welds are made.

As shown in Figure 1, tack welds fit into one of two broad categories: tack welds made in the joint, and tack welds made outside the joint. Each type presents specific challenges. This month, we'll discuss the basics of tack welds made in the joint. In Part 2, we'll look at tack welds and toughness requirements, as well as tack welds made outside the joint.

Same Quality Required

Regardless of where the tack weld is placed, it must be properly made. The AWS D.1.1: 2008 Structural Welding Code — Steel requires this in 5.18.1(1), which says “Tack welds…shall be made with a qualified or prequalified WPS and by qualified personnel.” One WPS requirement is preheat, which may be required depending on the steel thickness and composition.

Proper Size Required

A tack weld must be strong enough to resist the loads that will be transmitted through it. Some weldments have individual components that are massive, and the weight of such parts may be transferred through tack welds while the weldment is handled during fabrication. Tack welds are often required to hold parts in alignment while assemblies are being preheated for final welding. Thermal expansion, the corresponding strains, and resultant stresses may require tack welds of significant capacity.

The strength of tack welds, like other welds, is proportional to the throat size, and the length. Thus, a tack weld can be made stronger by using a larger throat, or longer length, or both. In most cases, tack welds are intermittent, and the strength across the joint can be made greater by increasing the number of intermittent tack welds, even to the point of a continuous tack weld. Additional factors to consider when selecting how to make the tack weld stronger are discussed below.

Tack Welds Made in the Joint

Examples of tack welds within a joint are shown in Figure 2. The tack weld may be completely remelted and become part of the final weld, or alternatively, part or most of the tack weld may remain within the joint, to be covered over by the final weld passes. These two types of tack welds, called remelted tack welds and incorporated tack welds, require fundamentally different approaches.