Last Updated on September 2020
Welding is among the most basic metal-joining techniques that date back to the 19th century. Any metallic construction is merely complete without the welding process. Welding is a metal fabrication process that uses extremely high temperatures to fuse parts. Filler metal is incorporated to create a strong bond between the base metal parts.
As simple as it is, welding is a crucial process which if done wrongly, results in dire consequences. These repercussions range from having weak welding joints to coming down of the whole construction; therefore, leading to property damage, injuries, and even loss of life.
Redoing a build-up is a frustrating experience that you should avoid at all costs. Quality over anything else is critical for any weld.
Various welding techniques exist, including gas metal arc welding (GMAW) and shielded metal arc welding (SMAW). Other terms you are likely to come across in the welding field are spot welding and tack welding. These are just other welding methods, whose only difference is the technicality applied.
What is Tack Welding?
Tack welding is quite a common term among most welders. Tack welds are weld fixtures that are temporarily put in place to hold two metals together in the proper location as you apply the final weld. They are typically small welds spaced out from each other that help you work on your final weld without worrying over the misalignment of the structure’s parts.
However, tack welds are different from temporary welds in that the former join part of a weldment while the latter joins the components of a weldment. Tack welds primarily hold edges together. This welding procedure allows for easy disassembly and re-welding if the final weld is misaligned.
Welding rules state that the same welding procedure must apply for both tack welding and the final welding. For instance, if you use clamping approach to hold components together in the final welding, this also applies to tack welding.
Have you ever noticed how the smallest of details result in the most hazardous problems? Similarly, tack welds, if incorrectly made, may propagate cracks and deficiencies to the final weld. The scientific definition of a tack weld does not define its size or length. However, a desirable tack weld should be relatively small sized.
Using Tack Welds in the Fabrication Process
Tack welds are placed either within the joint or outside the joint. For welds within the joint, in most cases, the tack weld is melted entirely and becomes part of the final weld. For tack welds outside the joint, you remove them after making the final weldment. You may also consider having these tack welds remain in place and joining them to the permanent weldment.
Tack welds within the joint are classified as re-melted or incorporated. When fabricating tack welds within the joint, place the final weld over the tack weld. Heat applied while applying the final weld melts the tack weld partially, or in some occasions, completely.
Re-melted tack welds apply the concept that heat passed over the final weld melts the tack weld entirely. Thus, there’s no evidence that the tack weld initially existed. Submerged arc welding (SAW), is mostly preferred for electrodes with a larger diameter and subsequent high current levels. Electrogas welding (EGW), is also another preferred technique for the same as it allows for deep heat penetration, consequently re-melting the tack welds.
Discontinuities in the tack weld should not be a bother since re-melting gets rid of these discontinuities. Ideal tack welds should be small in size because smaller tack welds are smoother to re-melt, unlike their larger counterparts. Longer tack welds are also preferred as they enable easy re-melting.
Contrarily, incorporated tack welds don’t involve re-melting. The tack weld, in this case, is the root pass of the final weld. The weld size, preheat, and heat input applied should be in accordance with the root pass. This type of weld should be entirely free from cracks, as there is no provision to correct the same.
Incorporated tack welds are generally more significant and shorter than the re-melted ones to allow for more effortless fusion. It’s essential to completely weld the spaces between the tack welds before creating subsequent layers to avoid disruptions that influence the final look of the weld.
Tack welds outside the joint, also known as non-incorporated joints, are considered as any final weld. The material and welding technique used for these joints should be applicable also for final welds. Permanent tack welds, just like final welds, should remain in place without any deficiencies whatsoever. These non-incorporated welds are continuous and should be strong enough to handle even high strain and stress.
Risks of Defective Tack Welding
As mentioned earlier, incorrect tack welding could result in damage to people and property. Tack welding should, therefore, be free from weld defects, including cracks, arc strikes, slag, or hard spots. As small as they seem, cracks may cause the failure of the whole structure.
Quality is critical for any tack weld. Tack welding should be performed by certified welders and according to the recognized qualified procedure to ensure unmatched quality. The tack welding procedure applied should be similar to that of the final weld.
Also, the tack weld sequence followed should control distortion. Tack welds should be positioned in such a way to ensure enough weld penetration. The welds must resist shrinkage to prevent overlapping of one sheet end over the other.
The tack welds must be cleaned thoroughly before moving on to the application of the final weld. Cleaning is essential to remove weld oxides formed during welding. Also, the ends of each tack weld are ground to create a more appealing blended look of the weld into the metal.
With the current improvement in technology, most welding techniques are either automatic or semi-automatic. Manual assistance is, however, necessary to ensure top-tier quality. Tack welding is needed for any successful welding project.
Tack welding ensures alignment of all assembled components while controlling distortion during welding. Therefore, not even the smallest of details should be left to chance.
You should keenly follow all due procedures to avoid the occurrence of any risk. Always remember that in welding, it’s the smallest mistake that creates the most catastrophic hazard.