April 11, 2006
Tack welding, a necessary preliminary step in many welding projects, must be performed correctly to achieve optimal results from the final weld and to minimize part defects. Quality is as important in tack welding as it is in the final weld. This article describes proper tack welding conditions.
|TIG Tack Welding|
Photo courtesy of Weldcraft
After items to be welded together have been positioned as required, generally by clamping them on suitable fixtures, tack welds are used as a temporarymeans to hold the components in the proper location, alignment, and distance apart, until final welding can be completed.
In short-production-run manual welding operations, tack welding can be used to set up the workpieces without using fixtures. Typically, tack welds are short welds. In any construction, several tack welds are made at some distance from each other to hold edges together.
An advantage of this provisionalassembly procedure is that if the alignment for final welding is found to be incorrect, the parts can be disassembled easily, realigned, and tack welded again.
In general, tack welding is performed by the same process that is used for the final weld. For example, aluminum-alloy assemblies to be joined by friction stir welding are tack-welded by the same process using a small tool developed for this purpose. Or electron beam tack welds, created with reduced power, are used to supplement or replace fixturing and to maintain the correct shape and dimensions during final electron beam welding.
If the final welding is performed while the elements are still clamped in a fixture, tack welding must keep the elements in place and resist considerable stresses, not sufficiently contrasted by clamping devices, that tend to separate the components.
The temporary nature of tack welds may give the false impression that the quality of these auxiliary joining aids is not as important as that of final weld and that this operation doesn't have to be properly programmed, performed, and inspected. This is not true.
Tack welding is real welding, even if the welds are deposited in separate short beads. It performs the following functions:
When hoisted, improperly tack welded assemblies can rupture, and portions or subassemblies can fall and endanger people or damage property.
Tack welding must not interfere with or degrade the quality of final welding. It must not introduce weld defects, such as arc strikes, craters, cracks, hard spots, and slag left in place.
Many steels used in fabricating pipes and vessels are sensitive to rapid cooling or quenching, especially following short tack welds, because of the limited heat input required to tack weld. Note:Higher heat input slows the cooling rate, which minimizes the occurrence of hard and brittle microstructures.
Hard, brittle, and crack-sensitive microstructures can be formed in the heat-affected zone (HAZ) if the metal is rapidly quenched. In this case, even removing the whole tack weld by grinding may leave dangerous, invisible cracks in the base metal.
The brittle metal can crack during solidification of the weld metal or when stressed. Underbead cracks cannot be readily detected by visual inspection, and more thorough nondestructive tests may not be performed if they are deemed unimportant for such limited welds. However, these small cracks can cause the whole structure to fail.
To ensure quality, most codes require that tack welding be performed only according to qualified welding procedures by welders fully certified in the process used for the final weld.
The requirements are applicable for any welding process used.
In all fusion welding processes, the sequence and the direction of the tack welds are important for distortion control. Besides maintaining the joint gap, tack welds must resist transverse shrinkage to ensure sufficient weld penetrations.
For a long seam, tack welding should start at the middle and proceed along the joint length, alternating in both directions, in proper back step or skip sequence to avoid stress buildup and deformation.
Tack welds also can be placed at the joint ends and then added in the middle of each resulting distance between those already done, until the whole length is covered with the required number at the needed spacing.
Why tack weld in sequences such as these? Because if tack welds are placed progressively from one end to the other, shrinkage can close the gap at the opposite end and might even cause one sheet end to overlap the other.
Because of greater thermal expansion in austenitic stainless steels, the spacing between tack welds on these materials should be much shorter than for mild steel.
Tack welding is an essential step in preparing pipes for welding. Thorough attention should be given to obtain adequate alignment and consistent root opening (joint gap) that control the success of the most important root pass. Although this work could be assigned to fitters, it should be supervised closely to make sure that the workers are properly qualified.
The number and size of tack welds depend on pipe diameter and wall thickness. Tack welds with complete fusion should be the same quality as the final weld.
All tack welds must be thoroughly cleaned before proceeding with the final weld.
Both ends of each tack weld, representing start and stop (which are weak points often having unacceptable defects), must be ground to remove possible flaws and to present a very gradual slope that blends the weld's sides into the metal.
When tack welding is used as fixturing for brazing, the area surrounding the tack must be thoroughly cleaned to remove oxides developed during welding.
In semiautomatic and automatic welding, the meeting points of the final weld electrode with tack welds can impair arc voltage control and filler wire feeding, making manual assistance especially important for maintaining quality.
Tack welding is an essential ingredient in a successful welding project, be it simple or complex. It is therefore very important to perform the process properly and minimize the risks associated with poor tack welding.