Tack weld
A tack weld is a short, temporary weld. It holds parts in the right position and secures the joint gap. The fast cooling of the small weld bead often results in a hard, crack-prone structure.
Physical-chemical mechanism of action
A tack weld brings only a little heat into the workpiece. The heat immediately flows into the cold base material. As a result, the weld cools down very quickly. Rapid cooling creates hard microstructures in the steel such as martensite. Additionally, the short weld shrinks during cooling. This creates tensile stresses in the workpiece. These stresses can cause cracks, especially in high-carbon steels.
Metallurgical relevance & risks when welding
Tack welds are often the first welds on the cold workpiece. They cool down the fastest. Therefore, they are especially prone to cracking. Hard spots in the tack weld can crack during the subsequent main welding. Also, hydrogen from the air or the filler metal can penetrate into the hard zone and cause hydrogen-induced cracking. If the tack weld is not completely remelted, it remains as a weak point in the workpiece.
Workshop practice & action instructions
Use the same filler metal and the same welding parameters as for the main weld. Preheat the workpiece if the material specification requires it (e.g., for plate thicknesses over 30 mm or high carbon equivalent). Make tack welds long enough: at least 3 to 4 times the plate thickness, for thin plates at least 20 mm, for thick plates up to 50 mm. Place tack welds at intervals of 200 to 300 mm. Avoid tack welds at the ends of the weld joint because they create stress peaks there. Check every tack weld for cracks before the main welding. Grind out cracked tack welds completely. Never weld over a cracked tack weld. For critical components, use clamping devices or ceramic backing instead.