When welding a joint, how many tack welds should be placed 180 degrees apart?

When welding a joint, placing two tack welds that are 180 degrees apart is a common practice to ensure adequate alignment and support for the pieces being joined. This arrangement allows for stability during the welding process, preventing any warping or misalignment as the joint is heated and cooled. The two tack welds effectively create a balanced hold that can help maintain the proper fit-up of the joint throughout the subsequent welding phases.

Using only one tack weld would not provide sufficient stability, while three tack welds could lead to uneven stresses and distortions in the joint. Four tack welds might be excessive for a typical joint that only requires two for effective anchoring. Therefore, the approach of using two tack welds placed 180 degrees apart strikes an optimal balance for achieving a strong and precise weld.