A common cause of center line weld cracking is:

Center line weld cracking is a phenomenon that occurs in welded joints, often related to the weld metal's susceptibility to cracking under stress. One of the primary contributors to this type of cracking is a highly restrained joint fit up. When the joint is heavily restrained, it limits the natural expansion and contraction that occurs during the welding process and the subsequent cooling phase. This restriction can create significant stress in the weld metal.

As the weld cools, the thermal contraction can lead to cracking along the center line of the weld. The constraint imposed by the surrounding material does not allow the weld to freely accommodate these changes, resulting in tensile stresses that can exceed the material's capability to handle them. This can lead to brittle fracture, particularly if the weld metal has low ductility or if there are detrimental conditions such as poor joint design or inadequate preheating.

High amperage, stress relieving, and incomplete side wall fusion can contribute to welding defects, but they are not the most common causes of center line cracking in this specific context. Factors like excessive heat can lead to issues like distortion or create undesirable microstructures, whereas stress relieving typically helps reduce residual stresses. Incomplete side wall fusion can affect the integrity of the joint overall but does not directly correlate with center line cracking