What are crater cracks in MIG welding, and how can they be prevented in collision repair?

• A. Crater cracks are caused by moisture in the wire; prevent by drying the wire.
• B. Crater cracks occur at the start of a weld from preheating.
• C. Crater cracks occur at the end of a weld from shrinkage; prevent by filling the crater or performing gradual stopping technique.
• D. Crater cracks are cosmetic and require no prevention.

Answer: (C)

Crater cracks form at the end of a weld as the molten metal in the end of the bead cools and contracts. When you stop welding abruptly, the last portion of metal hardens while still under stress from this shrinkage, creating a brittle crack that can propagate under service loads. In collision repair, this is especially a risk on thin sheet metal and near joints where stresses concentrate, so it’s important to manage the end of the weld wisely.

To prevent crater cracking in MIG welding, fill the crater as you finish the weld, or use a gradual stopping technique that tapered off current and travel into the crater so the final portion is welded. This keeps the metal from ending in an open, highly stressed crater and reduces the chance of cracks forming later.