Excess root penetration

When welding the root, the molten metal can sag or even drip through on the underside – that's called root sag. It usually happens when you use too much heat or weld too slowly. In a pipe, this means flow obstructions later, and stress peaks can form at the weld that lead to cracks. So keep the root of the weld nice and even and work cleanly, then the sag stays small.