Macroscopic root concavity

Root concavity is a typical defect on the underside of the weld when the molten pool isn't properly penetrated or supported. The weld is then not fully filled at the bottom, which significantly weakens the load-bearing capacity and can lead to cracks under load. Make sure the root is cleanly and fully penetrated, and adjust the pool control and heat input so the pool doesn't sag or run out. If you can't get the root clean, it often helps to adjust the joint preparation or choose a different welding position.