Root zone
The deepest region of the weld joint where the welding process begins. Achieving proper penetration here is critical for the structural integrity of the entire weld.
The root is the most critical part of the weld – that's where it's decided whether the joint holds or not. You have to make sure that both plate edges are cleanly melted and the weld pool completely fills the gap. If the root cools too quickly, the structure becomes hard and brittle, and cracks can form – especially with thick plates you have to counteract that. With an open root, air gets in from the back, causing oxides and porosity. Either a backing bar helps, or you have to grind out the root cleanly from the back side before welding the second layer. Finally, check the root carefully for lack of fusion and notches – these are typical starting points for fatigue cracks later.