Root face
Root face height is the unbeveled, straight land at the bottom edge of a weld joint. It acts as a heat and weld pool barrier and prevents burn-through at the root. The root face height determines penetration and thus the strength and toughness of the weld.
Physical-Chemical Mechanism of Action
With a V-groove or similar joint geometry, the plate is beveled at the top edge, but a straight land remains at the bottom edge. This land has a certain height, the root face. During welding, the arc melts the material. The land acts as a defined remaining wall thickness at the root. The molten weld pool penetrates into the groove and must fully melt the land to achieve full penetration. If the land is too thin, the pool burns through the back side. If the land is too thick, the root does not melt completely and unmelted remnants remain, acting as lack of fusion. Heat input and heat dissipation play a role: the land stores heat and conducts it away. With too rapid cooling, martensite can form in the land area.
Metallurgical Relevance & Risks in Welding
Too small a root face leads to burn-through, root sagging, uncontrolled weld pool loss, and possible root porosity. Too large a root face causes lack of fusion at the root, incomplete penetration, notching, reduced load capacity, and crack initiation. The root face indirectly affects the cooling behavior of the root zone: a thick land dissipates heat faster and can lead to rapid cooling with martensite formation, especially in higher-strength steels. This reduces toughness and increases the risk of cold or hydrogen-induced cracking. Hot cracking is also possible if the land is not melted properly and the weld pool solidifies under stress.
Shop Practice & How-To Instructions
The root face must be chosen depending on plate thickness and welding process. Typical guidelines: For plate thicknesses up to 8 mm: 1 to 1.5 mm. For 8 to 15 mm: 1.5 to 2.5 mm. Above 15 mm: 2 to 3 mm. With TIG welding rather smaller, with MAG welding somewhat larger. Before welding, check the root face with a gauge or caliper over the entire joint length – it must be uniform. If the root face is too large, you can rework the groove (reduce the land) or adjust the welding parameters: higher current or special root technique. If the root face is too small, back-welding or supporting the root with a ceramic or copper backing helps. Shop rule: choose the root face so that with the existing welding parameters you get reliable penetration without burn-through – usually about 1 to 2 mm, depending on heat input.