Square groove

Physical-Chemical Mechanism of Action

The I-groove is formed when two sheets butt together and the edges are at right angles to the surface. There is no bevel or radius. The welding process melts the edges directly. Heat conduction occurs perpendicular through the sheet. For thin sheets (up to approx. 3 mm), heat quickly reaches the underside and fully melts the root. A continuous weld forms. The narrow gap (usually 0–1 mm) limits the weld pool volume. This reduces filler metal consumption and shrinkage forces.

Metallurgical Relevance & Welding Risks

Main risk: incomplete penetration. If the plate thickness is too large or the welding current too low, the root stays cold. This creates a lack of fusion. Lack of fusion acts like a notch. Under load, stresses concentrate at this point. The weld can fail prematurely. If the gap is too wide (over 1 mm), air can enter the root. In gas-shielded welding, air displaces the shielding gas. Result: porosity and oxidation of the weld pool. For high-strength steels, rapid cooling in the HAZ can lead to martensite formation. The weld becomes hard and brittle. This happens especially with thick plates welded with I-groove – i.e., when the I-groove is misapplied.

Workshop Practice & Action Instructions

Use I-groove only for plate thicknesses up to 3 mm (MAG/MIG) or up to 4 mm (TIG). Beyond that, edge preparation (V-groove, U-groove) is required. Maintain the gap width exactly: for thin sheets 0–0.5 mm without filler, up to 1 mm with filler. Larger gaps lead to burn-through and porosity. Before welding, clean the edges thoroughly: remove grease, oil, rust, and scale. Set welding parameters so that heat reliably reaches the root. For MAG/MIG: short arc, slightly penetrating setting. For uneven plate thickness or single-sided access, I-groove is unsuitable. Choose a different groove design instead.