Convex contour symbol

The convex contour symbol is an arc drawn beyond the weld symbol's outer face, curving away from the reference line. It says the finished weld face should bulge outward rather than sit flat or dish inward. On a fillet, that means the face stands above the straight line joining the two toes; on a groove weld it means reinforcement above the plate surface.

A modest convexity is what most welds naturally form, so the symbol usually appears when the shape has to be guaranteed rather than merely allowed. That happens where extra metal is wanted for wear, where the joint sees mainly static load and reinforcement adds margin, or where the face will be machined later and needs stock left on it.

Convexity has limits, though the symbol itself does not state them. Piling weld metal too high creates a sharp angle where the face meets the base metal, and that abrupt change in section concentrates stress right at the toe. Under fatigue loading a heavily convex fillet can perform worse than a flat one with the same throat, which is exactly why fatigue-sensitive joints usually call for flush or concave instead.

The important measurement point is the throat, not the face. A convex fillet's extra crown does not add to the effective throat that carries load, since the throat is measured from the root to the straight line joining the toes. Adding convexity therefore costs consumable and heat without adding capacity, which is worth remembering before over-welding a joint that only asked for a shape.