What causes concave root surface?

• A. insufficient filler metal, or excessive heat is used in the overhead position (concavity results in reduced thickness)
• B. excessive filler metal in all positions
• C. too little shielding gas
• D. inadequate root bevel

Answer: (A)

Root concavity happens when the weld at the root isn’t built up enough, often because there isn’t enough filler metal deposited or the heat input is too high, particularly in the overhead position. In overhead welding, gravity pulls the molten metal downward, so if heat is excessive the pool can sag and the root ends up thinner, creating a concave surface. The combination of not enough filler metal and high heat tends to produce that sunken, thinner-root look you’ll see on the joint.

Other factors influence different aspects of weld quality. Adding too much filler metal typically overfills and can make the root flush or convex rather than concave. If shielding gas is insufficient, porosity and contamination may occur rather than shaping the root. An inadequate root bevel affects fit-up and penetration, not specifically the formation of a concave root surface.