In multipass welds, which pass is most susceptible to cracking?

In multipass welds, which pass is most susceptible to cracking?

• A. The Root Pass
• B. The Cap Pass
• C. The Fill Pass
• D. The Tie-In Pass

Answer: (A)

Cracking in multipass welds is driven by the stresses built up as the weld and surrounding metal cool and contract. The root pass is most vulnerable because it forms in a highly restrained zone: the base metal and the already-deposited material around it hold it in place as it cools, so the shrinkage contraction creates the highest tensile stresses right at the root. If hydrogen is present in the weld metal or heat-affected zone, those stresses can drive hydrogen-induced or cold cracking at the root during cooling. Since the root sets the initial geometry of the joint, a crack there can propagate into the weld metal itself and compromise the whole joint more readily than cracks that might start in later passes. The other passes tend to have slightly different cooling conditions and less restraint at their origin, so they are less prone to initiating cracks under typical welding conditions.