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 how the metal cools and contracts under restraint, and by hydrogen effects in the weld. The root pass is most susceptible because it sits at the bottom of the joint where the weld metal is highly restrained by the surrounding base metal. As the root cools and contracts, it develops high tensile stresses right at the root and in the adjacent heat-affected zone. If hydrogen is present in the weld or environment, those high stresses can drive hydrogen-induced cracking at the root. The combination of a steep thermal gradient, strong restraint, and potential hydrogen content makes the root pass the most prone to cracks, more so than the later passes, which can relieve some stress or have different microstructures.

Cracking in multipass welds is driven by how the metal cools and contracts under restraint, and by hydrogen effects in the weld. The root pass is most susceptible because it sits at the bottom of the joint where the weld metal is highly restrained by the surrounding base metal. As the root cools and contracts, it develops high tensile stresses right at the root and in the adjacent heat-affected zone. If hydrogen is present in the weld or environment, those high stresses can drive hydrogen-induced cracking at the root. The combination of a steep thermal gradient, strong restraint, and potential hydrogen content makes the root pass the most prone to cracks, more so than the later passes, which can relieve some stress or have different microstructures.