Which gas will cause a low ductility weld or underbead cracking?

• A. Oxygen
• B. Nitrogen
• C. Argon
• D. Hydrogen

Answer: (D)

Hydrogen embrittlement is the issue here. When hydrogen is present in the weld metal, it diffuses as the metal cools and tends to collect at grain boundaries and near the weld, creating brittle, high-stress sites. Under the tensile and shrinking stresses around the weld, these sites crack, often right beneath the bead as underbead cracking. This is especially a risk with high-hydrogen processes or damp consumables, so using low-hydrogen electrodes, drying and storing materials properly, and preheating when needed helps prevent it. The other gases don’t drive this particular cracking mechanism—argon is an inert shielding gas that helps prevent hydrogen pickup, while oxygen and nitrogen aren’t the culprits for underbead cracking in this context.