Subsurface porosity in a GMAW weld can result from what cause?

• A. Excessive shielding gas
• B. Nitrogen entering the weld pool
• C. Over-tensioning
• D. Using argon shielding

Answer: (B)

Gas entrapment in the molten weld metal can produce porosity, and subsurface porosity specifically happens when gas bubbles form below the surface as the metal solidifies. If nitrogen gets into the weld pool, such as from air leakage into the shielding gas or from a gas mixture that contains nitrogen, it increases the gas content in the molten metal. Nitrogen doesn’t stay dissolved well as the weld cools, so it tends to form gas bubbles that become trapped beneath the surface, creating subsurface porosity. Keeping a proper, clean shielding gas environment (preferably inert and appropriate for the material) helps prevent this issue.

Excessive shielding gas or using argon shielding alone aren’t the direct causes of subsurface porosity, and over-tensioning is a mechanical issue rather than a gas-related one.