If the carbon content in base metal and electrode is below 0.05%, what may happen under CO2 shielding?

• A. The weld may pick up carbon from CO2 shielding
• B. The weld will become fully decarburized
• C. The weld will have no carbon transfer
• D. The weld will automatically carburize the steel

Answer: (A)

Shielding gas can influence weld chemistry, and CO2 can act as a carbon source at welding temperatures. In the arc environment CO2 decomposes and carbon can dissolve into the molten weld metal. When base metal and electrode are very low in carbon (below about 0.05%), the weld pool readily accepts this carbon, leading to carburization of the weld metal. So the weld may pick up carbon from CO2 shielding. This is different from decarburization (loss of carbon) or no carbon transfer, and while carburization isn’t guaranteed in every case, CO2 shielding is a known avenue for carbon pickup in low-carbon materials.