What can cause porosity (tiny bubbles) to form on some welds on mild steel?

What can cause porosity (tiny bubbles) to form on some welds on mild steel?

• A. Using the wrong type of tungsten
• B. A chemical reaction between the filler metal and the mild steel
• C. Small pockets of CO2 trapped inside the steel during manufacturing
• D. Too much heat and the metal begins to boil

Answer: (C)

Gas porosity happens when gas gets trapped in the molten weld metal as it cools and solidifies, forming tiny bubbles in the weld. If there are CO2 pockets already inside the steel from manufacturing, heating the metal during GTAW can release those gases into the molten pool. If that gas can’t escape before the weld solidifies, it becomes porosity in the weld. This is why trapped CO2 in the steel explains porosity observed in some welds. The other options don’t fit as well because a tungsten choice mainly affects arc stability and contamination, not pre‑existing internal gas. A chemical reaction between filler and base metal would show up as different phases or oxides rather than gas bubbles from dissolved pockets. And excessive heat that makes the metal boil could cause other defects, but it isn’t the typical source of porosity caused by gas pockets trapped in the metal itself.

Gas porosity happens when gas gets trapped in the molten weld metal as it cools and solidifies, forming tiny bubbles in the weld. If there are CO2 pockets already inside the steel from manufacturing, heating the metal during GTAW can release those gases into the molten pool. If that gas can’t escape before the weld solidifies, it becomes porosity in the weld. This is why trapped CO2 in the steel explains porosity observed in some welds.

The other options don’t fit as well because a tungsten choice mainly affects arc stability and contamination, not pre‑existing internal gas. A chemical reaction between filler and base metal would show up as different phases or oxides rather than gas bubbles from dissolved pockets. And excessive heat that makes the metal boil could cause other defects, but it isn’t the typical source of porosity caused by gas pockets trapped in the metal itself.