A pressure vessel fabrication shop is using FCAW-G with E7XT-5M class electrodes to weld 12 mm (1/2 in.) thick low-alloy steel. During a pre-production run, the welder observes significant porosity along the root pass. The shielding gas is specified as 75% Argon / 25% CO2 at 20 L/min (40 CFH). What is the most probable cause of the porosity?

Question: A pressure vessel fabrication shop is using FCAW-G with E7XT-5M class electrodes to weld 12 mm (1/2 in.) thick low-alloy steel. During a pre-production run, the welder observes significant porosity along the root pass. The shielding gas is specified as 75% Argon / 25% CO2 at 20 L/min (40 CFH). What is the most probable cause of the porosity?

Answer options:

• Insufficient preheat temperature
• Excessive travel speed in the root pass ✅ Contamination from oil or grease on the joint faying surfaces
• Use of the wrong welding polarity (DCEN instead of DCEP)

Correct answer: Contamination from oil or grease on the joint faying surfaces

Explanation: Porosity, especially in the root pass, is frequently caused by surface contamination such as oil, grease, paint, or rust, which can release gases during welding that become trapped in the solidification. Excessive travel speed might cause lack of fusion or inadequate penetration, while insufficient preheat is more likely to cause cracking in low-alloy steels. The wrong polarity would lead to an unstable arc and poor penetration, but not necessarily porosity directly.