What is backgouging and why is it necessary?

Backgouging is the removal of metal from the back of a weld to expose sound metal before welding the final pass (root pass).

Why:

• The first weld pass (root pass) often contains defects: lack of penetration, inclusions, porosity, oxidation
• Backgouging removes these defects and exposes sound metal for the seal pass
• Required for all Complete Joint Penetration (CJP) welds — CSA W59 standard

Methods:

• Grinding: With a grinding wheel, for small sections. The most accessible
• Arc gouging (carbon arc gouging — CAG): A carbon electrode with an electric arc melts the metal, then a compressed air jet blows it away. Fast and efficient; the current is set from the carbon diameter (manufacturer's data sheet / WPS)
• Chipping: With a pneumatic chipping hammer — noisy and less precise

Criteria:

• Backgouging must be deep enough to expose metal with no visible defects (no porosity, no slag)
• The depth must be enough to reach sound metal across the whole affected section; it depends on the thickness and the defect being removed, and is checked visually
• The gouged surface must be smooth and clean before the seal weld
• MT (magnetic particle) or PT (dye penetrant) inspection is often required after backgouging