The backside of a plug weld should show melt through as evidence of penetration, but the melt-through height should be no greater than 1.5 mm (1/16").

• A. 1.5 mm
• B. 1.0 mm
• C. 2.0 mm
• D. 2.5 mm

Answer: (A)

The essential idea is controlling how much melt-through shows on the backside of a plug weld. You want evidence that the weld has fused through to the backing sheet, but you must limit that melt-through so you don’t burn through too much material. A maximum height of about 1.5 mm (1/16 inch) provides enough penetration to prove fusion without compromising the base metal. If melt-through climbs above 1.5 mm, it indicates excessive penetration, which can lead to burn-through, thinning, or weakening of the joint. If it’s noticeably less than that, penetration may be insufficient. So the acceptable limit is 1.5 mm, making that the correct guideline.