What is the maximum melt-through height on the backside of an open butt joint weld?

• A. 1.5 mm
• B. 0.5 mm
• C. 2.0 mm
• D. 3.0 mm

Answer: (A)

The concept here is controlling heat input to prevent melt-through on the backside of an open butt joint when performing a GMAW weld. In an open butt joint there’s no backing bar to support the molten metal, so it’s easy for the weld to burn through the far side if heat input is too high. The backside melt-through height measures how far the weld pool penetrates past the joint onto the backside.

The maximum allowed backside melt-through is 1.5 mm. Keeping within this limit ensures the weld fuses properly to both pieces without creating a through-thickness hole or weakening the sheet. If the backside melt-through were greater, you’d risk blow-through and damage that could require repair. Values smaller or larger than 1.5 mm aren’t appropriate for this standard Open butt joint setup: 0.5 mm would indicate overly conservative heat input in some cases and insufficient penetration in others, while 2.0 mm or 3.0 mm would show excessive heat and likely burn-through.