MT Melt-through
Melt-through · Soldadura en plano · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “V-groove joint with melt-through symbol in the tail” states a melt-through requirement.
What controls
Indication that the weld must fuse the full thickness and form a visible, controlled reinforcement on the back side of the joint, welding from one side only with no access from behind. It is a result requirement, not a recommendation: the root must exist, be continuous and stay within the excess limits set by the acceptance criteria. It is not the same as complete joint penetration with access from both sides, where the root is back-gouged and welded again.
How to verify
When there is visual access to the back side (pipe of sufficient diameter, with a borescope or mirror), the root is inspected for continuity, absence of undercut and excess within the limit, and measured with root gauges. When there is no access, the check is volumetric: radiography, which best shows the condition of the root in butt joints, or ultrasonic testing depending on the case. Leak tightness, if required, is a separate test.
Common mistakes when interpreting it
- Confusing it with complete penetration from both sides: here there is no access to the back side, which is why the root pass is critical.
- Accepting the root because the face bead looks good; they are two independent sides and the one that matters here is the back.
- Accepting any reinforcement on the back side: root excess also has a limit and can be grounds for rejection.
How to get it in the workshop / if it comes out of tolerance
It is demanding because you have to melt through to the other side without the puddle sagging. It depends above all on the joint preparation: correct root face and root opening, alignment with no high-low, and cleanliness. On stainless steels and pipe it is usually accompanied by back purging so the root does not oxidize. If there is no way to check the root afterward, control has to be in the process.
Discontinuous or unformed root: increase the root opening or reduce the root face in the preparation, and lower the travel speed on the root pass. Sagging root with excess: heat input too high or root opening too large; adjust parameters and check the joint fit-up. Oxidized, black root on stainless: insufficient purging, not a penetration problem. Do not try to correct a bad root by welding more on the face side, because the defect stays inside.
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Other symbols in Flat welding
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