Lack of penetration
The root of the joint never melts. The cap looks right, the weld measures right, and the joint carries a fraction of the load it was drawn for. Number 402 in ISO 6520-1, and not permitted at all at quality levels B and C.
Lack of penetration or lack of fusion?
They are named together and they are not the same thing. Lack of fusion (401) is a weld that touched the parent metal without melting it: the bead is where it should be, the bond is not. Lack of penetration (402) is a root that was never reached — the arc did not get to the bottom of the joint at all.
The difference matters because the fixes live in different places. Fusion is a heat and technique problem, solved at the machine and in the hand. Penetration is mostly a geometry problem, and it is usually solved at the fit-up bench, before the arc is struck. The other defect has a page of its own — lack of fusion.
Geometry decides more than current does
Before touching the current, look at what the joint gives the arc to work with. Root gap, root face and included angle decide whether the arc can physically reach the bottom. A 3 mm root face on 6 mm plate leaves half the section for the root run to melt through, and no amount of amperage makes that reliable in position.
| Thickness s, mm | Preparation | Angle α | Root face c, mm | Root gap b, mm | When |
|---|---|---|---|---|---|
| ≤ 3 … 4 | I — square edge | — | — | 0–3 | Thin material: the gap alone gives penetration |
| 3 … 12 | Single-V | 50–70° | 1–2 | 1–4 | The default when access is from one side only |
| 12 … 40 | Double-V (X) | 50–60° | 1–3 | 1–3 | Access from both sides: about half the deposit of a single-V |
| > 30 | Single-U | 8–12° | 1–3 | 1–3 | Heavy plate, one-sided access; needs machining |
| > 40 | Double-U | 8–12° | 1–3 | 1–3 | Very heavy plate, access from both sides |
| 5 … 20 | Single-bevel (½V) | 45–55° | 1–3 | 1–3 | T-joint with full penetration |
| > 15 | Double-bevel (K) | 45–55° | 1–3 | 1–3 | T-joint with full penetration, access from both sides |
| — | Fillet, no preparation | — | — | 0–2 | Where full penetration is not required |
Two numbers from the table do most of the work. The root gap is what lets the arc down into the joint — close it with a heavy tack and the root closes with it. The root face is what the root run has to melt through, and 1–2 mm is the figure. Measure it: a flame-cut edge is rarely what the drawing says.
Nine causes, and the sign that tells them apart
Each cause below shows itself differently on the plate, and that is what tells you which one you are looking at. Read the middle column first, then the third.
| Cause | What it looks like | What to change |
|---|---|---|
| The root gap has closed up | The root is unfused along the whole length, as an even band rather than in patches | Restore the gap: shorter and more frequent tacks, wedges against shrinkage, tack from the middle outwards |
| Root face too thick | The edges are fused but a dark line runs down the middle of the root | Root face of 1–2 mm, measured rather than eyeballed. Grind a thick one down before assembly |
| Current too low for the thickness | Narrow, convex bead, wedge-shaped penetration, slag hard to remove | Raise the current to the table value, and in MMA take one electrode size up for the root run |
| Travel speed too high | The pool never gets time to spread; penetration is narrow and ragged | Slow down until the pool is roughly twice as wide as the gap |
| Included angle too narrow | The cap is sound but the arc never reaches the root | Open it out to the ISO 9692-1 figures — the preparation table is further up this page |
| Electrode not on the root centreline | One edge fused, the other not — the whole way along | Track the centre of the gap and watch the pool rather than the arc; on pipe, reposition the hand more often |
| Long arc in MMA | The root is covered with metal from above but never melted | Hold an arc about one electrode diameter long, and in the root almost rest the electrode on the edges |
| MAG dip transfer on thick material | A good-looking bead on the surface, and an untouched root on the macro | Move to spray transfer or open a root gap. Dip transfer belongs on thin sheet and on a gapped root |
| Root not gouged before the sealing run | A slag-filled gap is left between the first run and the sealing run | Gouge or grind the root back to clean metal before the sealing run |
Seeing it before the inspector does
From the face side you cannot, and that is the whole difficulty: a root that never fused looks like a good weld from above. What is available is a visual check of the root from the back where there is access, a macro section on a test piece, and radiography or ultrasonics on production work.
On a single-sided pipe butt the root run is inspected before the fill runs go on top of it. After that, finding it costs a gouging job.
What the standard allows
ISO 5817 lists it as 402, incomplete penetration. At level B it is not permitted; at level C it is not permitted; only at level D is a limit given at all, 0.2·t up to 2 mm. On structural work of any consequence that means there is no allowance: the joint is either penetrated or it is repaired. the ISO 5817 quality levels.
When you only get one side
Pipework and closed sections give no second chance at the root, and three answers exist. A backing bar or backing ring turns the root into a fillet against a plate instead of an open butt. A consumable insert does the same and becomes part of the weld.
A TIG root run, filled out afterwards with stick or flux-cored wire, is the standard procedure on pipe: the TIG arc is controllable enough to place the root deliberately, and the slow first pass pays for itself by not having to be gouged out. Which of the three a job allows is a question for the procedure, not a preference.
Frequently asked questions
Can more current make up for a closed root gap?
Not reliably. Past a point the extra current widens the pool and undercuts the top edges before it reaches the bottom, and in the vertical or overhead position the pool falls out first. The gap is the cheaper fix: it is set once at fit-up and costs nothing to weld.
There is a dark line on my macro — which defect is it?
Depends where the line runs. Up the bevel, along the fusion boundary, it is lack of fusion. At the bottom of the joint, between the two root faces, it is lack of penetration. Neither is permitted at levels B and C; only 402 has an allowance at level D.
Why does it show up on pipe and mostly at the bottom?
Because the position changes underneath you. In 5G and 6G the same hand travels through overhead, vertical and flat inside one run, and the pool behaves differently in each. At the bottom of the pipe, welding overhead, the pool wants to sag away from the root — that is where the arc has to be kept shortest.
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