ISO 6520-1 group 5 · Imperfect shape and dimension
ISO 6520-1 reference 504 covers excess penetration at the weld root. Cause matrix, why radiography leads when the root cannot be seen, and the standards that set the acceptance limit for your material and code.
Excess penetration is the root-side counterpart to excess weld metal: too much filler arrives at the back of the joint instead of staying inside the groove. On an open, accessible root it is a nuisance found by turning the part over. On a pipeline girth weld, a vessel nozzle, or any joint welded from one side only, the root is never seen directly, and radiography becomes the definitive route rather than a backup to a visual check.
The imperfection also changes what a radiograph is read for: an excessively bulged root can obscure a smaller planar imperfection sitting close to it, part of why root profile is controlled as its own parameter rather than tolerated as a side effect of good fusion.
Every cause in the matrix — gap, land, current and travel speed, backing detail — is set before or during the root pass and is measurable there. A procedure that qualifies and records root parameters separately from fill passes is, in effect, qualifying directly against this imperfection, which is why it carries its own row in the ISO 5817 acceptance criteria rather than being folded into a general penetration check.
| Cause | Mechanism | Control |
|---|---|---|
| Root gap wider than the qualified joint design | A wide gap gives the arc a clear path through the joint, and the root bead grows on the far side faster than it can be controlled from the near side. | Hold root gap to the qualified tolerance and treat a recurring wide gap as a fit-up or tack-welding problem, not a welding one. |
| Current too high or travel speed too low for the root pass | Excess arc energy at the root melts through the land faster than the pool can freeze in place, and the bead grows on the back side of the joint. | Record current and travel speed for the root pass specifically; root parameters are usually qualified separately from fill passes for exactly this reason. |
| Insufficient root land, or a land ground thinner than specified | A thin land leaves little parent material to hold the pool back, so even a normal root pass produces excess protrusion on the far side. | Verify land dimension at fit-up rather than trusting the cutting or machining operation that produced the bevel. |
| Backing configuration too permissive for the process | A backing bar or ring set with excessive clearance, or the wrong groove for the backing, lets the root pass sag or bulge further than an unbacked joint at the same parameters. | Qualify the backing detail as part of the procedure rather than treating it as a fixturing convenience. |
The column that matters most is the last one. An inspection family marked as not finding this imperfection will not find it however carefully it is applied, and planning around that is cheaper than discovering it during a final examination.
| Family | Method | Capability | What it sees |
|---|---|---|---|
| Visual | Visual testing to ISO 17637 from the root side | Supporting signal | Finds it directly wherever the root side is accessible, but many joints — buried pipe, closed vessels, one-sided access — have no root-side view at all. |
| Thermal | In-process infrared thermography | Supporting signal | Root-side thermal profiling shows an overheating root pass while it is still being run, ahead of any post-weld inspection. |
| Camera | Root-side or high-speed weld pool imaging | Supporting signal | Shows the pool sagging through the root in real time, where camera access to that side of the joint exists. |
| NDT | Radiographic testing to ISO 17636 | Primary route | The definitive route regardless of physical access, which is why pipeline and pressure-vessel root profile is verified by radiography as standard practice. |
Therness pages never reproduce a standard's acceptance values: those tables are copyrighted and belong in the copy you hold. What follows is the pointer — which document governs, and which part of it to open.
| Standard | Clause topic | Note |
|---|---|---|
| ISO 5817 | Quality levels for excess penetration at the root, indexed by ISO 6520-1 reference number | The permitted profile depends on the selected quality level and on wall thickness. Read it in the standard. |
| API 1104 | Root profile acceptance for pipeline girth welds, verified primarily by radiography where the root cannot be seen | Pipeline acceptance is its own regime; do not carry a structural or vessel criterion into it. |
| ASME BPVC Section VIII | Root reinforcement acceptance for pressure vessels, with examination methods in Section V | Acceptance lives in the construction code, not in the qualification code. |
Most weld imperfections are produced by conditions that exist while the arc is running. Therness records those conditions on every weld, so a suspect run is known on the shift that made it rather than in a batch report.
Progetto cofinanziato nell'ambito del PR Piemonte FSE+ 2021-2027,
Priorità I, Obiettivo Specifico a), Azione 4 – "Sostegno alla nascita delle start up"
