ISO 6520-1 group 5 · Imperfect shape and dimension
ISO 6520-1 reference 5013 is undercut's root-side member (501), formed by shrinkage rather than arc erosion. Cause matrix, the root-concavity distinction, and where acceptance is set.
A shrinkage groove is ISO 6520-1’s root-side member of the undercut family: 501 covers 5011 (continuous), 5012 (intermittent) and 5013 (root). Undercut at the toe comes from arc erosion outrunning filler; a shrinkage groove comes from the root bead itself shrinking as it solidifies, opening along one or both edges of the root run — a different location from root concavity, the same bead’s centreline sagging inward.
The distinction that matters on the shop floor is lack of penetration: a shrinkage groove still has a thin, continuous fused root bead; lack of penetration has none. A radiograph reads the two apart on that feature.
Because the mechanism is volumetric, the fix sits upstream: a joint volume and consumables check run against the root gap and land catches a pass short on filler before the groove shows on film.
| Cause | Mechanism | Control |
|---|---|---|
| Autogenous or filler-light root pass on a joint welded from one side only | With little filler bridging the root, the bead itself is the only metal available, and as it solidifies and contracts it draws away from the sidewalls, opening a groove along one or both edges of the root run. | Add filler on the root pass rather than running it autogenous, and qualify root parameters on that basis. |
| Root pass travel speed too fast for the joint gap and land | A narrow, fast-run root bead freezes into a smaller cross-section than the pool occupied, and the shortfall opens along the edges where the bead meets the sidewall. | Measure root travel speed against the qualified procedure rather than judging bead width by eye. |
| High-shrinkage alloy welded to parameters set for carbon steel | Austenitic stainless and nickel alloys contract more on solidification, so a profile flush in carbon steel can still groove at the same parameters. | Qualify root parameters and filler addition per alloy family, not carried over from a carbon-steel procedure. |
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 where access exists | Limited | The root of a one-sided joint is usually the far side of the assembly, unreachable without sectioning — why radiography is the usual route. |
| Thermal | In-process infrared thermography of the root pass | Supporting signal | A root pass short on filler shows a different cooling signature before the groove sets into the bead. |
| Camera | Root-side weld pool imaging on orbital and mechanised heads | Limited | Useful only where the head views the root; most one-sided passes give no camera access there. |
| NDT | Radiographic testing to ISO 17636 | Primary route | Reads as two density-reduction lines flanking the root bead, not one centreline line (that is root concavity, ref. 515). Told apart from lack of penetration by a fused, thin bead still present between them. |
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 root-side imperfections, in a separate row from toe undercut (5011/5012) | Read the standard's own reference number; one row does not cover both. |
| API 1104 | Acceptance for pipeline girth weld root profile | Assessed together with root penetration and reinforcement. |
It is a member of the same family: ISO 6520-1 files 5011 (continuous), 5012 (intermittent) and 5013 (root) under 501. 5011/5012 come from arc erosion at the toe; 5013 comes from the root bead shrinking as it freezes and pulling from the sidewalls. Each keeps its own ISO 5817 row.
By where the depression sits. A shrinkage groove flanks the root run at the sidewall; root concavity (ref. 515) is the bead's own centreline sagging inward.
By whether a root bead is present. A shrinkage groove shows a thinned, continuous bead; lack of penetration shows a gap where the root never fused.
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"
