Shipbuilding and marine fabrication
Weld monitoring for shipbuilding
High-volume welding under classification-society rules. The codes that apply, the four monitoring pain points that cost yards most, and which Therness signal addresses each.
ISO 6520-1 group 1 · Cracks
Transverse crack (ISO 6520-1 ref. 102): causes and detection
ISO 6520-1 reference 102 covers cracks running across the weld axis, produced by restraint or by delayed hydrogen cracking. Cause matrix, why a delayed-inspection interval matters, and where acceptance is set.
- ISO 6520-1 reference
- 102
- Group
- 1 — Cracks
- Also called
- Transverse crack
Transverse crack runs across the weld axis rather than along it, and ISO 6520-1 reference 102 covers it in the weld metal, the heat-affected zone or the parent material, regardless of which of two mechanisms produced it.
The first is restraint-driven: a joint stiff across the bead resists shrinkage, and the strain opens a crack perpendicular to the weld. The second is delayed hydrogen cracking, hours after the weld has cooled. The two call for different controls, but share a detection consequence: a crack not yet formed at inspection time is not there to find — the case for a delayed-inspection interval and for treating interpass temperature as a qualified variable.
It is also why branching crack so often begins as an untreated transverse crack, and shares its ultrasonic-testing route with longitudinal crack.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| High restraint pulling across the weld axis | A joint stiff across the bead resists transverse shrinkage, and the crack opens perpendicular to the weld to relieve that strain. | Reduce restraint where design allows, or sequence welding so shrinkage occurs before the joint locks up. |
| Diffusible hydrogen in a hardenable heat-affected zone | Hydrogen diffuses to the region of highest triaxial stress and combines with a hard microstructure hours after welding. | Use low-hydrogen consumables, control storage and re-drying, and hold preheat/interpass temperature in the qualified window. |
| Insufficient preheat for the combined thickness and carbon equivalent | Fast cooling produces a harder, more crack-susceptible HAZ microstructure while also raising residual stress. | Set and verify preheat and interpass temperature from carbon equivalent and combined thickness, not a blanket figure. |
| Multi-pass sequence that locks in transverse stress | An early pass allowed to cool and restrain the joint carries the full transverse shrinkage of everything welded afterward. | Follow the qualified pass sequence and interpass temperature rather than a convenient shop order. |
Detection: which route finds it
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 | Supporting signal | Finds a transverse crack once it reaches the surface; a hydrogen crack forming later is not there yet. |
| Thermal | In-process infrared thermography | Supporting signal | Flags a cooling rate or interpass temperature outside the qualified window, hours before a cold crack could form. |
| Camera | High-speed weld pool imaging | Limited | The mechanism is largely post-solidification and delayed, so pool-stage imaging has little to see. |
| NDT | Ultrasonic or phased array testing to ISO 17640 and ISO 13588, magnetic particle or penetrant testing for a surface-breaking crack | Primary route | A hydrogen crack can take up to 48 hours to fully develop, so the delayed-inspection interval matters as much as the method. |
Where acceptance is defined
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 imperfections indexed by ISO 6520-1 reference number; cracks are excluded from every level. | A transverse crack is a mandatory-repair finding at any quality level, not a matter of degree. |
| AWS D1.1 | Structural welding acceptance, which mandates a delayed-inspection interval on hydrogen-susceptible steels. | Inspecting immediately after welding can miss a crack that has not finished forming. |
| ASME BPVC Section VIII | Pressure vessel acceptance, with examination methods in Section V and repair requirements for any crack found. | Acceptance and repair requirements sit in the construction code, not in the qualification code that trains the welder and procedure. |
See the condition, not the consequence
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.
