Most weld defects are easier to prevent than to repair. A bevel cut at the wrong angle, a root gap too wide, damp electrodes or skipped preheat can all be caught before the first pass. Once the weld is finished, the same problems cost grinding, repair welding and repeat NDT. Welding inspection from fit-up to final acceptance therefore starts well before welding and continues well after it. This guide covers the inspection stages for welding inspectors, QC engineers and welding supervisors.
What are the stages of welding inspection?
Welding inspection is usually divided into three stages: before welding, during welding and after welding, followed by repair control and final acceptance with complete records.
Who carries out welding inspection
Contractor welding inspectors cover most of the work, with client or third-party inspectors witnessing selected points. Both work from the same procedures, acceptance criteria and inspection plan.
Before welding
- Procedure approval. The welding procedure specification (WPS) is approved and supported by a qualified procedure qualification record (PQR).
- Welder qualification. Each welder holds a valid qualification for the process, position and material, and is identified on the weld record.
- Material verification. Materials match the specification, certificates and heat numbers, with PMI for alloys where required.
- Consumable control. Electrodes and filler wires are correct, stored properly and, for low-hydrogen electrodes, kept in heated ovens or holding containers.
- Joint preparation. Bevel angle, root face and cleanliness checked.
- Fit-up. Root gap, alignment and internal misalignment within tolerance.
- Preheat. Applied and verified where the procedure requires it.
During welding
- Welding parameters, such as current, voltage and travel speed, within the WPS range
- Interpass temperature controlled within limits
- Cleaning between passes to remove slag and prevent inclusions
- Back purging for stainless steel and other materials that require it
- Welding sequence followed to control distortion and residual stress
In-process checks are where welding inspectors add most value, because problems found here are corrected before they are buried under later passes.
After welding
- Visual inspection of weld profile, size, undercut, surface porosity and cracks
- NDT to the specified method and extent
- Post-weld heat treatment where required, with chart records reviewed
- Hardness testing where specified, often after heat treatment
- Dimensional checks where welding may have caused distortion
The NDT methods involved and when each is used are explained in when NDT is used in fabrication and pipeline work.
Repairs
Repairs follow an approved repair procedure: defect removal, confirmation that the defect is gone, re-welding by a qualified welder and re-inspection by the same method that found the defect. Repair rates by welder and procedure should be tracked weekly, because rising repairs signal a training, supervision or consumables problem.
Common welding inspection gaps
- Fit-up checked on a sample basis when the ITP requires every critical joint
- Electrode ovens on site but not used
- Preheat checked at the start of welding but not maintained
- Interpass temperature not measured at all
- Repairs welded without an approved repair procedure
Each of these is cheap to prevent and expensive to discover through NDT or, worse, in service.
Typical inspection points
| Stage | Typical point |
|---|---|
| WPS and welder qualification review | Review before work starts |
| Fit-up of critical joints | Witness |
| Preheat and in-process checks | Surveillance |
| Visual check of every completed weld | Contractor QC inspection |
| NDT | Review of reports |
| Heat treatment | Review of charts |
| Repairs | Re-inspection by the original method |
| Final acceptance | Release on the weld log |
These points are typically set out in the project ITP, as described in inspection and test plans for industrial construction.
Records and traceability
Each weld should be recorded in a weld log with its number, welder, procedure, date, inspection results, NDT reports, heat treatment and repairs. The log links the weld to the rest of the quality record, as explained in quality documentation for project handover. Shop-side welding controls are covered in fabrication quality controls that prevent field delays, and spool fabrication steps in pipe spool fabrication from isometrics to installation.
Apex Core Engineering supplies certified welding inspectors as part of its NDT and welding inspection support.
Frequently asked questions
Why are low-hydrogen electrodes kept in ovens?
To keep moisture out. Moisture introduces hydrogen into the weld, which can cause hydrogen-induced cracking in susceptible materials.
What is interpass temperature?
The temperature of the weld area just before the next pass is deposited. Procedures set limits to protect the material’s properties.
When is a weld finally accepted?
When visual inspection, required NDT, heat treatment and any repairs are complete and accepted, and the weld log shows it released.
Get certified welding inspectors through Apex Core Engineering.