14.3 NDT Method Selection, IQI & ISO 9712

14.3 NDT Method Selection, IQI & ISO 9712

Quick Answer: Choose NDT by what you need to find and what access allows. Surface methods (VT, PT, MT) address open or near-surface flaws; volumetric methods (RT, UT) address the weld volume. IQI proves RT sensitivity; each method has its own sensitivity demonstration. ISO 9712 Levels 1/2/3 certify NDT persons. IWI does not replace NDT certification — the inspector selects/verifies techniques and checks report adequacy, then uses quality documents for acceptance decisions (WI2.2, 2.8–2.11).

Chapters 13–14 methods only pay off when applied as a system. This section is the decision layer examiners love: given a joint, material, and risk, which methods, who may perform them, and how you prove the test was sensitive enough.

Surface vs Volumetric — Capability Map

Rule of thumb for exam scenarios

• Suspected toe crack open to surface on carbon steel fillet → VT + MT (or PT if non-magnetic)
• Root lack of fusion in thick butt weld → UT (often primary); RT may help depending on orientation
• Distributed porosity in aluminium TIG butt → RT often informative; UT characterisation may be harder
• Painted surface, need only surface-breaking after coating removal rules → follow procedure; PT/MT need proper surface condition

Never treat “we always do RT” as engineering. Critical review means asking: If the dangerous defect for this joint is planar LOF, does RT alone satisfy the risk?

Matching Method to Expected Discontinuity

Drive selection from process metallurgy and joint design, not habit:

1. List credible imperfections for the process/position (HICC risk → surface cracks after delay; high heat input thick section → LOF risk; aluminium → porosity risk)
2. Map each risk to a method that can detect it with required reliability
3. Check access (both sides? OD only? coating? temperature?)
4. Check code/client mandatory NDT (percentage, method, timing — e.g. after PWHT)
5. Write ITP with hold/witness points and acceptance standards (ISO 5817 quality level, product standard, or project specification)
6. Verify personnel and procedures before work, not after a dispute

IWI-S competence includes identifying and verifying relevant NDT techniques for a welded construction. On the exam, prefer answers that justify method by discontinuity type over answers that simply name the most expensive test.

Critical Review of Method Selection — Worked Patterns

Pattern A — Fillet welds on structural steel Volumetric RT of fillets is often impractical or low-value. VT + MT (magnetic materials) for surface cracking and profile, plus process control, may dominate. UT of fillets exists but needs specialised technique.

Pattern B — Butt welds in pressure piping Codes often mandate volumetric RT or UT by service class. Choose UT when planar flaw detection and depth for repair matter; choose RT when client specification or geometry favours radiography and porosity detection. Sometimes both staged (spot RT + UT, or production percentage rules).

Pattern C — Austenitic stainless MT does not apply. PT for surface; RT and specialised UT for volume. Coarse dendrites challenge conventional UT — procedure and operator skill become critical review items.

Pattern D — “NDT instead of WPS control” Invalid philosophy. NDT finds some imperfections after the fact; parameter control, fit-up, and consumable control prevent them. Inspectors who skip surveillance and only collect NDT reports fail the lifecycle model from Chapter 2.

IQI and Sensitivity Demonstrations Across Methods

Radiography — IQI / penetrameter As in 14.1, the IQI demonstrates that a defined sensitivity was achieved on that film. No required IQI feature visible → image not quality-adequate.

Ultrasonics — reference reflectors Side-drilled holes, notches, and DAC/TCG (or DGS) establish sensitivity. Transfer corrections link block to job. Encoded PAUT/TOFD systems still need procedure-defined sensitivity proofs.

MT — field adequacy Pie-field indicators, artificial shims, or quantitative field measurements (procedure-dependent) show that magnetisation was sufficient. Paint thickness and lift-off matter.

PT — system performance Known defect panels or procedure checks verify penetrant system sensitivity class.

VT — visual aids Lighting levels, weld gauges, and contrast (when specified) support reliable visual examination.

Inspector language: “Was sensitivity demonstrated and recorded?” is better than “Did someone look at it?”

Other Tests — Pressure, Dimensional, Production Testing

NDT is not the only verification tool in WI2:

• Pressure / leak tests (hydrostatic, pneumatic where allowed, sensitive leak tests) prove containment under controlled conditions — they do not size every planar weld flaw and can be hazardous if misapplied
• Dimensional inspection — tolerances, alignment, camber, nozzle orientation; essential for fitness and assembly
• Hardness surveys (often considered with DT/production tests) after PWHT or on HAZ — process control evidence
• Production test coupons welded with production parameters and then NDT + DT — used in some codes for ongoing proof

Critical review: a successful hydrotest does not mean RT/UT requirements can be ignored if the code still requires them. Conversely, perfect RT does not waive dimensional nonconformity.

ISO 9712 — NDT Personnel Levels Overview

ISO 9712 (and aligned national schemes) certifies persons in specific methods and often sectors (e.g. welds):

Exact privileges depend on the certification body’s scope and the employer’s written practice. For IWI exams, remember:

• Method-specific: UT Level 2 ≠ RT Level 2
• Certification has validity, renewal/recertification, and near vision requirements
• Employer authorisation (written practice) often sits beside certificate

IWI vs ISO 9712 — Do Not Collapse the Roles

Hard exam statement: IWI does not replace NDT certification. Holding IWI-S does not let you sign RT film interpretation as a radiographer. Holding UT Level 2 does not make you a welding coordinator under ISO 14731. Projects need both competencies in the right places on the ITP.

Putting It Together for ITPs

A coherent NDT package for a steel pressure butt weld might read:

• 100% VT all welds
• 100% MT after root (if accessible) or final surface as specified
• 100% UT volumetric after completion / after PWHT if required
• RT only if specified alternatively or for particular joints
• Hydrotest at system stage
• NDT personnel: ISO 9712 Level 2 minimum for evaluation in UT/MT as applicable
• Welding inspector: witness hold points, review reports, film quality check if RT used, NCR control

Adjust percentages and methods to code and contract. The inspector’s skill is critical review when someone proposes a method that cannot see the governing defect type.

Exam Focus

• Surface vs volumetric capability tables
• Method ↔ discontinuity matching with process risk
• IQI for RT; analogous sensitivity proofs for other methods
• ISO 9712 L1/L2/L3 overview
• IWI verifies adequacy and selects techniques; NDT certificates perform and interpret