14.4 Destructive Testing of Welded Joints

14.4 Destructive Testing of Welded Joints

Quick Answer: Destructive testing (DT) proves properties by breaking, bending, impacting, indenting, or sectioning welded specimens. It underpins WPQR (e.g. ISO 15614 route) and welder qualification (e.g. ISO 9606) together with required NDT. Common tests: tensile, bend, Charpy impact, hardness, macro (± micro). IWI-S witnesses tests when required, verifies coupon identity/traceability, and reviews lab certificates as quality documents — without fabricating mechanical values.

NDT asks “is this production weld free of rejectable discontinuities?” DT asks “does this qualified procedure/welder coupon meet mechanical and structural requirements?” Both appear in WI2.1 and in practical inspection competence.

Why Destroy Good Welds?

A qualification coupon is sacrificed so production can proceed with confidence. Properties that matter:

• Strength — will the joint carry design load (tensile, sometimes all-weld-metal tensile)
• Ductility / soundness — can it bend without cracking from LOF, inclusions, or brittle zones
• Toughness — resistance to brittle fracture at design temperature (Charpy V-notch energy or lateral expansion)
• Hardness — HAZ peaks linked to hydrogen cracking risk or wear applications; PWHT effectiveness
• Macrostructure — fusion, penetration, HAZ extent, multipass sequence, defects on the polished section

Production welds are usually not all destructively tested; sampling and NDT protect production. Qualification is the place where DT is intensive.

Core Test Types for Welding Inspectors

Tensile tests

Transverse tensile specimens pull the weld across the joint. Acceptance typically requires failure load/strength meeting parent metal or specified minimums, with failure location notes (weld, HAZ, parent). All-weld-metal tensile (when required) characterises deposited metal. Inspector checks: specimen machining from correct location, orientation, standard method on the certificate, and numeric results vs WPQR acceptance.

Bend tests

Face, root, and side bends (as required by thickness and standard) bend the specimen over a former of specified diameter. They expose lack of fusion, inclusions, and brittle regions as cracks on the tension surface. Partial small openings may be limited by acceptance criteria; open defects usually fail. Bends are highly visual for witnessing — you can see soundness failures without a densitometer.

Impact tests (Charpy V-notch)

A notched bar is broken by a pendulum at a specified temperature. Energy absorbed (joules) and sometimes lateral expansion or fracture appearance are recorded. Toughness qualification is temperature-critical: a pass at −20 °C does not automatically prove −40 °C service without the correct test temperature. Notch location (weld metal, FL, HAZ offsets) is defined by the procedure qualification standard. Inspector traps: wrong temperature, wrong notch location, or averaging tricks not allowed by the standard.

Hardness testing

Vickers or other methods map hardness across weld, HAZ, and parent on a macro section or production survey. Limits may cap maximum HAZ hardness (hydrogen cracking control) or require minimum hardness for wear. After PWHT, hardness can show whether heat treatment met the intent. Equipment calibration and indent placement matter; random single indents without a traverse may be noncompliant with the WPQR standard’s figure.

Macro and micro examination

Macro (low magnification, etched section) shows fusion line, penetration, number of passes, undercut, pores, cracks, and HAZ width. Acceptance may allow only imperfections within procedure standard limits. Micro (higher magnification) examines microstructure (e.g. grain size, phases in stainless, potential brittle constituents) when the standard or client requires it. Macro is frequent in WPQR packages; micro is more selective.

Use in WPQR and Welder Tests

Procedure qualification (WPQR / PQR) — e.g. under ISO 15614 series (know the concept even if you memorise the project’s exact part): a test piece is welded with recorded parameters; NDT and DT are performed per the standard’s table for material group, thickness, and joint type. Passing results support a WPS range of validity (thickness, diameter, positions, etc.).

Welder / welding operator qualification — e.g. ISO 9606 / ISO 14732: the person (or operator) demonstrates skill on a coupon. Tests often include visual, bend or fracture, and sometimes radiography/UT depending on the standard and product. Range of qualification (process, material group, position, thickness) follows the standard’s rules.

Inspector responsibilities in both streams:

1. Before welding the coupon — verify WPS draft/parameters, material certificates, consumable batch, joint preparation, position, essential variables understanding
2. During welding — witness as ITP requires; record actual parameters for the WPQR
3. After welding — ensure coupon ID, heat number, welder ID stamping; prevent mix-ups
4. NDT of coupon — before cutting specimens if required
5. DT laboratory — correct specimen extraction plan; chain of custody
6. Review certificates — accept/reject package; raise NCR if failed; control retest rules

Witnessing Tests as IWI-S Duty

Witnessing is not passive tourism. Effective witness includes:

• Confirming which standard and acceptance table apply
• Confirming specimen identity matches the welded coupon (stamp transfer, cutting sketch)
• Observing critical steps defined as hold points (e.g. bend execution, impact temperature verification)
• Checking lab equipment calibration status where accessible/required
• Recording time, place, results, deviations in the inspection report
• Stopping the process if identity is lost or the wrong specimen is about to be tested

You may not personally operate the tensile machine if company practice assigns that to the lab technician — but you own the verification trail when the ITP names welding inspection witness.

Laboratory Certificates — Critical Review

A lab certificate is a quality document. Incomplete certificates fail review even if someone says “it passed verbally.”

Minimum review points:

• Laboratory identity and accreditation statements as contract requires
• Customer, project, and unique specimen/coupon ID
• Welding process, base material, consumable references linking to WPQR
• Test standard method (e.g. which tensile/impact standard)
• Temperature for impact; notch location
• Numeric results with units and acceptance criteria used
• Date, technician/authorisation signatures
• Any deviations, retests, or broken grips notes

Trap: accepting a tensile certificate that omits specimen ID or shows parent metal strength without confirming failure location rules required by the qualification standard. Trap: Charpy results at ambient used to claim low-temperature qualification.

DT vs NDT — Keep the Distinction Sharp

Both feed quality documents on which IWI-S may decide. Neither replaces process control.

Link to Later Chapters

Chapter 15 develops ISO 15614 / ISO 9606 acceptance logic and ISO 5817 quality levels in more depth. Chapter 16 places DT/NDT witness points inside ITPs and NCR loops. For now, master the test menu, witness discipline, and certificate literacy.

Exam Focus

• Name tensile, bend, Charpy, hardness, macro/micro and what each proves
• WPQR vs welder test purpose of DT
• Witness and traceability duties
• Lab certificate completeness as a quality-document decision
• DT does not replace NDT on production welds required by code