11.3 Non-Destructive Testing (NDT) Methods

Sub-task F-23.04 (Performs non-destructive testing procedures) is one of six sub-tasks in the 11-question Task F-23. Millwrights routinely perform basic dye penetrant and magnetic particle checks on shafts, hooks, welds and castings, and they prepare components for, and act on the results of, tests performed by certified technicians.

Certification

In Canada, NDT personnel are certified under the Natural Resources Canada / CGSB certification program (CAN/CGSB-48.9712) in three levels:

Certification is per method and per sector. A millwright may perform a basic penetrant check as part of a maintenance inspection, but acceptance or rejection against a code criterion requires the appropriate certification.

The Five Principal Methods

Liquid Penetrant Testing (PT)

The sequence is fixed and each step matters:

1. Pre-clean thoroughly. Any paint, oil, scale or blasting residue bridges the discontinuity and hides it. Never sand-blast or shot-peen before PT — it smears metal over the flaw.
2. Apply penetrant and allow the specified dwell time (typically 10–30 minutes; longer for tight fatigue cracks and for cold parts). Capillary action draws penetrant into the opening.
3. Remove excess penetrant from the surface only — with a solvent-dampened cloth wiped in one direction, never by flooding the surface with solvent, which washes the penetrant back out of the flaw.
4. Apply developer, a fine white powder that acts as a blotter and draws penetrant back out.
5. Interpret within the specified development time. Visible-dye penetrant shows red indications on white; fluorescent penetrant shows green under ultraviolet light in a darkened area and is far more sensitive.
6. Post-clean the part, because penetrant residue can be corrosive to some alloys.

The absolute limitation: PT finds nothing that does not break the surface. A subsurface void or a crack that has been peened or smeared closed will not show.

Magnetic Particle Testing (MT)

A magnetic field is induced in a ferromagnetic part. Where a discontinuity interrupts the field, magnetic flux leaks out of the surface and attracts fine iron particles, forming a visible indication.

The critical technique point: flux leakage occurs only when the discontinuity lies across the magnetic field. A crack running parallel to the field is invisible. Therefore every area must be tested twice, with the field oriented roughly 90 degrees apart between the two inspections.

Wet fluorescent MT under ultraviolet light is the most sensitive variant and is standard for shafts, hooks, crane components and welds. Parts should be demagnetized after testing, because residual magnetism attracts wear particles to bearings and interferes with instrumentation.

Ultrasonic Testing (UT)

A transducer sends a high-frequency sound pulse into the material and times the returning echoes. UT is the workhorse of in-service inspection because it needs only one accessible side.

• Thickness gauging — the standard method for monitoring corrosion in piping, tanks, vessels and elbows; readings are trended over time to compute a corrosion rate and remaining life.
• Flaw detection — locates and sizes internal cracks, laminations and inclusions, including sizing crack depth in a shaft.
• Phased array and time-of-flight diffraction — advanced techniques for weld inspection.
• A couplant (gel or oil) is required to transmit sound into the part; an air gap reflects almost all of the energy.
• The instrument must be calibrated on a reference block of the same material and thickness range before use, because sound velocity differs between materials.

Radiographic Testing (RT)

X-rays or gamma rays pass through the part and expose film or a digital detector. Denser material absorbs more radiation, so voids, slag and porosity appear as dark areas.

RT excels at volumetric flaws in welds. Its limitations are practical: access to both sides is required, it is relatively poor at detecting tight planar cracks oriented across the beam, and the radiation hazard demands controlled exclusion zones, barriers, warning signs, dosimetry and licensed personnel, which is why plant radiography is usually done on off-shifts. Never enter or work near a posted radiography exclusion zone.

Other Methods a Millwright Encounters

• Eddy current (ET) — surface and near-surface flaws in conductive material; heat exchanger tube inspection.
• Magnetic flux leakage (MFL) — floor scanning in storage tanks (see section 14.3).
• Acoustic emission — listens for the stress waves emitted by growing cracks in a pressurized vessel.
• Airborne ultrasound — detects compressed air and vacuum leaks, steam trap faults and early bearing distress; overlaps with predictive maintenance in section 11.1.
• Hardness testing and replication metallography — assessing in-service material degradation.

Using NDT Results

An NDT report is an input to a decision, not the decision. A millwright uses it to:

• Confirm whether a suspect crack is real, and how deep it is, before deciding to repair or replace.
• Verify a repair weld before returning equipment to service.
• Track wall-thickness trend data against a retirement thickness so replacement is planned rather than reactive.
• Provide evidence for a failure investigation.

A finding that a component is cracked is not a licence to weld it. Cracks in rigging hardware, pressure-retaining components and registered vessels have specific code responses — usually scrapping the item or a certified repair procedure, not a field weld.