Question 6 of 7: NDT for Welded Pipe Cracks; Headlight Filament Forensics; Worn Carburized Gear

Question 6: NDT for Welded Pipe Cracks; Headlight Filament Forensics; Worn Carburized Gear (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. (a) A welded austenitic stainless steel pipe with suspected cracks oriented parallel to the pipe surface. (b) A headlight-filament dispute in an accident investigation; lab examination disproved the "not working" claim. (c) A carburized alloy-steel transmission gear, spec surface hardness 61 Rc, measured 31 Rc on an intact tooth from a companion gear.

Find. (a) Suitable NDT methods and why. (b) What test(s) were run and how the result differs for a lit vs. unlit filament. (c) Likely root causes and the confirming lab procedure.

Approach

Each part matches a physical mechanism to the observable evidence: (a) match NDT method physics to a planar, surface-parallel reflector; (b) match filament metallurgy (hot vs. cold tungsten) to impact response; (c) match a large surface-hardness shortfall to plausible case-hardening failure modes and the metallurgical evidence that would confirm each.

1. (a) Candidate methods and their fit to "parallel-to-surface" cracks. Liquid penetrant testing (PT) only reveals surface-breaking flaws, so it is of limited use if the crack lies below the surface. Radiography (RT) is good for volumetric defects but has poor sensitivity to planar cracks lying parallel to the beam/film plane — exactly the orientation described — so it tends to miss or badly under-report them. Eddy current testing (ET) is sensitive to near-surface flaws in a conductive material like stainless steel but has limited depth of penetration. Magnetic particle inspection (MT) is not applicable here because austenitic stainless steel is essentially non-ferromagnetic. The best-suited method is angle-beam (shear-wave) ultrasonic testing (UT): the sound beam is directed into the wall at an angle chosen to strike a parallel-to-surface reflector nearly perpendicular to its face, producing a strong, unambiguous echo — the standard volumetric method for this particular defect orientation, best paired with surface PT for any surface-breaking component.
2. (b) Test performed and result for a lit filament. A metallurgical/microscopic examination of the tungsten filament was performed, looking for evidence of hot plastic deformation. If the bulb was on (lit) at the moment of impact, the tungsten filament is close to its operating temperature and is soft/ductile at that temperature (much lower flow stress near the melting point), so the sudden deceleration stretches, sags, or distorts the coil plastically — visible as elongated/deformed coils under magnification, and, if the glass envelope also broke while the filament was still hot, localized oxidation discoloration from air reaching the glowing metal. Finding this hot-stretched, sometimes-oxidized filament is what disproved the claim that the lights were off.
3. (b) Result if the headlights had been off. A cold, unlit tungsten filament is near room temperature, where tungsten is comparatively brittle (a high ductile-to-brittle transition behaviour for the fine wire form). The same impact would instead fracture the filament cleanly with little to no plastic deformation, and no hot-oxidation discoloration would be present even if the envelope shattered, since the metal was never hot enough to react with any admitted air.
4. (c) Possible sources of the failure. A surface hardness of only $31$ Rc against a $61$ Rc specification is far too large a shortfall to be measurement noise, and points to a genuine case-hardening failure: (i) an inadequate/insufficient carburizing treatment (too low a temperature, too short a time, or the process omitted altogether, so the surface carbon content/case depth never reached spec); (ii) an improper post-carburizing heat treatment — too slow a quench (case never fully hardens to martensite) or too high a tempering temperature (over-tempers/softens the case); (iii) decarburization of the surface in a non-protective (oxidizing) furnace atmosphere before the final harden, removing the very carbon the case needs; (iv) the wrong steel grade substituted (insufficient hardenability even with correct carbon); (v) grinding burn after hardening, locally re-tempering/softening an otherwise correct case.
5. (c) Confirming laboratory steps. (1) Cross-section the tooth and run a microhardness traverse from the surface inward to establish the actual hardness-vs-depth (case-depth) profile against the drawing specification. (2) Prepare a metallographic cross-section, etch, and examine the surface microstructure — a properly hardened case should show tempered martensite; pearlite/ferrite or clearly over-tempered martensite at the surface confirms a heat-treatment or decarburization problem. (3) Carbon-content analysis (combustion/OES spot analysis or an SEM/EDS line scan) to check whether the surface carbon actually reached the target case-carbon level. (4) A temper-etch inspection (e.g. nital etch) to look for grinding-burn bands. (5) Bulk chemical analysis to confirm the correct alloy grade was used.