6.2 Typical Discontinuities by Product Form

Discontinuities by Product Form and the Method That Finds Them

Level III method-selection questions almost always arrive dressed as a product form: "a casting with…", "a forging suspected of…", "a butt weld containing…", "rolled plate with…". Knowing the signature discontinuities of each product form, and matching each to the method its geometry and material favor, is the fastest route to the right answer.

Castings

Molten metal freezing in a mold produces mostly volumetric discontinuities: gas porosity (rounded voids from trapped gas), shrinkage (macro- and microshrinkage cavities where the last metal to freeze pulls away), cold shut (two metal streams that meet but fail to fuse), hot tears (ragged cracks where solidification contraction is restrained by the mold), plus nonmetallic inclusions, dross, misruns, and unfused chaplets. Internal volumetric flaws such as porosity and shrinkage are RT targets; surface-connected ones such as surface hot tears and cold shut go to MT on ferromagnetic castings or PT. UT is difficult on many castings because coarse, as-cast grain structure scatters and attenuates the sound beam. PT is easily fooled on rough, porous cast surfaces, which bleed excessive background and mask real indications — a very common exam point.

Forgings

Hot working under pressure aligns the grain flow and produces mostly linear, working-direction discontinuities. Signature forging discontinuities are laps (metal folded over onto the surface and not fused, usually lying at a shallow angle), bursts (internal ruptures from working at too low a temperature or with excessive reduction), flakes (internal hydrogen cracks in heavy sections), and general forging cracks, plus elongated stringers carried over from the ingot. Surface laps and cracks in ferromagnetic steel are MT work — with the field oriented perpendicular to the discontinuity for maximum leakage and sensitivity; nonferromagnetic forgings use PT or ET. Internal bursts and flakes are UT targets. Because forging discontinuities follow the grain flow, the inspector must magnetize or scan in more than one direction to avoid missing flaws parallel to a single field.

Wrought Products (Rolled, Extruded, Drawn)

Primary working of plate, bar, pipe, and extrusions produces laminations (flattened inherent voids and inclusions lying parallel to the rolled surface), seams (longitudinal surface discontinuities in bar and wire), stringers (elongated nonmetallic inclusions following the working direction), and cupping (centerline bursts in drawn or extruded bar). Because laminations lie parallel to the surface, straight-beam (0-degree) UT is the preferred method — the flat, parallel reflector returns a normal-incidence beam strongly, while angle beam and RT (with the beam running parallel to the thin lamination) are poor. Longitudinal seams are surface-connected and best found with MT on ferromagnetic bar, ET, or PT.

Welds

Welds concentrate the widest variety of discontinuities and are the richest exam source. Volumetric weld discontinuities — gas porosity, slag inclusions, and incomplete penetration at the root — image well on RT. Planar weld discontinuities — lack of fusion (LOF) at the fusion face and cracks (longitudinal, transverse, crater, and HAZ) — are found far more reliably with angle-beam UT, which is why UT is preferred over straight beam or RT for tight fusion-face flaws. Surface weld discontinuities — undercut, overlap, toe cracks, and crater cracks — are caught by VT, MT, or PT. RT can miss a tight planar LOF oriented nearly parallel to the beam, the mirror image of UT's relative weakness with scattered rounded porosity — which is exactly why many codes pair the two methods on the same weld.

Discontinuity → Best-Method Reference Table

The One-Line Selection Rule

The whole chapter collapses to a single heuristic: surface-breaking → VT, PT, MT (ferromagnetic only), or ET (conductive only); subsurface/volumetric → UT (planar, favorably oriented) or RT (volumetric/rounded). Layer on material (MT works only on ferromagnetic metals; ET only on conductors; PT only on nonporous surfaces), plus access and safety, and you can answer nearly every product-form question. Service discontinuities obey the same logic — a fatigue crack is a surface planar flaw (MT/PT first, UT for depth), while general corrosion is volumetric wall loss (UT/MFL/ET) — so origin never overrides the geometry-plus-material test.