5.2 Processing Discontinuities

5.2 Processing Discontinuities

Quick Answer: Processing discontinuities are introduced while manufacturing the component—forming, machining, grinding, welding, heat treating, plating, and similar operations. They are not primary solidification features of the stock (inherent) and not in-service damage (service). Classic examples include machining cracks, grinding burns and cracks, weld defects, forming cracks, quench cracks, and plating-related cracking. Level III judgment is process → flaw → method → when to inspect.

If inherent flaws answer “what did the mill put in the metal?”, processing flaws answer “what did the shop do to the part?” Domain 4 of the Basic exam repeatedly tests that distinction with manufacturing scenarios.

Definition and Boundaries

A discontinuity is processing-related when the essential free surface, crack, fold, or volumetric defect is created by a fabrication or finishing process applied to stock or a semi-finished product.

Some defects look like service cracks under a microscope. Classification still follows when the crack formed relative to the life cycle, not only fracture-surface appearance.

Process → Flaw Map (High-Yield Table)

Memorize the left two columns for scenario questions; use the right columns for method selection.

Machining Cracks and Surface Integrity

Machining removes metal. When feeds, speeds, tool wear, or material hardness are wrong, the surface can develop:

• Cracks from excessive tensile residual stress or thermal shock at the tool tip
• Deep tool marks that act as notches for later fatigue (processing origin of a stress raiser even if the crack opens in service)
• Smeared metal that hides underlying flaws from PT

High-hardness alloys, quench-and-temper steels, and some cast irons are especially sensitive. Level III procedures often require surface NDT after final machining of critical features (threads, seal diameters, bearing journals) because machining both creates and reveals discontinuities.

Grinding Burns and Grinding Cracks

Abusive grinding—too aggressive a wheel, inadequate coolant, or glazed wheel—creates localized overheating:

1. Temper burn: over-tempered soft spots (dark etch response)
2. Re-hardening burn: thin martensite layer that can crack
3. Grinding cracks: typically shallow, often perpendicular to the grinding direction or in a network on the ground surface

For NDT:

• MT (wet fluorescent on steel) is classic for grinding cracks after finish grind of hardened parts
• PT for nonmagnetics or when MT is unsuitable
• Visual color change is a clue, not proof of crack depth
• Timing: many aerospace and gear procedures mandate inspection after final grind, not only after rough machine

Grinding cracks are processing even if the part later fails in fatigue from that crack—the origin class follows introduction during manufacturing.

Weld Defects as Processing Discontinuities

Fusion welding is a manufacturing process; weld discontinuities are therefore processing (unless the weld was sound and a service crack later initiates at the toe). Core families:

Method suites are code-driven, but Level III Basic logic remains: surface weld cracks and undercut → VT + MT/PT; volumetric porosity/slag/LOF → RT and/or UT depending on joint access and thickness. Chapter 4 deepens welding processes; here the classification point is origin = processing.

Forming Cracks

Cold or hot forming (bending, stamping, spinning, expanding) can crack material when strain exceeds local ductility:

• Outer fiber tensile cracks on bend radii
• Orange-peel surface rupture on coarse-grain material
• Luders or stretcher strains (more cosmetic, but can confuse VT)

Forming cracks are often surface-breaking and elongated parallel to the free edge of the bend or transverse to the principal stretch direction. MT/PT/VT dominate; UT is secondary unless thickness and access support subsurface checks at severe formed zones.

Heat-Treat Quench Cracks

Quench cracks form when rapid cooling produces tensile surface stresses (or internal stresses) that exceed the material’s strength while it is hard and brittle. Favored sites:

• Sharp corners, keyways, holes, and abrupt section changes
• High-hardenability steels quenched too severely
• Delayed cracking when hydrogen and residual stress combine after quench

NDT implications:

• Cracks may be deep relative to grinding cracks
• MT is highly effective on ferromagnetic parts once scale is controlled
• Procedures often specify inspection after quench and temper and again after final grind if grind is used to restore dimensions
• Residual stress and permeability changes after heat treat affect MT technique variables—Level III must approve parameters for the as-heat-treated condition

Plating and Related Processes

Electroplating, especially of high-strength steels, can introduce hydrogen that promotes hydrogen-assisted cracking. Process controls (baking after plate) are primary; NDT may be required after plating on fracture-critical parts. Coating processes can also create blisters or delamination at the coating–substrate interface—surface methods for open defects, and specialized approaches when interface integrity is critical.

Inspection Timing and Method Selection

Level III thinking for processing discontinuities:

1. Identify the last process that can create the flaw of concern
2. Inspect after that process (and after surface preparation that reveals the surface)
3. Match surface vs volumetric methods to whether the flaw is expected open to the surface
4. Do not rely on receiving inspection of raw stock alone if welding, quench, or grind follows

Exam Traps

• Calling a quench crack “inherent” because the steel chemistry was high-carbon — chemistry is material; the crack is processing.
• Calling weld porosity “service” because the vessel is already installed — if it formed during welding, origin remains processing (discovery time ≠ origin class).
• Using only RT for suspected tight grinding cracks — wrong morphology/method match; surface methods first.
• Ignoring orientation: forming and grind cracks have preferred directions relative to tools and strain.

Processing discontinuities close the manufacturing half of the origin triad. Section 5.3 covers what happens after the part enters service.