3.2 Forging, Rolling, and Forming

3.2 Forging, Rolling, and Forming

Quick Answer: Forging and rolling deform solid metal under pressure, refining structure and creating grain flow. Benefits come with process-specific discontinuities—laps, folds, bursts, flashes in forgings; laminations, seams, and stringers in rolled products; cracks from bending and forming. NDT success depends on knowing flaw orientation relative to the product form.

Wrought manufacturing dominates structural metals: shafts, flanges, pressure vessel plate, bar stock, and formed shells. Level III candidates must contrast wrought discontinuities with cast ones and explain why inspection direction matters for UT and RT.

Forging Fundamentals

Forging plastically deforms heated (or sometimes cold) metal between dies or tools. Deformation closes some cast porosity, breaks up dendrites, and aligns grains and inclusions along the flow direction.

Grain Flow Benefits

Proper grain flow follows the contour of the part so fibers run parallel to expected tensile stresses (e.g., around a crankshaft fillet). Compared with a machined-from-billet part that cuts across grain, a well-forged part usually shows superior fatigue resistance and toughness in the flow direction. Transverse properties (across flow and inclusion stringers) can be poorer—important for mechanical test orientation and for understanding anisotropic UT responses.

Forging Discontinuities

Laps and folds form when metal folds over itself without welding—often from improper die design, excess or misplaced stock, or cold surface scale. They are typically surface or near-surface, elongated along the flow, and may be oxide-filled (tight, hard to wet with penetrant if very tight).

Bursts are internal ruptures from excessive deformation, low ductility (wrong temperature, segregate-rich centers), or secondary tensile stresses during forging. Central bursts in drawn or forged bar are classic. They act as internal cracks—UT is usually preferred.

Flashes are excess metal squeezed from the die parting line. Flash itself is trimmed; problems arise when flash is folded back into the part or when the flash line traps oxide (flash-line defects). Parting-line cracks and mismatch steps are inspection focus zones.

Shear cracks / die mismatch and underfill (incomplete die fill) produce geometric and crack-like indications at die locks and corners.

Scale pits and seams from surface oxidation during heating can be forged into the surface as linear indications—often detected by MT/PT/VT after descaling or machining.

Rolling Products and Discontinuities

Rolling reduces thickness and elongates metal between rolls. Starting stock may be ingot or continuous-cast slab/bloom/billet.

Laminations

Laminations are planar separations parallel to the rolling plane, often originating from cast shrinkage, porosity, or inclusion clusters that are flattened and elongated. In plate, they can be large and UT plate scanning (normal-beam) is the standard detection approach because the lamination faces the beam as a strong reflector. RT is less sensitive to tight planar laminations oriented parallel to the film (little thickness change).

Seams

Seams are longitudinal surface or near-surface discontinuities, often from elongated surface defects, cracks, or folded overfill on billets that are then rolled into bar or pipe. They run parallel to the rolling/drawing direction. MT (ferromagnetic bar), PT, ET, and specialized seam-detection systems are common. Orientation: long linear, parallel to axis.

Stringers

Stringers are elongated nonmetallic inclusions aligned with the working direction. They may not be “cracks” but reduce transverse toughness and can appear as linear UT indications or MT background. Chemistry and deoxidation practice (e.g., sulfide shape control) influence stringer severity.

Pipe and Related Internal Defects

In rolling/forging terminology, pipe can mean centerline residual shrinkage cavity from the original ingot that was not cropped adequately—elongated into a central discontinuity. Distinguish this metallurgical “pipe” from extruded “extrusion pipe” discussed in the next section; both are central voids with process-specific origins.

Forming and Bending

Forming includes press-brake bending, roll forming, spinning, deep drawing, and stretch forming. Outer fibers of a bend are in tension; inner fibers compress. Forming cracks typically start on the outside of the bend when bend radius is too tight for the material’s ductility, when work hardening is excessive, or when surface defects act as starters.

Other forming issues:

• Orange peel / surface roughness from coarse grain—VT and surface method background.
• Wrinkling on the compressive side—geometric, usually VT.
• Springback is dimensional, not a discontinuity, but can leave residual stress affecting crack behavior.
• Sheared edges with work-hardened burs may crack during subsequent forming—edge condition matters before PT/MT.

Heat-affected ductility (prior welds, prior heat treatment) can make formed parts crack at unexpected locations—Level III procedure writers should know prior process history.

Directionality and Inspection Orientation

Wrought discontinuities are anisotropic. Inspection geometry must place energy paths so they intersect the expected plane or line of the flaw.

Ultrasonic Testing (UT)

Coarse grain in large forgings (especially stainless or heavy sections) increases noise—procedure may require lower frequency, dual probes, or alternative methods.

Radiographic Testing (RT)

RT detects volumetric loss of thickness or density. Tight laminations parallel to the radiation beam path (i.e., parallel to the plate surface when shot through thickness) may show little contrast. Crack-like forging bursts may need multiple angles. RT of rolled plate for laminations is generally inferior to UT for planar mid-wall separations.

Surface Methods (MT, PT, VT, ET)

Laps, seams, and forming cracks that break the surface are excellent candidates for MT (ferromagnetic), PT (nonporous metals), VT, and ET on conductive products. Magnetization direction for MT should be roughly perpendicular to expected seam/lap orientation so leakage fields form across the discontinuity.

Process → Flaw → Method Scenarios

1. Closed-die steel forging, oxide scale folded at die radius → lap/fold → MT/PT after clean; machine if scale hides opening.
2. Hot-rolled plate from continuous-cast slab with centerline porosity → flattened laminations → automated or manual UT normal beam.
3. Cold-drawn bar with longitudinal surface crack elongated from billet defect → seam → MT continuous or wet fluorescent on bar line.
4. Press-brake bend, tight radius on cold plate, crack on outer fiber → forming crack → VT + PT/MT on tension side.
5. Upset forging at too-low temperature, center rupture → burst → UT through the section.

Why Level III Care About Grain Flow and Specs

Acceptance standards and customer drawings may require grain flow inspection (macroetch of a sectioned forging) as a process check separate from NDT of discontinuities. NDT procedures should still note that indications may follow flow lines and that “fiber-like” MT indications from stringers may be nonrelevant or acceptance-limited per code. The Level III decides relevance rules in the written procedure and training for Level II evaluators.

Study Focus

Link open vs closed die to surface vs internal risks; memorize wrought discontinuity names (lap, fold, burst, flash-line, lamination, seam, stringer); always ask “which way does the flaw run?” before choosing UT angle or MT field direction.