7.2 Wrought and Primary Forming Discontinuities

7.2 Wrought and Primary Forming Discontinuities

Primary Mechanical Working and Grain Flow Dynamics

Wrought manufacturing processes convert cast ingots or continuously cast blooms and billets into structural shapes, bars, plates, forgings, and seamless tubing through severe plastic deformation. Primary working operations include hot rolling, open-die forging, closed-die forging, extrusion, and cold drawing.

The Formation of Fiber Texture and Grain Flow

During plastic deformation above the recrystallization temperature, the coarse, randomly oriented equiaxed and columnar cast dendrites are broken down, refined, and elongated. Non-metallic inclusions (such as ductile manganese sulfides), segregation bands, and remaining micro-voids are stretched in the direction of plastic extension. This microstructural re-orientation establishes a directional fibrous pattern known as grain flow (or fiber texture).

Transformation of Inherent Flaws: Mechanical working fundamentally alters the physical geometry of ingot discontinuities:

• Three-dimensional globular voids or inclusions are squeezed and elongated into two-dimensional planar or one-dimensional linear discontinuities.
• Discontinuities become oriented parallel to the direction of principal metal flow.
• Unoxidized internal gas blowholes and microshrinkage voids can forge-weld shut under hot compressive hydrostatic pressure. Conversely, oxidized voids, non-metallic inclusions, and surface-connected cracks cannot weld and instead lengthen into severe primary forming discontinuities.

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Primary Forming Discontinuity Types and Formation Mechanisms

Seams (Rolling, Forging, and Drawing Seams)

Seams are among the most common and structurally dangerous discontinuities found in rolled bar stock, drawn wire, and finished shafts.

• Origin Mechanisms:
  1. Opened Subsurface Blowholes: Near-surface gas blowholes in the ingot that oxidize when heated in the soaking pit break open during the initial rolling passes, elongating into continuous longitudinal surface fissures.
  2. Ingot Surface Tears: Thermal stress cracks or mold scabs on the ingot surface that oxidize and are rolled out into long, tight crevices.
  3. Roll Pass Underfill and Mechanical Grooving: Defective rolls, improper roll pass drafting, or mechanical guide scratching that tears the bar surface during processing.
• Morphology: Seams are straight, tight, linear cracks running strictly parallel to the longitudinal axis of the wrought product. They are typically perpendicular to the outer surface in depth and can extend intermittently or continuously for lengths ranging from several millimeters to tens of meters.
• Detection Physics: Because seams run parallel to the length of a bar or cylinder, circular magnetization (direct current passed through the part via headstocks, or through a central conductor) is mandatory. Circular magnetization establishes magnetic flux lines traveling circumferentially ($360^\circ$ around the bar), cutting across the longitudinal seam at an optimal $90^\circ$ angle to create maximum flux leakage. Longitudinal magnetization (encircling coil) directs flux parallel to the seam, generating zero leakage and causing complete inspection failure.

Longitudinal Seam in Rolled Bar Stock

  Circular Flux (Headstock Shot)          Longitudinal Flux (Encircling Coil Shot)
          [Optimal: 90° Cut]                     [Ineffective: Parallel to Flaw]

         +-----------------+                    +-----------------+
         |  ============   |  Seam              |  ============   |  Seam
         |  ============   |                    |  ============   | 
         +-----------------+                    +-----------------+
              |       ^                              ----->  ----->
              v       |                             Flux Lines (Parallel)
           Circular Flux                             NO FLUX LEAKAGE
         SHARP MT INDICATION                        FLAW CANNOT BE DETECTED

Laps (Forging and Rolling Laps)

Laps are mechanical folding defects that occur during hot working operations.

• Formation Mechanism: During rolling or closed-die forging, an overfill occurs when excessive metal enters a roll pass or die cavity, squeezing out between the dies as a protruding ridge, fin, or flash. In the subsequent pass or forging blow, this protruding fin is bent over and flattened into the surface of the workpiece. Because the surface of the fin is coated with iron oxide mill scale, the folded metal cannot forge-weld to the base material.
• Morphology: Unlike seams, laps enter the metallic surface at an oblique or acute angle (often $10^\circ$ to $45^\circ$ relative to the surface plane). On forgings, laps often follow the contour of the parting line, die fillets, or web-to-flange transitions, presenting a curvilinear, crescent-shaped, or wavy visual appearance.
• Magnetic Particle Response: Because laps penetrate at an angle, the magnetic flux leakage field is slightly broader than that of a vertical crack or seam. However, because the lap boundary reaches the surface, high-sensitivity fluorescent wet suspensions reveal distinct, sharp, continuous linear indications following the fold trace.

Non-Metallic Stringers

During ingot rolling, ductile non-metallic inclusions—principally manganese sulfides ($\text{MnS}$) and certain plastic silicate glasses—deform plastically alongside the steel matrix.

• Morphology: These inclusions are flattened and elongated into thin, microscopic ribbons or discontinuous parallel chains of non-metallic particles running parallel to the direction of rolling. Under magnification, stringers resemble broken pencil lines running through the microstructure.
• Magnetic Indication Characteristics:
  • Inclusions have low magnetic permeability ($\mu_r \approx 1$), creating small localized leakage fields when cutting perpendicular flux lines.
  • Magnetic particle indications appear as fine, faint, intermittent, broken linear chains parallel to the grain flow.
  • Level III Engineering Distinction: Non-metallic stringers are inherent microstructural features present to some degree in all commercial steels (especially resulfurized free-machining grades like AISI 1117 or 1215). The Level III must establish rigorous acceptance criteria: isolated, short stringers are generally harmless in low-stress regions, whereas dense clusters of stringers in high-strength aerospace components or critical rotating journals act as fatigue crack initiation sites and must be rejected.

Forging Bursts (Centerline Bursts and Chevron Cracking)

Forging bursts are internal mechanical ruptures formed during forging, upsetting, or extrusion.

• Root Metallurgical Causes:
  1. Improper Forging Temperature: Forging metal when it is too hot (within the "hot-short" temperature range where grain boundary liquid films form) causes intergranular tearing. Forging metal when it is too cold (below recrystallization temperatures) lowers ductility, causing brittle cleavage ruptures under heavy reduction.
  2. Excessive Tensile Stresses: In open-die forging or extrusion, improper die angles, excessive draft, or poor feed-to-height ratios ($h/d$) generate severe secondary tensile triaxial stresses along the central neutral axis of the billet. When these internal tensile stresses exceed the ultimate tensile strength of the hot core, the core tears apart internally.
• Morphology: Bursts are internal, jagged, star-shaped, or chevron-patterned cavities centered along the longitudinal centerline of the forging.
• MT Detectability: Bursts are entirely internal during primary forging. Standard surface MT cannot detect forging bursts in as-forged billets because the solid metal outer shell prevents magnetic flux leakage from reaching the exterior. Bursts are only detectable by magnetic particle testing when subsequent machining, drilling, or boring operations cut through the core, exposing the burst cavity to the surface. Prior to machining, ultrasonic testing (UT straight beam) is the primary method required for burst detection.

Hydrogen Flakes (Shatter Cracks / Fish Eyes)

Hydrogen flakes represent one of the most catastrophic internal defects encountered in heavy alloy steel forgings, thick blooms, and railroad rails.

• Physical Formation Mechanism:
  1. Dissolution in Molten Steel: Molten steel dissolves atomic hydrogen ($H$) from atmospheric humidity, damp raw materials, or furnace refractories.
  2. Solubility Drop on Cooling: Hydrogen solubility is high in molten steel and liquid austenite, but drops drastically as the steel cools and transforms to body-centered cubic (BCC) ferrite or martensite. In heavy cross-sections, rapid surface cooling traps atomic hydrogen in the interior, preventing outward diffusion.
  3. Recombination and Hydrostatic Pressure: Trapped atomic hydrogen diffuses to internal micro-voids, inter-dendritic boundaries, and inclusion interfaces, where it combines into molecular hydrogen gas ($2H \rightarrow H_2\uparrow$). Because molecular hydrogen cannot diffuse through the metal lattice, gas molecules accumulate in micro-cavities, developing immense localized hydrostatic internal pressures exceeding thousands of megapascals ($>1000\text{ MPa}$).
  4. Rupture Under Transformation Stresses: When this internal gas pressure combines with high cooling stresses and transformation stresses (austenite to martensite/bainite volume expansion), the internal metal matrix ruptures along grain boundaries, forming micro-fissures.
• Morphology: Hydrogen flakes are internal, disc-shaped, circular cleavage cracks. In fractured test specimens, they appear as bright, silvery, oval patches termed "fish eyes." In cross-sections, they appear as fine, hairline cracks randomly oriented in the central and mid-radius zones of heavy forgings, rarely extending into the outer rim.
• MT Detection: Like bursts, flakes are volumetric internal defects undetectable by surface MT until the forging is sectioned or bored. When deep hole boring or rough machining exposes flakes to the inner or outer bore surface, high-sensitivity fluorescent wet continuous MT reveals clusters of sharp, hairline, multi-directional cracks.

Laminations in Rolled Plate

Laminations are extensive, flat, two-dimensional internal separations found in rolled plate, strip, and sheet.

• Origin: Laminations originate from uncropped primary or secondary pipe cavities, large oxidized blowholes, or dense planar clusters of non-metallic inclusions in the original ingot. When the ingot or slab is hot-rolled into flat plate, the internal cavity is flattened and spread out laterally, producing a wide planar separation located in the mid-thickness plane parallel to the plate surfaces.
• The Plane of MT Inspection Trap:
  • Laminations lie flat, parallel to the top and bottom rolled faces of the plate.
  • When magnetic fields are introduced into the plate (via electromagnetic yokes or prods), the magnetic flux lines travel through the plate thickness along planes parallel to the lamination.
  • Because the flux runs parallel to the planar separation, no magnetic flux leakage is forced outward through the top or bottom surface. Surface magnetic particle examination of rolled plate faces is completely incapable of detecting internal laminations.
  • Laminations can only be detected with MT when they break out at cut plate edges, sheared ends, or beveled weld preparations. At a cut edge, an applied magnetic field traveling across the plate thickness cuts the lamination at $90^\circ$, producing a distinct linear particle indication along the mid-thickness centerline. Volumetric lamination screening across the plate body must be performed using ultrasonic straight-beam testing per ASTM A435 or ASTM A578.

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Summary Comparison Table: Wrought and Primary Forming Discontinuities

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Practical Level III Engineering Scenarios and Exam Traps

Scenario 1: The Encircling Coil Seam Inspection Error

A fabrication shop receives a shipment of 3-inch (76 mm) diameter AISI 4140 hot-rolled bars. The customer specification requires 100% magnetic particle testing for longitudinal seams. The Level II technician places each bar horizontally inside a 5-turn encircling coil, energizes the coil with 2500 Amperes AC, sprays wet fluorescent bath, and reports zero indications across 50 consecutive bars. During subsequent CNC turning, longitudinal seams up to 0.040 inch (1.0 mm) deep open up on nearly 30% of the shafts.

Level III Technical Root-Cause Analysis:

• Magnetic Physics Failure: An encircling coil generates a longitudinal magnetic field parallel to the cylindrical axis of the bar. Because the seams run longitudinally along the bar, the magnetic flux lines travel completely parallel to the seam crevices. Zero flux leakage is created, rendering even deep seams completely invisible.
• Mandatory Technique Modification: The inspection must be redesigned to utilize circular magnetization. The bar must be clamped between copper headstocks, and an electrical current passed directly through the bar ($I = 20\text{ to }30\text{ A/mm}$ of diameter, or $\approx 1500\text{ to }2200\text{ A}$ per ASTM E1444), establishing circumferential flux that intersects the longitudinal seams at $90^\circ$. Re-inspection of the quarantined lot using headstock contact reveals distinct, brilliant fluorescent linear indications across all defective bars.

Scenario 2: Plate Lamination at Weld Bevel Preparation

During the fit-up of a 2-inch (50 mm) thick pressure vessel shell plate conforming to ASME SA-516 Grade 70, the welding inspector tests the double-V weld prep bevel using an AC yoke and dry red powder. A sharp linear indication appears along the exact mid-thickness centerline of the bevel, extending continuously for 18 inches (450 mm). The shop foreman claims the indication is merely a non-relevant "machining mark from the bevel planer."

Level III Disposition Protocol:

• Verification: The Level III lightly dresses the bevel surface with fine emery paper (600 grit) to eliminate tool chatter marks and re-tests using an AC yoke with the poles straddling the bevel edge ($90^\circ$ to the edge). The indication reappears with identical intensity.
• Flaw Classification: The indication is confirmed as an edge-breakout lamination originating from an uncropped ingot pipe cavity flattened during plate rolling.
• Remediation: Because the lamination provides a direct leak path and risk of lamellar tearing under weld contraction stresses, the Level III performs straight-beam ultrasonic testing (UT per ASTM A578) to map the lamination's depth into the plate body. The UT survey reveals the lamination extends 4 inches (100 mm) back from the weld bevel, exceeding ASME Section VIII acceptance thresholds and requiring gouging and weld repair or plate section replacement.