7.1 Ingot and Casting Discontinuities

7.1 Ingot and Casting Discontinuities

Metallurgy of Ingot Solidification and Primary Inherent Flaws

In non-destructive testing, discontinuities are categorized chronologically by the manufacturing stage in which they originate: inherent, primary processing, secondary processing, and in-service. Inherent discontinuities originate during the initial liquid-to-solid phase transformation when molten metal cools and crystallizes within an ingot mold or foundry casting mold. The physical and chemical mechanisms governing ingot solidification establish internal structures and material anomalies that persist through subsequent mechanical forming operations or directly affect the performance of finished castings.

Dendritic Crystallization and Solidification Dynamics

As molten steel cools below its liquidus temperature, solid crystals nucleate at the mold walls where heat extraction is most rapid. These crystals grow inward toward the molten core as branched, tree-like structures called dendrites. The progression of dendritic growth creates three distinct structural zones across the cross-section of a cast ingot:

1. Chill Zone: A thin outer layer of fine, equiaxed, randomly oriented grains formed immediately adjacent to the cold mold wall.
2. Columnar Zone: Long, parallel, directional grains that grow perpendicularly inward from the chill zone along the path of steepest thermal gradient.
3. Central Equiaxed Zone: A core of randomly oriented, coarser equiaxed grains that form when the center of the ingot gradually cools below the liquidus temperature without a steep directional thermal gradient.

The volume reduction that accompanies cooling from liquid to solid, combined with the progressive rejection of solute elements and dissolved gases into the remaining molten liquid ahead of the advancing dendritic freeze front, generates several characteristic inherent flaws.

Primary and Secondary Pipe

Liquid iron and steel undergo a volumetric shrinkage of approximately $3%$ to $5%$ during solidification, followed by continuous solid-state thermal contraction. Because the outer walls and bottom of the ingot solidify first, the volume of remaining molten metal decreases continuously relative to the rigid outer shell.

• Primary Pipe: As the upper surface of the ingot cools and solidifies, the central molten metal recedes downward, leaving a deep, central, inverted cone-shaped shrinkage cavity at the top of the ingot. In commercial steelmaking, an insulated or exothermic "hot top" (sinkhead) is placed above the mold to maintain a reservoir of liquid metal that feeds the shrinkage cavity. If the hot top is undersized or omitted, primary pipe penetrates deep into the usable ingot body.
• Secondary Pipe: Occurs along the central axis beneath the primary pipe when bridging of dendritic branches cuts off the downward feeding of liquid metal to the lower interior portions of the ingot. This leaves isolated, elongated internal voids along the centerline.

Significance in NDT: Primary pipe exposed to ambient atmosphere becomes heavily oxidized internally by atmospheric oxygen. Because iron oxide scale prevents metallic bonding during hot working, oxidized pipe cannot weld shut during subsequent rolling or forging. If the piped top of the ingot is not cropped off completely, the defect is rolled out into an extensive internal separation known as a lamination in plate or an internal seam / burst in bar stock.

Non-Metallic Inclusions (Indigenous vs. Exogenous)

Molten steel always contains non-metallic impurities that become trapped in the solidifying dendritic matrix. These are classified into two broad metallurgical groups:

1. Indigenous (Endogenous) Inclusions: Chemical reaction products precipitated directly out of the melt during cooling and solidification. These include deoxidation products such as alumina ($\text{Al}_2\text{O}_3$), silica ($\text{SiO}_2$), and manganese silicates ($\text{MnO}\cdot\text{SiO}_2$), as well as sulfides formed by the reaction of sulfur with iron and manganese (manganese sulfide, $\text{MnS}$, and iron sulfide, $\text{FeS}$).
2. Exogenous Inclusions: Mechanical impurities entrained from external sources during teeming, such as eroded refractory brick particles from the ladle, tundish nozzles, or furnace linings, as well as entrapped mold sand, dross, and oxidizing slag.

Magnetic Reluctance and Flux Displacement: Non-metallic inclusions are non-ferromagnetic or weakly paramagnetic, possessing a relative magnetic permeability near unity ($\mu_r \approx 1$). The surrounding ferromagnetic steel matrix exhibits relative permeability values typically ranging from $\mu_r = 500$ to $\mu_r > 2000$. According to the magnetic reluctance relationship: $R = \frac{l}{\mu A}$ where $l$ is path length, $\mu$ is magnetic permeability, and $A$ is cross-sectional area. Because $\mu$ of the inclusion is several orders of magnitude lower than that of steel, the inclusion represents a zone of extreme magnetic reluctance. Magnetic flux lines are forced to detour around the non-magnetic obstacle. When the inclusion lies at or near the examination surface, this flux displacement creates a magnetic leakage field ($B_y$ and $B_x$) in the air space above the metal, attracting applied ferromagnetic particles.

Chemical Segregation and Solute Banding

Solute elements—most notably carbon ($C$), sulfur ($S$), phosphorus ($P$), manganese ($Mn$), and silicon ($Si$)—have lower solubility in solid austenite and ferrite than in liquid steel. As dendrites freeze, pure iron crystallizes first, rejecting solute elements into the surrounding liquid. This process, known as segregation, produces non-uniform chemical compositions throughout the ingot:

• Macrosegregation: Large-scale compositional differences between the exterior and interior of the ingot. This includes positive centerline segregation (enrichment of carbon and sulfur at the core), "A"-segregation and "V"-segregation channels formed by buoyant upward convection of solute-rich liquid, and negative segregation at the bottom cone.
• Microsegregation: Localized concentration differences across the inter-dendritic spaces (spacing of fractions of a millimeter). During subsequent mechanical rolling, microsegregation flattens into alternating parallel microstructural bands of ferrite and pearlite (solute banding).

NDT Level III Consequence: Heavy chemical segregation does not create physical voids, but the resulting microstructural variations cause localized differences in magnetic permeability ($\mu_r$) and coercive force ($H_c$). Under high magnetizing currents, these permeability boundaries can create faint, diffuse magnetic flux leakage that manifests as non-relevant magnetic particle indications (banding indications). The Level III must distinguish between physical flaw indications and non-relevant metallurgical segregation patterns.

Blowholes, Pinholes, and Gas Porosity

Molten steel dissolves substantial quantities of gases, primarily hydrogen, nitrogen, and oxygen. The solubility of these gases decreases abruptly during the phase transition from liquid to solid:

• Gas Evolution: As liquid steel cools, dissolved oxygen reacts with carbon to form carbon monoxide gas bubbles ($C + O \rightarrow CO\uparrow$). Simultaneously, dissolved atomic hydrogen and nitrogen precipitate out of solution.
• Blowholes: If the evolving gas cannot escape through the liquid metal before the dendritic network solidifies, it becomes trapped as rounded, cylindrical, or pear-shaped voids called blowholes. Deep internal blowholes with unoxidized surfaces often weld shut under hot compressive rolling. However, subsurface blowholes located just beneath the ingot skin frequently break open to atmospheric oxygen during soaking and rolling, producing severe longitudinal seams.

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Foundry Casting Discontinuities: Formation Mechanisms and Morphology

When molten metal is poured directly into a sand mold, investment shell, or permanent mold to create a net-shape component, the resulting casting can develop unique inherent discontinuities governed by fluid flow, thermal gradients, and mold-metal reactions.

Hot Tears (Solidification Cracking)

Hot tears represent one of the most critical structural defects found in castings.

• Formation Mechanism: Hot tears occur at elevated temperatures—near the solidus temperature—during the final stages of solidification when the metal is in a semi-solid, dendritic "mushy" state or immediately after complete solidification. As the casting cools, it undergoes solid-state thermal contraction. If this thermal contraction is mechanically restrained by rigid sand cores, hard molds, or adjacent heavy structural sections that freeze at different rates, high localized tensile stresses develop.
• Morphology: Because the tears follow weak, liquid-filmed inter-dendritic grain boundaries, hot tears exhibit an unmistakable ragged, heavily branched, jagged, and stepped linear morphology. They follow the thermal contours of the mold, frequently propagating from sharp re-entrant fillets, internal corners, rib intersections, or abrupt transitions between thin and thick cross-sections.
• Surface Connectivity: Hot tears almost invariably break out to the surface, as thermal contraction stresses peak at the cooler outer surfaces and sharp geometric transitions.

Cold Shuts and Misruns

Cold shuts and misruns are planar discontinuities caused by fluidity failure during pouring.

• Misrun: A casting that solidifies before the mold cavity is completely filled, resulting in missing edges, incomplete sections, or unfilled webs.
• Cold Shut: Occurs when two converging streams of molten metal meet inside the mold cavity, but their leading edges fail to fuse or coalesce into a continuous metallic bond. This lack of fusion is driven by low pouring temperatures, slow pouring rates, inadequate gating, or the presence of a tenacious refractory oxide film covering the advancing liquid fronts.
• Morphology: Unlike hot tears, cold shuts exhibit a smooth, rounded, curvilinear planar boundary. The edges of the discontinuity are rounded rather than razor-sharp, and the opposing faces are typically separated by a thin oxide layer. In cross-section, a cold shut resembles a continuous, curved seam that extends inward from the casting surface.

Shrinkage Cavities and Microshrinkage

Because metals contract upon freezing, castings require properly positioned reservoirs of molten metal—called risers or feeders—to supply liquid metal to the freezing zones until solidification is complete.

• Macroshrinkage (Shrinkage Cavity): Large, irregular, jagged internal voids formed in heavy sections, boss intersections, or thermal hot spots that lack adequate riser feeding. The interior surfaces of shrinkage cavities are typically rough, dark, and lined with protruding dendritic crystals (dendritic arborization).
• Microshrinkage (Spongy Shrinkage / Feathery Shrinkage): A dispersed network of microscopic inter-dendritic voids found in cast alloys that freeze over a wide temperature range between the liquidus and solidus (such as high-alloy steels and ductile iron). The feeding channels between advancing dendritic arms become blocked, leaving fine, interconnected capillary voids throughout the cross-section.

Foundry Inclusions (Sand and Slag Entrapment)

• Sand Inclusions: Loose mold sand dislodged from the gating system, sprue, or mold walls by the turbulent impingement of molten metal is swept into the casting cavity and trapped during freezing. They appear as irregular, chunky, non-metallic particles embedded at or near the surface.
• Slag Inclusions: Molten flux, dross, or deoxidation slag poured from the ladle into the mold cavity. Slag inclusions are usually globular or elongated glassy masses composed of iron-manganese silicates and aluminum oxides.

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Magnetic Particle Testing Indication Characteristics and Physics

Detecting and correctly classifying casting discontinuities using magnetic particle testing requires understanding the difference between surface and subsurface magnetic leakage fields.

Surface-Breaking vs. Subsurface Magnetic Leakage Fields

The geometry, sharpness, and holding power of a magnetic particle indication are governed by the spatial gradient of the magnetic leakage field ($dB/dx$ and $dB/dz$).

Surface-Breaking Discontinuity (Hot Tear)        Subsurface Discontinuity (Shrinkage Cavity)

          Air Space (Leakage Field)                        Air Space (Weak Leakage Field)
           S               N                                  S                 N
      +---------+     +---------+                        +---------------------------+
      |  Steel  |  |  |  Steel  |                        |     Solid Steel Bridge    |
      |  Matrix |  |  |  Matrix |                        +---------+       +---------+
      |         |  |  |         |                        |  Steel  | Void  |  Steel  |
      |         | Crack         |                        |         |       |         |
      +---------+     +---------+                        +---------+       +---------+
      High Gradient (dB/dx >> 0)                         Low Gradient, Diffuse Leakage
      Sharp, Crisp, Pinned Indication                    Broad, Fuzzy, Weak Indication

1. Surface-Breaking Flaws (Hot Tears, Cold Shuts, Surface Sand Tears):

   • The discontinuity creates an open air gap directly at the surface boundary. Magnetic flux lines traversing the steel must exit into the air immediately at the edge of the gap.
   • Because the permeability transition from steel ($\mu_r > 500$) to air ($\mu_r = 1$) is instantaneous, the leakage flux is concentrated in a tight spatial zone directly over the flaw. The spatial field gradient $\frac{dB}{dx}$ is extremely steep.
   • Applied magnetic particles experience an intense localized magnetic attractive force ($F_m \propto \nabla B^2$), pinning them into a sharp, well-defined, tightly bound, high-contrast linear ridge. The indication faithfully traces the jagged branching of a hot tear or the smooth line of a cold shut.

2. Subsurface Flaws (Shrinkage Cavities, Internal Inclusions, Deep Porosity):

   • The discontinuity is separated from the examination surface by a layer of intact, high-permeability ferromagnetic steel. The magnetic flux lines divert around the void, but much of the flux remains contained within the solid metal bridge.
   • Only a fraction of the displaced flux leaks out into the air above the surface. As the depth ($d$) of the void beneath the surface increases, the flux leakage field spreads out over a much wider surface area. The peak normal leakage field ($B_y$) decays rapidly (inversely proportional to $d^2$ or $d^3$), and the field gradient $\frac{dB}{dx}$ becomes very shallow.
   • Applied magnetic particles experience only weak magnetic forces. The particles form a broad, diffuse, fuzzy, low-contrast band with soft edges. The powder is loosely held and easily dislodged by light air flow.

Technique Selection: Rough Sand Castings vs. Precision Investment Castings

A critical Level III responsibility is matching the MT technique, current waveform, and particle medium to the surface condition and metallurgical structure of the casting:

1. Sand Castings (Rough As-Cast Surfaces):

   • Surface Texture: Sand castings exhibit rough, textured surfaces ($Ra > 12.5\ \mu\text{m}$ / $500\ \mu\text{in}$) containing embedded mold scale, micro-pits, and parting line flash.
   • The Wet Method Trap: Applying wet suspension (fluorescent or visible) to a rough sand casting results in severe non-relevant particle trapping. Wet carrier liquid pools in surface depressions, and surface tension holds the fine particles in micro-crevices, creating heavy fluorescent background noise that completely masks true flaw indications.
   • Mandatory Technique: Dry Ferromagnetic Powder with Half-Wave Rectified Direct Current (HWDC).
     • HWDC Waveform Advantages: HWDC delivers pulsating unidirectional current (a rectified 60 Hz half-sine pulse). The peak current provides deep volumetric magnetic penetration to detect near-surface shrinkage voids and tears, while the pulsating wave creates a mechanical vibration ("particle mobility") that causes dry particles to jump across the rough surface. Particles bounce out of harmless sand depressions and migrate exclusively to true flux leakage fields.
     • Dry Powder Formulation: Dry particles are larger ($50\ \mu\text{m}$ to $150\ \mu\text{m}$) and treated with anti-caking agents, enabling them to resist mechanical entrapment on rough cast surfaces.

2. Investment Castings (Precision Thin-Wall Castings):

   • Surface Texture: Investment (lost-wax) castings possess very smooth surface finishes ($Ra < 3.2\ \mu\text{m}$ / $125\ \mu\text{in}$) and complex, intricate geometries with tight fillet radii.
   • Mandatory Technique: High-Sensitivity Fluorescent Wet Continuous Method with Alternating Current (AC) or Full-Wave Rectified DC (FWDC).
     • Smooth investment surfaces produce zero mechanical entrapment. Fine fluorescent wet particles ($1.5\ \mu\text{m}$ to $10\ \mu\text{m}$) suspended in light petroleum distillate or conditioned water are drawn into the tightest micro-hot tears in fillet radii, delivering maximum visual contrast under UV-A illumination.

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Summary Comparison Table: Ingot and Casting Discontinuities

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Practical Level III Engineering Scenarios and Exam Traps

Scenario: Valve Body Casting Evaluation

A Level II technician is inspecting heavy sand-cast carbon steel valve bodies for pressure service using a portable AC electromagnetic yoke and visible black wet suspension. The technician notes heavy black particle accumulation across all fillet radii and rejects the entire production lot for "generalized hot tearing."

Level III Technical Investigation & Root-Cause Audit:

1. Physics Audit: AC yoke current produces an electromagnetic skin depth of less than $1\text{ mm}$ in ferromagnetic steel. While excellent for fine surface cracks on machined surfaces, applying visible wet suspension to a rough as-cast sand finish ($Ra \approx 15\ \mu\text{m}$) causes severe mechanical trapping. Capillary action retains the liquid vehicle in the cast fillets, and particles settle into surface asperities without any magnetic flux leakage.
2. Corrective Examination Design: The Level III removes the wet bath, lightly sandblasts the cast surfaces to remove loose scale, and switches to Half-Wave Rectified Direct Current (HWDC) prods or an HWDC yoke using dry red ferromagnetic powder.
3. Diagnostic Findings: Under HWDC dry powder examination, the false fillet indications disappear entirely. However, a true hot tear is uncovered at a sharp transition between the valve flange and neck. The indication is distinct, razor-sharp, heavily branched, and remains tightly pinned when excess dry powder is blown away with a gentle air stream ($1.5\text{ to }2.0\text{ psi}$ bulb pressure).
4. Cold Shut Distinction: In an adjacent web, a smooth, curved, continuous line without branches is detected. A cross-sectional metallographic grind confirms a cold shut resulting from low pouring temperatures during mold filling. The hot tear is rejected as a severe crack-like planar defect; the cold shut is evaluated against ASME Section VIII Division 1 linear acceptance standards.