3.1 Casting Processes and Cast Discontinuities
Key Takeaways
- Casting solidifies liquid metal in a mold; gating feeds metal while risers feed solidification shrinkage
- Gas porosity is rounded and scattered; shrinkage porosity is dendritic/angular and located at last-to-freeze regions
- Cold shuts, hot tears, misruns, and inclusions each map to distinct process causes and preferred NDT methods
- RT and UT address volumetric cast flaws; PT/MT/VT catch surface-breaking openings once access and material allow
- Level III scenarios expect process → expected discontinuity → method selection, not method trivia alone
3.1 Casting Processes and Cast Discontinuities
Quick Answer: Casting pours liquid metal into a mold. As the metal freezes, volume contracts and gases evolve. Where feeding is inadequate or process control fails, discontinuities form—porosity, shrinkage cavities, cold shuts, hot tears, inclusions, misruns, and segregation. Level III candidates must link process to flaw type and choose NDT methods that match flaw morphology and location.
Manufacturing process knowledge is a core part of the ASNT NDT Level III Basic exam (materials science and process technology). Inspectors who understand how a part was made predict where discontinuities hide and which methods will find them.
Why Casting Matters for NDT
Castings start as liquid and end as solid shapes that may be near-net or heavily machined later. Unlike wrought products, castings lack directional grain flow from mechanical working. Discontinuities are therefore often volumetric (voids, inclusions) or solidification-related cracks, and they follow thermal and feeding patterns rather than rolling or forging planes.
A Level III writing or approving procedures must:
- Identify the casting process family (sand, investment, die, continuous, and variants).
- Predict likely discontinuity types and locations.
- Select methods sensitive to those morphologies (open to surface vs internal; gas vs planar).
- Specify technique limitations (rough as-cast surfaces, coarse grain, geometry).
Common Casting Processes
| Process | Mold / tooling | Typical products | NDT-relevant traits |
|---|---|---|---|
| Sand casting | Expendable sand mold (green sand, chemically bonded) | Large housings, pump bodies, valves | Rough surfaces; possible sand inclusions; slow cool → coarser structure |
| Investment (lost-wax) | Ceramic shell over wax pattern | Precision aerospace / turbine parts | Fine detail; thinner walls; ceramic inclusions risk |
| Die casting | Permanent metal die under pressure | High-volume nonferrous parts (Al, Zn, Mg) | High productivity; gas entrapment; thin-wall cold shuts |
| Continuous casting | Water-cooled mold, continuous withdrawal | Billets, blooms, slabs for later rolling | Centerline segregation/porosity; subsurface cracks if cooling unbalanced |
| Centrifugal casting | Rotating mold | Pipe, rings, cylindrical shells | Dense outer wall; inclusions migrate toward ID |
Sand casting remains the workhorse for large ferrous and nonferrous shapes. Metal is poured through a gating system (sprue, runners, gates) into the mold cavity. Risers (feeders) are reservoirs of liquid metal that feed the casting as solidification shrinkage occurs. Chills, pads, and mold design control freeze order so solidification progresses toward the riser (directional solidification).
Investment casting produces complex geometries with better surface finish and dimensional control. The ceramic shell can fracture or shed particles, creating nonmetallic inclusions. Thin sections freeze quickly and may show misruns or cold shuts if pour temperature or mold preheat is wrong.
Die casting injects metal under pressure into a steel die. Air and lubricant vapors can be trapped, producing gas porosity. High-velocity fill can create cold shuts where metal streams meet without full fusion. Die castings are often thin-walled aluminum or zinc alloys used in automotive and consumer products.
Continuous casting freezes a strand withdrawn from a mold. Defects often align with the centerline (last to freeze) or with secondary cooling patterns. Downstream rolling may elongate or close some voids but can also convert them into laminations or stringers in plate and sheet—linking casting quality to later wrought inspection.
Solidification, Gating, and Risers
Liquid metal occupies more volume than solid metal for most alloys. If liquid cannot feed the contracting solid, shrinkage cavities or shrinkage porosity form in last-to-freeze regions: heavy sections, hot spots, junctions of thick and thin walls, and poorly designed bosses.
Key process concepts:
- Gating system: Delivers clean metal with controlled velocity. Turbulent fill entrains air and oxide films.
- Risers: Supply feed metal. An undersized or poorly placed riser leaves shrinkage at the thermal center of a section.
- Hot spots: Local regions that stay liquid longest—corners, thick junctions, poorly chilled bosses.
- Chills: Conductive inserts that accelerate freeze and pull solidification away from critical volumes.
- Pour temperature and mold temperature: Too cold → misruns and cold shuts; too hot → more gas absorption, coarser grain, sand burn-on.
Exam questions often ask which region solidifies last or why a cavity sits at a section change. Answer from heat-flow and feeding logic, not from memorizing brand names of cast alloys.
Cast Discontinuities: Morphology and Cause
Porosity — Gas vs Shrinkage
Gas porosity forms when dissolved gases (hydrogen in aluminum, nitrogen/hydrogen in steels and irons) come out of solution during freezing, or when mold moisture/binders generate gas. Bubbles are typically rounded or slightly elongated, may be scattered or clustered under the cope (upper) surface, and often have smooth walls.
Shrinkage porosity (including microshrinkage) forms when feeding fails. Morphology is angular, dendritic, or sponge-like, and location tracks last-to-freeze volumes. Macroscopic shrinkage cavities may be large open voids under riser connections or in heavy bosses.
Distinguishing gas from shrinkage on radiographs or fracture surfaces is a classic Level III skill: round + random → gas; jagged + thermal center → shrinkage.
Cold Shuts
A cold shut is a discontinuity where two metal streams meet without complete fusion—often a thin oxide film or incomplete weld-like junction. Causes include low pour temperature, slow fill, interrupted pour, or long flow paths. Cold shuts may be surface-breaking or subsurface and can act as crack starters in service.
Hot Tears
Hot tears (hot cracks) form while the casting is still in the mushy temperature range and has low strength. Contraction is restrained by the mold, cores, or already-solid sections. Tears are often irregular, interdendritic, and located at hot spots or sharp section changes. They differ from cold cracks that form after full solidification under residual stress.
Inclusions
Nonmetallic inclusions include sand, slag, dross, refractory, and oxide films. Sand inclusions often appear near mold surfaces; slag and dross may float or be trapped by turbulence. Inclusions are usually denser or less dense than the matrix on RT (depending on composition) and scatter ultrasound. They reduce fatigue life and can seed corrosion.
Misruns
A misrun is incomplete filling of the mold cavity—missing edges, incomplete thin walls, or short features. Causes: low temperature, insufficient metal volume, blocked gates, or inadequate venting. Misruns are usually obvious visually; NDT may still be needed on remaining metal for associated cold shuts or porosity.
Segregation and Related Issues
Segregation is chemical nonuniformity as solute is rejected during freezing (macrosegregation in heavy sections; microsegregation between dendrites). Continuous cast product may show centerline segregation. Segregation itself may not be a “void,” but it can create hard/soft bands, inclusion clusters, and preferential corrosion—and it influences heat treatment response and NDT background (e.g., ultrasonic attenuation, MT background).
Hot spots are not discontinuities by themselves; they are process conditions that produce shrinkage and hot tears. Procedure writers use hot-spot maps from casting design to set inspection focus zones.
Matching NDT Methods to Cast Flaws
| Discontinuity | Typical location | Preferred methods | Notes |
|---|---|---|---|
| Gas porosity | Scattered; often near surfaces or cope | RT (excellent for volumetric gas); UT if geometry/grain allow | Rough as-cast surface degrades UT coupling |
| Shrinkage cavity/porosity | Thermal centers, heavy sections | RT, UT | RT shows dark volumetric indications; UT may lose backwall |
| Cold shuts | Stream junctions, thin sections | PT/MT if open to surface; RT/UT if subsurface | Partial fusion may be tight—sensitivity matters |
| Hot tears | Hot spots, restrained sections | PT/MT/VT surface; RT/UT volumetric | Crack-like; orientation vs beam/radiation important |
| Inclusions | Near mold wall, gates, pour stream | RT, UT | Density contrast drives RT detectability |
| Misruns | Incomplete geometry | VT primary | May accompany cold shuts |
| Surface sand/scale | As-cast exterior | VT, then clean for PT/MT | Surface condition controls penetrant/magnetic success |
Radiographic testing (RT) is the classic volumetric method for castings: gas porosity, shrinkage, and many inclusions produce density differences. Sensitivity depends on thickness, energy, geometry, and film/digital technique quality.
Ultrasonic testing (UT) can find volumetric and planar flaws but struggles with coarse as-cast grain (attenuation, noise), complex geometry, and rough surfaces. Large critical castings often combine RT of primary volumes with UT where access and metallurgy permit.
Liquid penetrant (PT) finds surface-breaking openings on nonporous materials after proper cleaning—cold shuts, hot tears, and surface-connected shrinkage. As-cast roughness traps penetrant and causes background; machining or blasting may be required before PT.
Magnetic particle testing (MT) applies to ferromagnetic castings for surface and near-surface cracks/tears. Residual magnetism and irregular geometry affect technique selection (yoke, prod, coil).
Visual testing (VT) remains first-line for misruns, surface tears, obvious porosity, and mold defects. Optical aids and surface prep standards belong in procedures.
Eddy current (ET) is less common on rough bulk castings but can apply to thin-wall nonferrous die castings or machined surfaces for surface cracks.
Exam Scenarios: Process → Flaw → Method
Think in chains the Basic exam rewards:
- Thick sand-cast steel valve body, boss at flange junction, no chill under boss → last freeze at boss → shrinkage → RT (and design fix with riser/chill).
- Aluminum die casting, thin wall, low die temp → incomplete fusion of metal fronts → cold shut → PT after machining/cleaning; RT if subsurface concern.
- Moist sand mold, turbulent pour of aluminum → hydrogen/mold gas → rounded gas porosity → RT.
- Restrained sand casting with sharp fillet, high solidification stress → hot tear at fillet → MT/PT surface exam; UT/RT if depth unknown.
- Continuous cast slab centerline defects rolled into plate → elongated laminations/porosity remnants → UT plate scan (often plate-mill automated UT).
Level III responsibility includes knowing when a method is inappropriate: PT on uncleaned, porous as-cast iron surface; UT through thick coarse-grain austenitic castings without specialized techniques; MT on nonmagnetic aluminum die castings.
Procedure and Quality Links
Manufacturing process knowledge feeds acceptance criteria and inspection plans. Codes and customer specs may require RT of critical zones, 100% VT of as-cast surfaces, or machining of surfaces before final PT/MT. When casting process changes (new foundry, different riser design, switch from sand to investment), the Level III should reassess discontinuity risk and update procedures—not assume prior NDT coverage still fits.
Summary for Study
Master four process families, the role of gating/risers/hot spots, the morphology differences among gas porosity, shrinkage, cold shuts, hot tears, inclusions, and misruns, and the method table above. Exam items rarely ask only “what is porosity?”—they ask which process produces which flaw and which method finds it.
On a radiograph of a cast steel housing, a cluster of rounded, dark indications is found near the cope surface, away from heavy section junctions. Which discontinuity is most consistent with this description?
A thick boss on a sand casting freezes after adjoining thin walls because no chill or riser feeds that volume. Which discontinuity is most likely at the boss thermal center?
Which NDT method pairing is most appropriate for detecting subsurface volumetric gas porosity in a critical steel casting and surface-breaking hot tears after machining?
Two metal streams meet in a thin investment-cast section without complete fusion, leaving a film-like discontinuity. What is this defect called, and what process factor most often contributes?