Study guide
3.1 Casting Processes and Cast Discontinuities
ASNT NDT Level III Basic Examination Exam
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1 Introduction & Exam Overview
1.1 Current Exam Facts 1.2 Level III Role and Certification Path 1.3 Eligibility, Fees, Scheduling, and Retakes 1.4 Study Strategy and Exam-Day Approach
2 Chapter 2: Materials Fundamentals for NDT
2.1 Ferrous Metals and Iron-Carbon Phases 2.2 Non-Ferrous Metals and Alloys 2.3 Mechanical Properties and Crystal Structure
3 Chapter 3: Manufacturing Processes
3.1 Casting Processes and Cast Discontinuities 3.2 Forging, Rolling, and Forming 3.3 Extrusion, Drawing, and Machining 3.4 Heat Treatment Effects
4 Chapter 4: Welding and Joining Processes
4.1 Fusion Welding Processes 4.2 Weld Metal and HAZ Discontinuities 4.3 Brazing, Soldering, and Mechanical Joining
5 Chapter 5: Discontinuity Classification
5.1 Inherent Discontinuities 5.2 Processing Discontinuities 5.3 Service Discontinuities
6 Chapter 6: Specific Discontinuity Types
6.1 Cracks and Fracture Modes 6.2 Porosity, Inclusions, and Lack of Fusion 6.3 Laminations, Seams, Laps, and Undercut 6.4 Relevant vs Non-Relevant Indications
7 Chapter 7: NDT Method Selection and Inspection Planning
7.1 Method Selection Principles 7.2 Surface vs Volumetric Methods 7.3 Inspection Planning and Flaw Expectations
8 Chapter 8: Ultrasonic and Radiographic Testing
8.1 Ultrasonic Testing Principles and Applications 8.2 UT Limitations, Calibration, and Evaluation 8.3 Radiographic Testing Principles and Applications 8.4 RT Limitations, Image Quality, and Safety Basics
9 Chapter 9: Magnetic Particle, Liquid Penetrant, and Visual Testing
9.1 Magnetic Particle Testing 9.2 Liquid Penetrant Testing 9.3 Visual Testing
10 Chapter 10: Electromagnetic, Leak, Acoustic Emission, and Infrared Testing
10.1 Electromagnetic / Eddy Current Testing 10.2 Leak Testing 10.3 Acoustic Emission Testing 10.4 Thermal / Infrared Testing
11 Chapter 11: Specialty Methods and Method Comparison
11.1 Neutron Radiography 11.2 Magnetic Flux Leakage 11.3 Method Comparison and Supplemental Examinations
12 Chapter 12: SNT-TC-1A Recommended Practice
12.1 SNT-TC-1A Overview and Written Practice 12.2 SNT-TC-1A Training and Experience 12.3 SNT-TC-1A Examinations and Recertification
13 Chapter 13: CP-189 Standard Practice
13.1 CP-189 Overview and Scope 13.2 CP-189 Mandatory Requirements 13.3 SNT-TC-1A vs CP-189 Comparison
14 Chapter 14: CP-105 and NDT Training Programs
14.1 CP-105 Purpose and Topical Outlines 14.2 Developing and Approving NDT Training 14.3 OJT, Mentoring, and Instructor Oversight
15 Chapter 15: Building and Managing Certification Programs
15.1 Building Qualification and Certification Programs 15.2 Administering Examinations and Evaluating Competency 15.3 Program Audits and Managing Certifications
16 Chapter 16: Codes, Standards, Procedures, and Quality
16.1 Interpreting Codes, Standards, and Specifications 16.2 Developing and Approving NDT Procedures 16.3 Acceptance Criteria and Quality Control 16.4 External Audits and Inspection Documentation
3.1 Casting Processes and Cast Discontinuities
Key Takeaways Explain This Topic AI
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
- Last updated: July 2026
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.