casting-defects

name	casting-defects
description	Casting defects — shrinkage porosity, gas porosity, cold shuts, misruns, hot tears, inclusions, detection methods, remedies, ASTM E446 (radiography), sand/die/investment casting.
metadata	{"priority":7,"promptSignals":{"phrases":["casting defects","casting porosity","shrinkage defect","casting inspection","sand casting defects","die casting defects","investment casting defects"],"minScore":3}}

Casting Defects — Complete Skill

Defect Classification

Porosity

Shrinkage porosity: Origin: metal shrinks as it solidifies; insufficient liquid feed → voids in final solidified region Location: at last-to-solidify hot spots; inside thick sections; at feeding cut-off zones Morphology: irregular, dendritic-network, non-spherical; interconnected in severe cases Prevention:

• Risers (feeders): liquid reservoir provides metal as shrinkage occurs; design to solidify last
• Riser design: Chvorinov's rule V/A ratio; riser V/A > casting V/A × 1.2
• Directional solidification: taper casting → solidification from thin to thick toward riser
• Exothermic riser compounds: maintain riser liquid longer

Gas porosity: Origin: dissolved gases (H, N, CO) released during solidification; trapped as spherical bubbles Location: throughout casting; more at hot spots (last solidified) Morphology: spherical or rounded; isolated bubbles Causes (by metal):

• Aluminum: hydrogen absorption from moisture in sand, flux, mold wash; H₂ solubility drops at solidification → porosity
• Steel: CO from SiO₂ + C reaction; also nitrogen from atmosphere
• Iron: hydrogen; blow holes from mold gas

Prevention:

• Degassing: rotary degassing with inert gas (Al); vacuum degassing (steel)
• Dry molds and cores; bake before pour
• Rapid pour (minimize gas absorption time)
• Deoxidation: Al/Si addition to steel (binds O₂)

Chvorinov's Rule (solidification time): t_s = B × (V/A)^n [s; B = mold constant; V/A = volume-to-surface area ratio; n ≈ 2 for sand casting]

Cold Shut

Origin: two streams of metal meet but don't fully fuse; low temperature at junction Appearance: visible line or seam on casting surface; like a crack but rounded Cause: low pouring temperature; slow pouring speed; poor gating (two streams from different gates) Prevention: increase pouring temperature; redesign gates for single-fill pattern; top gates for thin sections

Misrun

Origin: metal solidifies before filling mold completely; short pour Location: typically at furthest point from gate, thin sections, extremities Prevention: increase pouring temperature; improve gate design; reduce section changes

Hot Tear (Hot Cracking)

Origin: tensile stress during solidification (shrinkage); occurs at grain boundaries when metal is semi-solid Location: acute corners, abrupt section changes, where mold constrains free contraction Morphology: branching, intergranular cracks with rough oxidized surfaces Prevention:

• Design: round corners (min radius t/5); reduce section changes; avoid constrained geometry
• Processing: reduce mold/core rigidity; increase knockout temperature
• Alloy selection: alloys with wide freezing range (e.g., 7075 Al → prone; 380 → less prone)

Inclusions

Sand inclusions: mold erosion → loose sand carried into casting; common in ferrous castings Oxide films (bifilms): entrained oxide layer (Al₂O₃ in Al); severe effect on elongation and toughness Slag inclusions: oxidized flux/slag entrapped in metal stream Prevention:

• Filter systems: ceramic foam filters in gating; removes inclusions > filter pore size
• Quiet pouring: avoid turbulence (Fr < 1 for Al filling)
• Slag removal before pour
• Inert atmosphere for reactive metals

Blow Holes

Origin: gas evolved from mold/core binders during pour → large spherical voids at mold surface Mold gas sources: organic binders in resin-bonded sand; moisture Prevention: bake molds/cores; vent design; dry materials; use gas-permeable mold

Shrinkage Pipes (Centerline Shrinkage)

Solidification from outside in: liquid core shrinks → pipe forms along centerline Common in: ingots, large cylindrical sections Prevention: risers at top of casting; top-pouring to feed downward

Detection Methods

Method	Detects	Sensitivity
Visual inspection	Surface defects, cold shuts, misruns	Qualitative
X-ray radiography	Internal porosity, inclusions	≥ 1–2% wall thickness
Computed tomography (CT)	3D porosity map, internal cracks	≥ 0.1 mm voids
Dye penetrant (PT)	Surface-connected cracks	≥ 0.1 mm
Magnetic particle (MT)	Surface + near-surface cracks (Fe alloys)	≥ 0.5 mm
Ultrasonic (UT)	Internal defects; thick sections	Varies; ≥ 1–5 mm

Radiographic grading (ASTM E446): Radiograph of casting → compare to reference standards (E155 for Al, E186 for steel) Grade 1 (best) to Grade 5 (worst); specify acceptance (e.g., "Grade 3 or better for Class A zones")

Porosity measurement: Reduced pressure test (Al): freeze sample under vacuum; visible gas porosity relative index Archimedes density test: bulk density vs. theoretical; Δρ/ρ_theoretical ≈ porosity volume fraction

Remediation

Impregnation: vacuum impregnate with polymer sealant (Loctite Resinol 90C); seals micro-porosity in hydraulic components Pressure test after impregnation; accepted practice for pressure-tight castings NOT acceptable for structural (mechanical) load-bearing applications

Welding repair: fill surface-connected shrinkage by welding; pre-heat; PWHT Weld repair procedure per ASTM A370 or MIL-STD-278; requires NDT after repair

HIP (Hot Isostatic Pressing): Eliminates internal porosity in premium castings (aerospace, medical) Al alloys: 520°C / 1000 bar / 4 hr; Ti alloys: 920°C / 1750 bar / 2 hr Porosity closure → improved fatigue life 50–100%

Standards

Standard	Scope
ASTM E446	Radiographic reference standards for steel casting
ASTM E155	Reference radiographs for Al and Mg castings
ASTM E186	Heavy steel casting radiography
ASTM A802	Surface acceptance criteria for steel castings
ISO 4986	Radiographic examination of castings

Output

Provide: defect type and likely cause, detection method and sensitivity, ASTM E446/E155 acceptance grade specification, remedy (process change vs. impregnation vs. HIP vs. weld repair), prevention measures (riser sizing [V/A ratio], gate design, pouring temperature [°C], degassing procedure), required NDT type, and applicable standard (ASTM E446, E155).