Casting defects and inspection

Why it matters

A foundry's profit is decided by its scrap rate, and most scrap comes from a small family of defects with well-known causes. An engineer who can look at a casting, name the defect and trace it to the sand, the gating, the risering or the metal can fix the process; one who cannot will only increase inspection. Inspection methods matter just as much — each one sees some defects and is blind to others.

Key ideas

Grouping defects by cause is the fastest way to diagnose them.

Gas defects — gas trapped in or absorbed by the metal.

• Blowholes — smooth-walled, rounded cavities near the surface, from steam and gas generated in the mould that cannot escape. Causes: high moisture, low permeability, hard ramming, poor venting, damp cores. Fixes: control moisture, open the sand, vent moulds and cores.
• Gas porosity (pinholes) — many small holes from gas (mainly hydrogen in aluminium, hydrogen and nitrogen in steel) dissolved in the melt and rejected on freezing, because solubility drops sharply at solidification. Fixes: degassing (inert gas, vacuum), dry charge and ladles, not overheating the melt.

Shrinkage defects — volume loss during freezing not fed by liquid.

• Shrinkage cavity (pipe) — rough, dendritic-walled void at the last region to freeze, often at a hot spot (thick section, junction). Fixes: riser of adequate modulus and volume, directional solidification toward the riser, chills on thick sections.
• Centre-line or micro-shrinkage porosity — spread-out porosity in long-freezing-range alloys. Shrinkage and gas cavities are told apart by their walls: gas holes are smooth and round, shrinkage voids are rough and dendritic.

Pouring-metal defects — the metal does not fill or fuse.

• Misrun — cavity not completely filled because the metal froze too early. Causes: low pouring temperature (low fluidity), slow pouring, thin sections, poor gating.
• Cold shut — two streams meet but do not fuse, leaving a visible seam. Same causes as misrun, plus interrupted pouring.
• Inclusions — slag, dross, oxide films or sand in the metal. Fixes: skimming, slag traps, filters, gentle non-turbulent gating, clean moulds.

Mould-material defects.

• Scab — a patch of sand face expands and buckles off, metal fills behind it, giving a rough raised patch. Rat tail is a milder form (a line or depression). Caused by sand expansion with low hot strength; fixed by adding cushioning material (wood flour) and avoiding hard ramming.
• Metal penetration and burn-on — metal enters coarse, loose sand; fixed by finer facing sand and mould coatings.
• Swell — enlargement where the mould wall gave way under metallostatic pressure (soft ramming).
• Drop — sand from the cope falls into the cavity.
• Fusion — sand fuses to the casting due to low refractoriness.

Shape and stress defects.

• Hot tear — ragged crack formed at high temperature, when the casting's contraction is resisted by a strong mould or core while the metal is still weak. Fixes: collapsible cores and sand, uniform sections, fillets at junctions.
• Shift (mismatch) — cope and drag (or core) misaligned at the parting line.
• Warpage — distortion from uneven cooling; handled by design and distortion allowance.
• Fin (flash) — thin metal at the parting line from a poorly closed mould.

Inspection.

• Visual and dimensional — surface defects, mismatch, misruns, fins.
• Liquid (dye) penetrant — surface-breaking cracks and porosity on any non-porous material.
• Magnetic particle — surface and near-surface cracks, only in ferromagnetic materials (steels, cast irons — not aluminium or austenitic stainless).
• Radiography (X-ray, γ-ray) — internal volumetric defects (gas holes, shrinkage, inclusions) show as density differences; gives a permanent record. Planar cracks are seen only if roughly aligned with the beam.
• Ultrasonic testing — internal defects by echo; best for planar defects and thickness measurement; difficult in coarse-grained castings and complex shapes.
• Pressure (leak) testing — for valve bodies and pump casings.
• Destructive tests on sample bars or sectioned castings — tensile, hardness, metallography.

Formulas

d = v·t / 2 — pulse-echo ultrasonic depth of a reflector (m); v = sound velocity in the material (m/s; about 5900 m/s longitudinal in steel, 6300 m/s in aluminium — take values from your data book), t = round-trip time (s).

V_shrink = β × V_c — volume of feed metal needed (mm³); β = solidification shrinkage (fraction, from data book), V_c = casting volume.

η·V_r ≥ β·(V_c + V_r) → V_r ≥ β·V_c / (η − β) — riser volume needed so that the usable feed (fraction η of the riser, the riser efficiency) covers shrinkage of both casting and riser.

Worked examples

Example 1 (standard — ultrasonic depth). A pulse-echo probe on a steel casting receives a flaw echo 20 µs after the pulse. Velocity in steel is 5900 m/s. Find the depth of the flaw.

1. d = v·t/2 = 5900 × 20 × 10⁻⁶ / 2.
2. d = 0.059 m.

Answer: d = 59 mm. (Forgetting the factor 2 gives 118 mm — the sound travels there and back.)

Example 2 (GATE level — shrinkage volume). A steel casting of 200 × 150 × 100 mm is to be free of shrinkage cavities. Solidification shrinkage of the steel is 3%, and an open cylindrical riser can deliver 14% of its volume as feed metal (data-book value). Find the minimum riser volume and the size of a riser with H = D.

1. V_c = 200 × 150 × 100 = 3.0 × 10⁶ mm³.
2. V_r ≥ β·V_c / (η − β) = 0.03 × 3.0 × 10⁶ / (0.14 − 0.03) = 9.0 × 10⁴ / 0.11 = 8.18 × 10⁵ mm³.
3. For H = D, V_r = πD³/4 → D = (4 × 8.18 × 10⁵ / π)^(1/3) = 101.4 mm.

Answer: V_r ≈ 8.2 × 10⁵ mm³, D = H ≈ 102 mm. The riser must also pass the freezing-time (modulus) check from the risering topic; use the larger of the two sizes.

Example 3 (diagnosis). An aluminium casting shows many small, smooth, round holes evenly spread through the section, even in thin walls far from hot spots. Smooth and round → gas, not shrinkage; spread through the section and present in thin walls → dissolved gas, not mould steam. Diagnosis: hydrogen gas porosity; remedy: degas the melt and use a dry charge.

Common mistakes

• Calling every cavity "porosity". Distinguish gas (smooth, round) from shrinkage (rough, dendritic, at hot spots).
• Confusing misrun (cavity not filled) with cold shut (filled but not fused).
• Using magnetic particle inspection on aluminium or austenitic stainless steel.
• Forgetting the factor of 2 in pulse-echo depth.
• Blaming hot tears on fast cooling alone; the root cause is hindered contraction — a non-collapsible mould or core.
• Treating scab and blowhole as the same; a scab is a sand-expansion defect at the surface.

For GATE PI

Most questions here are one-mark matching or cause–effect items: defect → cause (blowhole → low permeability/high moisture, hot tear → poor collapsibility, scab → sand expansion, cold shut → low fluidity), defect → remedy, and defect → suitable NDT method. Occasional numericals use ultrasonic depth or shrinkage-volume and riser-volume calculations. Practise the cause tables until you can classify a described defect in one reading.

Quick check

1. Which NDT method cannot be used on an aluminium casting: penetrant, magnetic particle or radiography?
2. Rough, dendritic cavity at the junction of two thick sections — gas or shrinkage?
3. What sand property, if too low, causes hot tears?
4. An echo returns after 34 µs in steel (v = 5900 m/s). Depth?
5. Name the defect where metal streams meet but do not fuse.

Answers: 1. Magnetic particle. 2. Shrinkage. 3. Collapsibility. 4. 5900 × 34 × 10⁻⁶ / 2 ≈ 0.100 m = 100 mm. 5. Cold shut.