Shrinkage porosity in iron is one of the most damaging defects in gray/grey and ductile (SG/EN‑GJS) iron castings. This guide gives you reliable detection methods, actionable prevention, and a proof package buyers can trust—so you can go from problem to PPAP‑ready.
What is shrinkage porosity in gray/grey & ductile iron?
Definition. Shrinkage porosity (also called shrinkage cavities or shrinkage sponge) forms when the metal cannot be properly fed during solidification. In gray iron, graphite flakes alter heat flow and feeding; in ductile iron, Mg treatment and graphite nodules change solidification behavior—so the hot spots, morphology, and location of shrinkage can differ between the two.
Typical locations. Thick sections, junctions (rib‑to‑wall, boss bases), abrupt transitions, and regions far from risers or with a weak metallostatic head.
Impacts. Leak failure, machining breakthrough, reduced fatigue life, paint blistering, and assembly rework.
Why it happens
- Hot spots/modulus imbalance. Thick sections freeze last and need directional feeding.
- Undersized or misplaced risers. Inadequate modulus, no insulating/exothermic sleeves, and wrong neck size.
- Gating & filling. Turbulent flow and trapped air can couple with shrinkage to create mixed cavities.
- Chemistry & inoculation. Carbon equivalent (CE), Si, residual Mg, and inoculation timing affect nucleation and feeding.
- Pour température et temps. Excess superheat concentrates shrinkage; too low causes misruns/cold shuts.
- Molding & permeability. Moisture/compaction/venting inconsistency restricts feeding paths and local pressure.
DETECT — find shrinkage early, not after assembly
Machining & leak tests
- Trial cuts and in‑process visual checks at critical faces (per control plan).
- Pressure or vacuum leak tests after machining for fluid‑handling parts (typical 0.5–1.5 bar / 7–22 psi, 30–60 s hold—set per drawing).
NDT method selection
- UT (ultrasonic) — good sensitivity to volumetric defects in ductile iron (>10–12 mm / 0.4–0.5 in walls). Gray iron’s flake graphite increases attenuation; use tuned procedures and reference blocks.
- RT (X/γ radiography) — visualizes shrinkage clusters; economical above ~8–10 mm (0.31–0.39 in) walls.
- CT (industrial computed tomography) — 3D quantification; excellent for samples/failure analysis; batch use as periodic audits.
- DPI/MPI — surface‑breaking indications; MPI suits ferritic/pearlitic ductile iron; DPI for open porosity/cracks (mind cleaning to avoid dye entrapment).
Metallography & density checks
- Graphite evaluation per ASTM A247 (nodularity/flake class) to understand feeding tendency.
- Sectioning & density comparison for root cause during sampling.
Buyer tip: On the print or spec, define the method + acceptance level + sample rate. For leak‑tight parts, specify allowable leak rate and 100% leak test where necessary.
PREVENT — 10 measures that actually work (design + process)
- Uniform walls & generous radii. Avoid sharp corners; as a rule of thumb, R ≥ 3–5 mm (0.12–0.20 in).
- Riser modulus & sleeves. Use insulating/exothermic sleeves and controlled necks; add chills to pull solidification.
- Laminar gating. Reduce turbulence, avoid stagnant pockets; use well‑sized sprues/runners and a calm pouring basin.
- Place risers on hot spots. Shorten feeding paths; prioritize thick‑end feeding.
- Chemistry control. CE, Si, Mn, S, residual Mg within window; late/stream inoculation for nucleation.
- Pour temperature window. For ductile iron, keep within your foundry’s qualified range (often ~1370–1450 °C / 2498–2642 °F).
- Consistent sand. SPC on compactability, moisture, permeability; vents/risers designed for air release.
- Standardized riser/chill library. Reuse proven patterns to reduce trial‑and‑error.
- Solidification simulation first. Use MAGMAsoft/ProCAST‑class tools to flag risk hot spots before tooling cuts.
- In‑process witness points. Record pour temp, inoculation add‑on, riser‑neck temp; photo logs for traceability.
PROVE IT’S GONE — documentation buyers accept
Sampling stage (PPAP / FAI)
- FAI report. CMM results on criticals (probe spec, temperature, datum scheme).
- Material. OES chemistry, hardness (HBW), tensile per ASTM A536 (ductile iron) / EN 1563; graphite per ASTM A247.
- NDT evidence.
- RT per EN 12681‑1 (Radiographic testing of castings) or reference images ASTM E446/E186.
- UT per EN 12680‑3 (Spheroidal graphite iron castings — UT).
- For leak‑tight parts, attach pressure‑time curves tied to part serials/barcodes.
- Process docs. Control plan, PFMEA, MSA, melt/heat traceability, pour batch records, riser/chill layout sketch.
- CT report (when used). Porosity volume %, minimum remaining wall maps.
Mass production (Run‑at‑Rate)
- Sampling frequency. Define 100% leak test where needed + periodic NDT on critical zones; tighten on risk batches.
- Process capability. Key dims Cpk ≥ 1.33 target; set PPM goals for leaks.
- Change control. Any change in risers/inoculation/pour temperature window triggers re‑qualification.
Data tables & quick‑use checklists
Values are typical for guidance. Always confirm with your drawing, testing standard, and your foundry’s qualified window.
Table A — Typical capability by process (tolerances & surface)
| Process | Typical ISO 8062‑3 CT grade | Linear tolerance at 100 mm (indicative) | Surface roughness Ra | Notes |
|---|---|---|---|---|
| Green sand | CT9–CT10 | ±1.6–2.4 mm (±0.063–0.094 in) | 6.3–12.5 μm (250–500 μin) | Most cost‑effective; higher porosity risk at hot spots |
| No‑bake / resin sand | CT8–CT9 | ±1.2–1.8 mm (±0.047–0.071 in) | 3.2–6.3 μm (125–250 μin) | Good compromise of finish vs cost |
| Shell mold | CT6–CT7 | ±0.6–1.0 mm (±0.024–0.039 in) | 1.6–3.2 μm (63–125 μin) | Best finish among sand routes |
Table B — Recommended machining allowances (iron castings)
| Casting mass | Suggested stock on machined faces |
| 5–10 kg (11–22 lb) | 1.5–2.5 mm (0.06–0.10 in) |
| 10–50 kg (22–110 lb) | 2.5–3.5 mm (0.10–0.14 in) |
| 50–200 kg (110–441 lb) | 3.5–5.0 mm (0.14–0.20 in) |
| >200 kg (>441 lb) | 5.0–8.0 mm (0.20–0.31 in) |
Table C — NDT applicability for shrinkage porosity
| Section thickness | Gray iron (UT) | Ductile iron (UT) | RT (X/γ) | CT |
| <8 mm (<0.31 in) | Limited | Limited | Limited sensitivity | Good for small parts |
| 8–20 mm (0.31–0.79 in) | Challenging | Moderate | Good | Good (sample audit) |
| >20 mm (>0.79 in) | Poor | Good | Needs higher energy | Selective (cost) |
Leak test quick spec (example for pump/valve bodies)
- Medium: air under water or dry‑air mass flow
- Pressure: 0.8–1.0 bar (11.6–14.5 psi) unless otherwise specified
- Hold/measure: 30–60 s; define max leak rate at temperature X°C/°F
- Traceability: log curve + part barcode/serial
FAQs
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