Sorting gas holes from shrinkage in grey iron impellers
A foundry casts 6 kg grey iron impellers (CE 3.9) in green sand, with a 40 mm diameter hub, 12 mm blades, and a linseed-oil-bonded sand core forming a 30 mm bore. After shot blasting and machining, 14% of one week's impellers show internal cavities. Three are sectioned. Two show rounded cavities with smooth, bright walls, 2–6 mm across, clustered 3–5 mm below the cope surface near the core print. The third shows an irregular cavity with rough, dendritic walls inside the 40 mm hub, and a pressure test shows it connects to the bore. That week's sand tests were in specification (moisture 5.0%, permeability 130, green strength 0.95 kg/cm²). The core shop reports oil-sand cores baked at 200 °C for 40 minutes and then stored for 10 days in humid monsoon air; the core vents were cut but never connected to the mould vent. Pouring temperature was 1400 °C.
1. Classify the two types of cavity and justify the classification from the evidence, ruling out the moulding sand as the source.
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The two cavity types are gas holes (blowholes) and shrinkage porosity, and the evidence separates them cleanly.
Rounded, bright, smooth-walled cavities just under the cope surface near the core print: round shape and smooth, oxidised-bright walls mean gas was trapped as a bubble while the metal was liquid; a shrinkage cavity has torn, dendritic walls because it forms while the metal is pasty. Their position near the core print and just below the cope, where gas collects, points to the core as the gas source.
Irregular, rough, dendritic cavity in the 40 mm hub, connected to the bore: this is a hot spot. The 40 mm hub is the heaviest section and is surrounded by thin 12 mm blades that cool and freeze first, isolating the hub from any feed metal, so a contraction cavity forms there. The connection to the bore is just the machining breaking into the pipe, not a gas path.
Ruling out the sand: green sand moisture 5.0%, permeability 130 and strength 0.95 kg/cm² are all in specification that week, so the mould wall is not the culprit. The common factor linking both defects is the core, which is under-vented (gives the gas holes) and removes a feed path in the hub (helps create the shrinkage).
2. Give the root cause and the corrective action for each defect, including what should be done with the cores already in store.
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Gas holes:
- Root cause: the oil-bonded core is damp and under-vented. Baked at 200 °C for 40 minutes is at the low end for an oil-sand core of this size, and 10 days in humid monsoon air re-absorbs moisture into the core. With core vents cut but not led to the mould vent, gas generated by the binder has no exit and is forced through the metal at the core print, just under the cope.
- Action: re-bake the stored cores (or scrap them) before use; store baked cores in a dry, closed rack with a maximum stated shelf life; connect core vents to the mould vent and add a vent wire through the cope above the core print; reduce binder/water in the core mix; keep pouring temperature under control, since 1400 °C is high for a CE 3.9 iron and increases gas.
Shrinkage in the hub:
- Root cause: isolated hot spot. The hub is heavier than the blades, so it freezes last with no reservoir.
- Action: feed the hub with a riser or a side feeder, or place a chill in the hub bore, or pad the section so the blades and hub cool more evenly. Slightly raising CE to promote graphite expansion helps, but do not rely on it alone for a 40 mm section.
3. Which inspection methods would you specify to detect these defects before machining, and why did the gas holes appear only after machining?
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Detection before machining:
- Radiography is the first choice for both defects: gas holes appear as round, sharply defined dark spots, shrinkage as irregular, diffuse, often branching indications. It also locates them relative to the bore, which matters for the pressure-tightness requirement.
- Ultrasonic testing is useful on the 40 mm hub for shrinkage, but is limited on the thin, curved blades.
- Pressure test (air or water) is the functional check the customer cares about; it reliably finds interconnected shrinkage but not isolated gas holes.
- Dye penetrant would not help here; both defects are internal and do not reach the surface before machining.
Why the gas holes appeared only after machining: they sat 3–5 mm below the cope surface, inside the machining stock. Before machining the skin was sound and pressure-tight, so they were invisible to visual inspection and even to a pressure test; the tool removed the skin and opened them. That is exactly why the sampling must be by radiography of the as-cast part, not by visual inspection alone. Sampling frequency should also rise while the core drying and venting problems are being corrected, since the defect rate is 14% and the defective parts look sound.