Iron and Steel Castings

§1Pouring a shape

Casting forms metal by melting it, pouring it into a mould shaped like the wanted part, and letting it solidify. It makes complex shapes in one step — shapes that would be hard or wasteful to machine or forge.

The appeal is that almost any shape, however intricate — an engine block, a pump housing, a bracket with internal passages — can be made directly by filling a cavity, at any size from grams to many tonnes, and often cheaply in quantity. The challenges all trace to two physical facts. First, metal shrinks as it cools and solidifies, so a casting comes out smaller than the mould and can pull voids into itself (§3, §5). Second, thick sections solidify last, which decides where those voids form and how they must be fed (§4). The whole craft of casting — the pattern’s size, the risers, the gating — is arranged around managing shrinkage and solidification, and this page follows that thread from the mould (§2) to the metals (§6).

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§2The sand mould

The commonest mould is made of bonded sand, packed around a pattern in two halves, then parted to leave a cavity the metal is poured into.

In sand casting, a pattern — a model of the part — is packed in moulding sand within a two-part box (the flask): the top half is the cope, the bottom the drag. The pattern is withdrawn, leaving a cavity, and channels are cut to fill it: a sprue down which metal is poured, a gate leading into the cavity, and a riser reservoir (§5). Internal hollows are made with sand cores set into the cavity. The halves are closed and the metal poured (the hero shows the cross-section). Two features of the pattern deserve note: it carries a draft — a slight taper on vertical faces — so it can be withdrawn without tearing the sand, and it is made oversize to allow for shrinkage (§3). Sand moulds are cheap and take any size, but are destroyed to release each casting; permanent metal moulds (die casting) suit high volumes of smaller parts.

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§3Shrinkage allowance

Because metal contracts as it cools from pouring to room temperature, the pattern must be made larger than the finished casting by a shrinkage allowance — different for each metal.

Example 1 — making the pattern oversize

A casting solidifying and cooling shrinks by a characteristic amount — roughly 1% for grey cast iron and about 2% for steel — so the pattern is scaled up to compensate. A steel casting meant to finish 500 mm long needs a pattern 500 × 1.02 = 510 mm long; the same part in cast iron would need about 505 mm. Patternmakers use a “shrink rule” — an oversized ruler — so every dimension is laid out already enlarged by the metal’s shrinkage. Get it wrong and the whole casting comes out undersize. This is solid shrinkage, the contraction after freezing; it is distinct from the solidification shrinkage of the liquid-to-solid change, which forms voids and is dealt with by risers (§5). Both stem from metal shrinking, but one is fixed by an oversize pattern and the other by feeding.

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§4Solidification and Chvorinov's rule

How long a section takes to freeze depends on its bulk relative to its surface — captured by Chvorinov's rule, which is the key to feeding a casting soundly.

t = C (V / A)² — t solidification time, V volume, A cooling surface area

Example 2 — which section freezes last

Heat leaves a casting through its surface, so a section with much volume behind little surface freezes slowly, and one with little volume and much surface freezes fast. Chvorinov’s rule makes this exact: solidification time goes with the square of the ratio V/A, called the modulus. A 100 mm cube has volume 10⁶ mm³ and surface 6 × 10⁴ mm², so its modulus is 16.7 mm; a thin plate of the same volume, with far more surface, has a much smaller modulus and freezes far sooner. The consequence is decisive: the thickest part of a casting, with the largest modulus, is the last to solidify — and that is exactly where solidification shrinkage will pull a void unless fresh liquid can be fed in as it freezes. Chvorinov’s rule tells the founder where the trouble will be and how to beat it (§5).

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§5Risers and feeding

To stop the last-freezing region pulling a shrinkage void, a riser — a reservoir of molten metal — is attached to feed liquid into the casting as it solidifies. The riser must freeze after the casting.

As a section solidifies it draws in liquid to make up the volume lost in freezing; if none is available, it forms an internal cavity or surface sink. The riser supplies that liquid — a pocket of molten metal, connected to the heavy section, that stays liquid long enough to feed it. The governing condition comes straight from Chvorinov’s rule: the riser must have a larger modulus (V/A) than the casting section it feeds, so that it solidifies later and can keep supplying liquid until the casting is fully frozen. A riser that froze first would be useless. So risers are made chunky (high volume, low surface) and placed on or near the thickest sections, and the gating is arranged so those sections stay hot and fed. Sound feeding — the right riser in the right place, sized by modulus — is what separates a solid casting from one riddled with shrinkage porosity.

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§6The cast irons

Iron castings dominate because cast iron is cheap, pours easily and, in its several forms, spans a wide range of properties — from brittle and wear-resistant to tough and ductile.

The materials pages set out how carbon form governs cast iron; in the foundry the forms are chosen for the part. Grey iron, with its carbon as graphite flakes, is the everyday casting metal — cheap, fluid, easy to machine, excellent at damping vibration and in compression (machine bases, blocks), but brittle in tension. Ductile (nodular) iron, with the graphite formed into spheres by treatment, keeps grey iron’s castability but gains real toughness and ductility, so it takes shock and tension — crankshafts, gears, pipes. White iron, with its carbon locked as hard iron carbide, is intensely hard and wear-resistant but very brittle, used where abrasion resistance is all. Malleable iron, made by heat-treating white iron, is a tougher old alternative to ductile. Cast steel, poured where a casting must have steel’s full strength and toughness, shrinks more (§3) and is harder to cast but gives properties grey iron cannot. The founder picks the metal for the duty — grey for stiffness and economy, ductile for toughness, white for wear, steel for strength.

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§7Quick reference

The working core of the page on one card rack.

Sand mould

cope + drag · sprue · gate · riser

pattern with draft, oversize

Shrinkage

iron ~1% · steel ~2%

500 mm steel → 510 mm pattern

Chvorinov

t = C(V/A)² · modulus V/A

thickest freezes last

Risers

modulus > casting → freezes later

feeds shrinkage void

Cast irons

grey · ductile · white · steel

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Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Iron and Steel Castings. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Apply Iron and Steel Castings by beginning with the duty, not the component or software command. Convert the key ideas—mould, pouring, sand, shrinkage, allowance—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.

Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.

Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.

Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.

Step-by-step operating method

Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.

Illustrative design review record

Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.

Evidence class	Question	Release expectation
Requirement	What must the design do and under which conditions?	Approved and traceable
Input	Where did the load, property, tolerance or process limit come from?	Source, unit and revision recorded
Analysis	Which model and assumptions connect input to result?	Checkable calculation or simulation
Verification	How will conformity be demonstrated?	Method and acceptance criterion agreed
Validation	Will the solution work for intended users and conditions?	Representative use evidence

Common failure modes and recovery actions

1. Watch for

Starting detailed design before interfaces and operating limits are agreed.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Using catalogue or typical values as though they were certified project inputs.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Confusing verification of requirements with validation of user need.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Releasing drawings or procedures without configuration, inspection and change controls.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.

Review checklist

What function and failure consequence govern this decision?
Which inputs are measured, specified, assumed or illustrative?
How will conformity be demonstrated and recorded?
What change would invalidate the current evidence?
Are mandatory requirements distinguished from recommendations and illustrative values?
Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
Is there a named owner and a trigger for review, escalation, change or retirement?

Questions for deeper application

What is the most important distinction a practitioner must preserve when applying Iron and Steel Castings?