Cast Iron vs Cast Steel: Differences, How to Tell Them Apart, Which to Choose

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What Are Cast Iron and Cast Steel?

Both are iron-carbon alloys and both are melted and poured into moulds. The only decisive difference is carbon content — and that single difference transforms the character of the two materials:

• Cast iron: carbon above 2.1% (typically 2.5–4.0% in practice). The excess carbon cannot stay dissolved and separates out as free graphite.
• Cast steel: carbon below 2.1% (typically ≤0.30% in structural cast steels). All carbon remains dissolved; there is no free graphite.

In short: free graphite means cast iron; no graphite means cast steel. This is exactly why cast iron is brittle and cast steel is tough — the graphite flakes act like micro-cracks distributed through the material.

Don't confuse the two: Pig iron is the raw intermediate tapped from the blast furnace; cast iron is the finished product made by remelting pig iron and pouring it into moulds.

Carbon Content and Composition

Element	Grey cast iron	Ductile (nodular) iron	Cast steel (carbon)
Carbon (C)	2.5–4.0	3.2–3.8	≤ 0.30
Silicon (Si)	1.0–3.0	1.8–2.8	≤ 0.60
Manganese (Mn)	0.4–1.0	0.1–0.5	0.5–1.0
Free graphite	Flake (lamellar)	Spheroidal	None

Use our Chemical Composition tool to compare analyses grade by grade.

Mechanical Properties

Property	Grey iron (EN-GJL-250)	Ductile (EN-GJS-400-15)	Cast steel (≈A216 WCB)
Tensile strength (Rm)	250–350 MPa	≥ 400 MPa	485–655 MPa
Yield strength (Rp0.2)	No distinct yield	≥ 250 MPa	≥ 250 MPa
Elongation (A)	< 1%	≥ 15%	≥ 22%
Hardness	180–220 HB	130–180 HB	130–190 HB
Impact toughness	Very low	Moderate	High
Vibration damping	Excellent	Good	Poor
Density	~7.1–7.2 g/cm³	~7.1 g/cm³	~7.8 g/cm³

The real story in this table is the elongation column: grey iron stretches less than 1% — it fails without warning. Cast steel stretches 22%; it deforms before it breaks and gives you time. That single fact is why safety-critical parts are made from cast steel.

To convert hardness between HB/HRC/Rm, use the Hardness Converter.

Casting and Manufacturing Behaviour

Carbon governs not only strength but how the metal behaves in the foundry:

	Cast iron	Cast steel
Pouring temperature	~1350–1450 °C	~1550–1650 °C
Fluidity (filling thin sections)	Excellent	Poor
Solidification shrinkage	Low (graphite expands and compensates)	High — risers essential
Machinability	Excellent (graphite acts as a lubricant)	Moderate
Weldability	Poor	Good
Relative cost	Low	High

Cast iron is the foundry's friend: it melts cooler, fills thin walls, shrinks little and machines easily. Cast steel is stronger — but it charges you at every step.

How to Tell Them Apart (on the shop floor, in a minute)

1. Chip test (most reliable): Take a chip with a file or drill. Cast iron produces powdery, crumbling chips (because of the graphite). Cast steel produces continuous, curling chips.
2. Spark test: On a grinding wheel, cast iron throws short, dull reddish sparks with dense fine bursts near the tip. Cast steel throws longer, bright yellow-white and straighter streaks.
3. Sound test: Strike the part with a hammer. Steel rings. Grey cast iron gives a dull, dead thud — direct proof of the excellent damping that makes it the material of choice for machine bases.
4. Fracture surface: Grey cast iron fractures matt grey and granular. Steel fractures bright and fibrous/silky.
5. Density: Cast iron ~7.1 g/cm³ vs steel ~7.8 g/cm³ — for the same volume, steel is noticeably heavier. Run the numbers with our Steel Density tool.

⚠️ The magnet test does NOT work. Both cast iron and carbon cast steel are ferromagnetic; a magnet attracts both. Magnetism is not a way to tell them apart.

Welding Cast Iron to Steel

A common job, and one that is frequently botched. It is possible — but do not attempt it with an ordinary steel electrode. The weld zone turns into hard, brittle white cast iron (carbide) and cracks.

The correct method:

• Use a nickel-iron (ENiFe-CI) electrode; nickel tolerates carbon and prevents brittle carbide formation.
• Preheat to 200–300 °C and cool the part slowly (in ash or under a blanket).
• Run short beads and let the part cool between them — low heat input is essential.
• Peen each bead while hot to relieve stress; most cracking comes from shrinkage stress.

If the joint carries critical load: consider a bolted/mechanical connection instead. Welding cast iron is a repair technique, not a design solution.

Which Should You Choose?

Choose cast iron if:

• The part works in compression (machine bases, slideways, engine blocks)
• Vibration damping matters (machine tool structures — something steel simply cannot do)
• The geometry is complex and thin-walled (fluidity advantage)
• Wear resistance and easy machining are required
• Cost is the primary constraint

Choose cast steel if:

• The part sees impact, tensile or dynamic loading
• It is safety-critical — it must deform before it breaks (cranes, rail, pressure parts)
• Welded fabrication or later weld repair is needed
• Low-temperature toughness is required (cast iron gets even more brittle in the cold)
• The part will later be heat treated to raise strength

Ductile iron (EN-GJS) is the middle ground: the castability and cost of iron plus meaningful ductility (15% elongation). That is why it dominates crankshafts, gearbox housings, pressure pipe and fittings. What turns the graphite from flakes into spheres is the FeSiMg nodulariser and inoculants added during casting.

Applications

Sector	Cast iron	Cast steel
Machinery	Machine beds, slideways, flywheels	Gears, couplings, heavy-load parts
Automotive	Engine blocks, brake discs, cylinder liners	Suspension components, shafts
Energy / valves	Bodies (low pressure)	Valve bodies, high-pressure parts
Infrastructure	Sewer pipe, manhole covers, ductile pipe	Rail fittings, crane components
Foundry industry	Grey/ductile casting range	Steel castings, grinding balls

Supply from Aktif Çelik

Aktif Çelik supplies the full chain foundries need to hit their target chemistry: pig iron (basic, foundry and nodular grades), inoculants and FeSiMg nodularisers, ferroalloys and cast/grinding balls. Our technical team supports you on charge calculation, sulphur/magnesium adjustment and grade selection. Contact us for analysis reports and samples.