Choose cast steel when the part must take impact, bending or internal pressure and yield before it breaks; choose cast iron when castability, machining, vibration damping or price decide — and keep ductile iron on the list, because it covers most of the ground between the two. The line between the families is carbon: below about 2 % the alloy is steel, above it cast iron, and that one number sets nearly every row of the comparison below.
what is Cast Steel
Cast steel is an iron–carbon alloy with less than about 2 % carbon poured to shape instead of rolled or forged — and in the grades you will actually specify, carbon is capped at 0.25–0.35 %. It is held that low on purpose: higher carbon cracks in heat treatment and in welding. In return you get wrought-steel behavior in a cast shape.
Three ASTM specifications cover nearly every carbon steel casting we pour. A27 is the general-purpose spec, with minimum tensile strength of 60 to 70 ksi (415 to 485 MPa) and 22–24 % minimum elongation. A216 covers pressure-containing parts — valve bodies, flanges, fittings — for high-temperature service; WCB runs 70–95 ksi (485–655 MPa) tensile with a 36 ksi (250 MPa) minimum yield.
A148 takes over when the part is stressed harder, climbing to 150 ksi tensile. The trade is ductility: at grade 150-135 the minimum elongation has fallen to 7 %, and toughness falls with it. Stronger cast steel is not simply better cast steel.
what is cast iron
Cast iron is the iron–carbon alloy above about 2 % carbon — typically 2 to 4 % — and the carbon the metal cannot hold in solution comes out during solidification as graphite. That carbon also lowers the melting point, so iron pours about 400 °F cooler than carbon steel and flows further into a mold before it freezes.
In gray iron the graphite forms flakes. They are what make the metal cheap to cast and easy to machine, and they are also why it is brittle: every flake tip is a ready-made crack starter.
Ductile iron is the same base iron treated with magnesium in the ladle so the graphite precipitates as spheroids instead of flakes. A round nodule does not start a crack the way a flake tip does, so the casting keeps iron’s castability and machinability and gains real ductility: 18 % minimum elongation in the base grade against roughly zero for gray.
Gray iron is bought to ASTM A48, which grades on tensile strength alone and leaves chemistry to the foundry; the class number is the minimum tensile in ksi from a separately cast test bar, so Class 35 means 35,000 psi. Ductile iron is bought to A536, whose grade names read tensile–yield–elongation: 60-40-18 is 60 ksi tensile, 40 ksi yield, 18 % elongation, and the grades run up to 120-90-02.
Cast Steel vs Cast Iron: Key Differences
The difference that decides most parts is failure mode: cast steel stretches 22–24 % before it breaks, gray iron essentially does not, and ductile iron at 18 % sits far closer to steel than to gray. That is why the table splits cast iron into its two working forms — treating it as one material gets the toughness, ductility and damping rows wrong.
| Property | Cast steel | Gray iron | Ductile iron |
|---|---|---|---|
| Typical carbon content | 0.25–0.35 % max in A27 and A216 grades (2 % is the metallurgical ceiling) | 2–4 % | 2–4 %, magnesium-treated |
| Tensile strength | 60–70 ksi min (A27); 70–95 ksi (A216 WCB); up to 150 ksi (A148) | Class number = minimum ksi (Class 35 = 35,000 psi) | 60–120 ksi across A536 grades |
| Toughness | Excellent | Low — graphite flakes nucleate cracks | Good |
| Impact resistance | Excellent | Poor | Good |
| Ductility (min. elongation) | 22–24 % (A27 60-30, A216 WCB) | Near zero; no proportional limit | 18 % (60-40-18) down to 2 % (120-90-02) |
| Wear resistance | Good; rises with carbon | Excellent | Good |
| Compressive strength | About equal to tensile | 3–4× its tensile strength | High |
| Castability | More difficult | Excellent | Good |
| Fluidity when molten | Lower; pours at 2850–3100 °F (1565–1700 °C) | Highest of the irons; pours at 2450–2700 °F (1340–1480 °C) | High; same pouring band as gray |
| Shrinkage during casting | About 2 % pattern allowance; about 3 % solidification shrinkage | About 1 % pattern allowance; graphite expansion offsets solidification shrinkage | About 1 % pattern allowance |
| Machinability | Moderate | Excellent | Good |
| Weldability | Good while carbon stays low | Difficult | Difficult |
| Vibration damping | Lowest of the three | Excellent (1.0 reference) | 0.14 relative to gray — closer to steel |
| Heat resistance | Good; A216 is written for high-temperature pressure service | Good | Good |
| Typical cost | Highest | Lowest | Between |
| Typical applications | Pressure valve bodies and flanges; impact-loaded structural parts | Machine bases, counterweights, flywheels | Pump, hydraulic and gearbox housings; bearing housings; valve bodies |
Every strength and elongation figure above is a minimum from a separately cast test bar, and a test-bar result represents the quality of the metal, not necessarily the properties of the casting itself. Inside a heavy section, expect less.
The shrinkage row looks backwards to anyone who has fed a steel casting, but it is right. As gray iron solidifies the graphite precipitating out of the melt expands, and that expansion offsets most or all of the liquid-to-solid contraction — high-carbon gray iron shows essentially zero net solidification shrinkage. Low-carbon cast steel shrinks about 3 % on solidification with nothing to offset it, so it carries risers that gray iron often does not.
The fluidity row is not a setting you can turn up. Carbon steel already pours at 2850–3100 °F against a sand refractory limit of roughly 3000–3330 °F, and the Steel Founders’ Society is blunt that fluid life does not increase with superheat. The gap comes from steel’s higher liquidus and freezing behavior; a thin wall that fills in gray iron does not become fillable in steel by pouring hotter.
Damping is a gray iron property, not a cast iron property. The American Foundry Society puts ductile iron’s damping capacity at 0.14 against gray iron’s 1.0 — better than steel, but nowhere near gray. A ductile iron housing specified for its quietness will disappoint.
Which Should You Choose
Specify cast steel when the part carries impact, bending or internal pressure and must deform rather than fracture; gray iron when the load is compressive or static and the part must be cheap, quiet and easy to machine; ductile iron for the housings and bodies in between, where you want most of steel’s ductility at iron’s price and castability.
Steel wins where the failure mode is the whole story: pressure-containing valve bodies and flanges to A216, mining and construction parts that see rock impact, and anything to be welded into a fabrication. The condition is carbon: weldability goes away as carbon and strength climb, so if a part needs both A148 strength and a weld, settle the weld procedure with the foundry before the grade goes on the drawing.
Gray iron wins where the load is compressive or static: machine bases, counterweights, flywheels, housings whose only job is to hold bearings in line. Its compressive strength is three to four times its tensile, so a base that only ever sees weight on it gives up nothing by being iron, and it is the only one of the three with damping worth designing around. The price is near-zero elongation — nothing on the part should see a tensile or shock load you have not calculated.
Ductile iron is the default for pump housings, hydraulic housings, bearing housings and valve bodies, because it survives shock a gray part would crack under while casting and machining like iron. Its A536 grades run from 60-40-18 to 120-90-02, so where a part lands between gray iron and steel is set by the grade you write, not by the metal family — and the elongation you give up for strength is the trade to watch.
Conclusion
Carbon draws the line — under about 2 % is cast steel, over it is cast iron — and failure mode draws the decision. Cast steel yields before it fractures and welds; gray iron does neither but casts, machines, damps and costs least; ductile iron carries most of steel’s ductility at iron’s price. Decide from the overload case: if the part must bend rather than break, steel or ductile iron; if it can be stiff, quiet and cheap and never sees tension it was not designed for, gray iron.