Cast iron comes in five types — gray, ductile, compacted graphite, white and malleable — and what separates them is the form the carbon takes when the casting solidifies: graphite flakes, graphite nodules, compacted worms, iron carbide, or temper carbon.
All five carry more than about 2% carbon, the line that divides cast iron from cast steel.
The Five Types of Cast Iron
The five types are gray, ductile, compacted graphite, white and malleable iron, sorted by the form the carbon takes in the solid casting.
The high-alloy irons sit outside this scheme: Ni-Resist and the other austenitic grades and the high-silicon irons carry over 3% alloy content and are classified commercially as their own family.
Gray Cast Iron
Gray iron carries its carbon as graphite flakes, and those flakes are why it machines easily, damps vibration and runs against a mating surface without galling. Gray iron fails in tension with no significant plastic deformation: no useful yield point, effectively no elongation.
At the pour it is fluid and expands as graphite forms during solidification, so thin sections fill and feed with modest risering.
ASTM A48 grades it by class from 20 to 60, and the class number is the minimum tensile strength in ksi on a separately cast test bar. Machine bases, counterweights, flywheels and housings under compressive load are gray iron castings by default.
Ductile Cast Iron
Ductile iron is the same base iron given a magnesium treatment in the ladle, which pulls the graphite into rounded nodules. The casting gains real tensile strength, a yield point and measurable elongation as-cast.
ASTM A536 runs from grade 60-40-18 to 120-90-02, and the three numbers are the minimum tensile strength in ksi, the minimum yield in ksi and the minimum elongation in percent. Which grade fits a given part is a separate decision.
It expands on solidification the way gray iron does, so it feeds with risers far smaller than steel or malleable iron needs.
Pump and valve bodies, gearbox and bearing housings, hydraulic manifolds and high-load flywheels are ordered as ductile iron castings.
Compacted Graphite Iron
Compacted graphite iron sits between the other two. A controlled, deliberately small magnesium treatment leaves the graphite as short, thick, interconnected worms, and strength and fatigue resistance land between gray and ductile iron while thermal conductivity stays close to gray.
It is a specialist pour: a 2017 study of CGI production puts the residual magnesium window at roughly 0.006 to 0.01%, against 0.02 to 0.06% for ductile iron.
ASTM A842 accepts the iron on graphite form, 80% minimum Type IV graphite in the test coupon. It is specified for heavy-duty truck cylinder heads and similar thermally cycled parts.
White Cast Iron
White iron holds all of its carbon as iron carbide. It comes out hard and abrasion resistant, too brittle for structural use and too hard to machine — wear surfaces are ground.
Hardness is the property that gets tested: alloyed white irons run 450 to 800 HB. ASTM A532 covers only those alloyed abrasion-resistant grades, the Ni-Hard and high-chromium irons, and specifies chemistry and hardness with no tension test. Mill liners, crusher parts and slurry pump wear parts are bought against it.
At the pour it behaves like steel: the casting shrinks and needs large feeders, and the white structure gets harder to hold as the section thickens. Chilled iron is the same structure placed on purpose — white at a chilled face over a gray core, mottled in between.
Malleable Cast Iron
Malleable iron starts as a white iron casting and is then held through an extended anneal until the carbide breaks down into clusters of temper carbon, which brings ductility and toughness into ductile iron’s range.
ASTM A47 covers ferritic malleable iron and carries a single grade, 32510: 50,000 psi minimum tensile, 32,500 psi minimum yield and 10% minimum elongation.
The casting has to solidify white through its whole section, which caps how thick and how heavy a malleable part can be, and it shrinks like steel doing it.
Ductile iron displaced it for machinery castings — the same ductility as-cast, at any section, with no anneal — and malleable production has fallen to small custom orders in hardware and appliance parts.
Which Type of Cast Iron Is Best for Your Part
None of the five is best. The part’s loading, wear, machining and section thickness decide, and for the medium-to-large machinery parts we pour as iron castings, the answer is gray iron unless the part needs yield strength or impact resistance, and then it is ductile.
Gray iron covers compressive load, vibration and the lowest cost per pound, while ductile iron is specified for tensile load, shock or internal pressure. Most machinery parts come down to gray against ductile.
White iron is for sliding abrasion with no impact behind it, and compacted graphite iron for parts cycling hot and cold under load. Malleable iron is worth raising only for small hardware that ductile iron has not already taken.
| Type | Carbon form | ASTM spec | ASTM minimum | Elongation | Typical parts |
|---|---|---|---|---|---|
| Gray | Graphite flakes | A48, Class 20–60 | 20–60 ksi tensile | Negligible | Machine bases, counterweights, flywheels |
| Ductile | Graphite nodules | A536, 60-40-18 to 120-90-02 | 60–120 ksi tensile | 18% down to 2% | Pump and valve bodies, housings, manifolds |
| Compacted graphite | Compacted worms | A842 | 80% min Type IV graphite | Between gray and ductile | Truck cylinder heads |
| White | Iron carbide | A532, alloyed grades only | 450–800 HB hardness | None | Mill liners, crusher and slurry wear parts |
| Malleable | Temper carbon | A47, grade 32510 | 50 ksi tensile | 10% | Small hardware, appliance parts |