🪨 CAST IRON
Cast Iron: The Material Engineered for the Foundry Itself
Cast iron is the one material on this list that exists because of casting rather than in spite of it, the very name tells you the foundry is the point. Its high carbon content (over 2 percent, mostly precipitating as graphite) gives it superb fluidity, near-zero solidification shrinkage, low cost, and properties no wrought material matches: vibration damping, thermal stability, and excellent machinability. Gray iron, ductile iron, and A48 Class 40 each tune the graphite shape to a different job.
ISO 9001ISO 14001AS9100
1
Graphite morphology is everything: flakes versus nodules
What separates the cast irons is not chemistry so much as the shape the graphite takes as the iron solidifies, and that shape dictates the mechanical behavior. In gray iron, carbon precipitates as interconnected graphite flakes. Those flakes are why gray iron is so good at damping vibration (they absorb and dissipate energy) and so machinable (they break the chip and lubricate the cut), but they are also internal stress raisers, so gray iron is strong in compression and stiff but weak and brittle in tension, with essentially no ductility. It fractures rather than yields.
Ductile iron (also called nodular or spheroidal-graphite iron) is gray iron's chemistry treated with a trace of magnesium that forces the graphite to form spheres instead of flakes. Spherical nodules are far less effective as stress raisers, so ductile iron retains good castability and machinability while gaining real tensile strength (60 to 120 ksi) and ductility (up to 18 percent elongation in ferritic grades). This single metallurgical trick, discovered in the late 1940s, is why ductile iron replaced steel and gray iron in countless safety-critical parts like crankshafts, suspension components, and pressure pipe.
A48 Class 40 is a specific gray iron designation, ASTM A48 grades the iron by minimum tensile strength in ksi, so Class 40 means 40 ksi minimum tensile, a strong, dense gray iron used for machine bases, engine blocks, and heavy castings where stiffness and damping matter. The buyer's first decision is graphite morphology: if the part is loaded in tension, takes shock, or must not crack, specify ductile iron; if it needs damping, stiffness, and economy and sees mainly compression, gray iron (and a class like A48 Class 40) is the proven, cheaper choice.
2
Why iron is the cheapest, soundest thing a foundry can pour
Cast iron's foundry behavior is the envy of every other casting metal. Its near-eutectic composition gives it a low melting point (around 1,150 to 1,200 C, well below steel) and outstanding fluidity, so it fills thin sections and fine detail beautifully. Most importantly, as graphite precipitates during solidification it expands, and that expansion largely offsets the liquid-to-solid contraction that plagues steel. The result is near-zero net shrinkage, which means gray iron needs minimal risering, the opposite of steel's heavy riser systems. Less risering means higher yield (more of the poured metal ends up as part), less cleaning, and lower cost.
This is why cast iron is the cheapest cast metal per pound of finished part, often $1.50 to $3.50 per pound, and why it dominates high-volume applications where cost matters: automotive blocks and heads (still cast iron in many heavy-duty and diesel engines), brake rotors and drums, pipe and fittings, machine tool bases, manifolds, and countless industrial castings. Green sand casting is the dominant route, automated high-pressure molding lines pour millions of iron castings a year at very low unit cost.
The machinability bonus compounds the economics. Gray iron rates among the most machinable of all metals (often 100 percent or better relative to free-machining steel) because the graphite flakes break chips and act as a built-in lubricant, so machining cast iron is fast, with good tool life and excellent surface finish. Ductile iron machines nearly as well. For a buyer, cast iron offers a rare combination: cheap metal, sound castings with minimal defects, and easy machining, which is why it remains the highest-tonnage casting material in the world.
3
Choosing gray vs. ductile, and the grades that matter
The practical selection comes down to load type and the property you cannot live without. Gray iron (ASTM A48, classes 20 through 60 by tensile strength) is the choice when you want maximum vibration damping, dimensional stability, thermal conductivity, wear resistance under lubrication, and lowest cost, and the part is loaded mainly in compression or bending rather than tension. Classic gray-iron parts: machine tool bases and beds (the damping kills chatter), engine blocks, cylinder heads, brake rotors (graphite aids heat dissipation and the rotor wears well), flywheels, and counterweights. Class 40 (A48 Class 40) is a common high-strength gray iron for these heavier-duty uses.
Ductile iron (ASTM A536, grades like 60-40-18, 65-45-12, 80-55-06, 100-70-03, the numbers being tensile/yield/elongation) is the choice when the part sees tension, shock, fatigue, or pressure and cannot be allowed to crack. The ferritic grades (60-40-18) maximize ductility and impact toughness; the pearlitic grades (100-70-03) maximize strength and wear at the cost of ductility. Ductile-iron staples: crankshafts, steering knuckles and suspension parts, gears, pressure pipe and valve bodies, wind-turbine hubs and large structural castings.
There are further variants worth knowing: austempered ductile iron (ADI), heat treated to reach 130 to 230 ksi with good ductility, competes with cast and forged steel at lower weight for gears and suspension parts; compacted graphite iron (CGI), with a graphite shape between flake and nodule, splits the difference for high-strength engine blocks. The buyer's job is to specify the ASTM grade and class that meets the controlling tensile, yield, and elongation, and to confirm whether the part's loading demands ductile iron's tensile capability or whether gray iron's damping and economy are the better fit.
Frequently Asked Questions
The difference is the shape of the graphite in the iron, and it changes the mechanical behavior completely. Both are cast irons with over 2 percent carbon, but in gray iron the carbon precipitates as interconnected graphite flakes, while in ductile iron a trace magnesium treatment forces the graphite into spheres (nodules). Those flakes in gray iron act as internal stress raisers and crack paths, so gray iron is strong and stiff in compression, has excellent vibration damping and machinability, but is brittle with essentially zero ductility, it fractures rather than bends, and is weak in tension (20 to 60 ksi). The nodules in ductile iron are far less effective as stress raisers, so ductile iron keeps good castability and machinability while gaining real tensile strength (60 to 120 ksi) and ductility (up to 18 percent elongation in ferritic grades), meaning it can yield and absorb shock without cracking. Practically: choose gray iron for machine bases, engine blocks, brake rotors, and flywheels where damping, stiffness, thermal stability, and low cost matter and loading is compressive. Choose ductile iron for crankshafts, suspension knuckles, gears, pressure pipe, and any part loaded in tension, fatigue, or shock where cracking is unacceptable. Ductile iron costs a little more due to the magnesium treatment and tighter process control, but it behaves much more like steel.
Because cast iron solidifies with near-zero net shrinkage, while steel contracts substantially. The reason is graphite expansion. As cast iron freezes, carbon precipitates out as graphite, and graphite is less dense than the surrounding iron, so its formation causes the solidifying metal to expand. That expansion largely cancels the normal liquid-to-solid contraction, leaving cast iron with almost no net volume change, gray iron is nearly neutral or even slightly expanding. Steel has no such graphite precipitation; it just contracts 1.6 to 2.5 percent as it solidifies, concentrating that shrinkage in the last regions to freeze, which is why steel castings need large riser systems to feed molten metal into shrinking sections and avoid centerline porosity. Cast iron's near-zero shrinkage means it needs minimal risering, which has big practical benefits: higher casting yield (far more of the poured metal becomes finished part rather than risers that get cut off and remelted), less gating and cleaning labor, fewer shrinkage defects, and lower cost overall. This self-feeding behavior, combined with iron's low melting point and excellent fluidity, is a major reason cast iron is the cheapest and highest-tonnage casting material in the world. It is, almost literally, a metal engineered to be cast.
A48 Class 40 is a gray iron specified to ASTM A48, the standard specification for gray iron castings, and the class number is the minimum tensile strength in thousands of psi (ksi). So Class 40 means the iron must achieve at least 40 ksi (about 276 MPa) minimum tensile strength when tested on a separately cast test bar. ASTM A48 runs from Class 20 (20 ksi, a soft, highly machinable iron with maximum damping) up through Class 25, 30, 35, 40, 45, 50, 55, and 60 (60 ksi, a strong, dense, harder gray iron). Higher class numbers mean higher strength and hardness but somewhat lower machinability and damping, achieved by adjusting chemistry (lower carbon equivalent) and cooling rate to produce a finer, more pearlitic matrix with smaller graphite flakes. Class 40 is a common high-strength gray iron, strong enough for demanding machine bases, engine blocks, hydraulic components, and heavy industrial castings, while still offering gray iron's signature damping, machinability, and thermal stability. Remember the strength is measured on a standard test bar; the strength in a thick section of an actual casting will be somewhat lower because the slower cooling produces coarser graphite, which is why section thickness is part of gray-iron design. For tension or shock loading, you would move to ductile iron rather than a higher gray-iron class.
Cast iron is among the most machinable engineering metals, which is a major part of its economic appeal. Gray iron rates around 100 percent or better relative to free-machining steel because the graphite flakes break the chip into small pieces (no long stringy chips to manage) and act as a built-in solid lubricant at the cutting edge, giving long tool life, low cutting forces, and excellent surface finish. It machines dry in many operations and produces a clean, easily handled chip. Ductile iron machines nearly as well, slightly tougher because the nodular graphite and stronger matrix raise cutting forces a bit, but still very good. A few practical notes: the as-cast skin of an iron casting contains hard scale, sand inclusions, and sometimes chilled (white iron) edges that are abrasive and hard on tools, so the first roughing cut should be deep enough to get below the skin in one pass, and carbide tooling handles the skin better than HSS. Watch for hard spots (localized white iron) at thin sections, fillets, and chill locations where the iron cooled too fast and formed brittle iron carbide instead of graphite, these dull tools quickly and may need annealing or grinding. Use carbide or ceramic tooling for production, keep speeds moderate to high, and collect the fine graphite-laden dust. Overall, cast iron's machinability is a feature, not a problem, and a big reason it stays competitive for high-volume machined parts.
Use cast iron when you want low cost, excellent castability, vibration damping, thermal stability, good machinability, and wear resistance, and the loading suits the grade. Gray iron wins for machine tool bases and beds (damping kills chatter and the stiffness holds tolerance), engine blocks and heads, brake rotors and drums, flywheels, and counterweights, applications dominated by compressive and bending loads where its brittleness in tension does not matter. Ductile iron wins where you need real tensile strength, ductility, fatigue resistance, or shock tolerance at lower cost and weight than steel: crankshafts, suspension knuckles, gears, pressure pipe and valve bodies, and large wind-turbine hubs, with austempered ductile iron (ADI) reaching 130 to 230 ksi to compete directly with forged steel for gears and suspension parts. Choose steel instead when you need maximum tensile strength and toughness combined, weldability into fabricated structures, service at temperatures or in environments where iron's properties fall short, or when the part is a simple plate weldment better fabricated than cast. Steel weld-fabricates readily; cast iron is difficult and risky to weld and is generally not chosen for welded structures. The honest rule: for complex, machined, damping-or-stiffness-driven parts in moderate-to-high volume, cast iron is usually the most economical and capable choice; for high-tensile, weldable, or fabricated structures, steel is the answer. Ductile iron is the bridge that lets iron compete with steel on strength while keeping iron's cost and castability advantages.
Last updated: July 2026
Find Cast Iron Casting Suppliers
Search verified shops that handle Cast Iron casting.
No logins. No email gates. Just results.