Cast Iron

What is Cast Iron

Cast iron is a group of ferrous alloys whose carbon content, typically 2.5 to 4.0 percent, exceeds the maximum solubility of carbon in austenite, which is roughly 2.1 percent. That excess carbon cannot stay in solid solution, so during solidification it precipitates either as graphite or as the hard iron carbide cementite (Fe3C). Silicon, present at 1 to 3 percent, is the decisive lever: it is a powerful graphite-stabilizing element, and at around 3 percent silicon almost no carbon remains locked up as cementite. The combination of high carbon and graphite formation is what separates cast iron from carbon steel and gives the family its low melting point, excellent castability, and distinctive damping and wear behavior.

The defining commercial advantage of cast iron is castability. Its eutectic composition near 4.3 percent carbon equivalent melts at roughly 1,150 to 1,200 degrees Celsius, well below the 1,500 degrees Celsius needed for low-carbon steel, and it flows readily into thin, complex molds with low shrinkage. This is why cast iron has dominated complex near-net-shape parts, such as engine blocks, machine tool beds, pump and valve bodies, manhole covers, and pipe fittings, for nearly two centuries. The trade-off is that, unlike steel, most cast irons cannot be hot or cold worked: the part is poured to shape, typically into a sand casting mold, and finished by machining, not forged or rolled.

The industrial history is long. Cast iron was produced in China by the 5th century BCE, but the material entered mass industry with Abraham Darby's coke-fired blast furnace at Coalbrookdale around 1709, which made cheap cast iron the structural backbone of the Industrial Revolution, from the 1779 Iron Bridge to early machine frames. The modern era began in 1948 when researchers at the International Nickel Company patented the magnesium treatment that produces spheroidal graphite, creating ductile iron and giving designers a cast material with steel-like strength and ductility. Compacted graphite iron and austempered ductile iron followed in the late 20th century as engines and drivetrains demanded more from castings.

Cast iron remains, by tonnage, the most-cast metal in the world. Global castings output runs to roughly 100 million tonnes per year, and iron castings (gray plus ductile plus malleable) account for the large majority of that volume, far exceeding all non-ferrous castings combined. Ductile iron alone underpins most of the world's buried potable-water and sewer pressure pipe. The material is cheap, abundant, fully recyclable, and produced from scrap and pig iron in the cupola furnace or electric induction furnaces at every industrial scale.

Four engineering attributes determine whether a given cast iron suits a job: graphite shape (which sets ductility and toughness), matrix structure (ferrite versus pearlite, which sets hardness and strength), section sensitivity (how much properties vary with wall thickness), and the controlling material standard that the foundry will certify against. The rest of this guide works through each.

The Five Cast Iron Families

Engineering cast iron divides into five families distinguished by the form their carbon takes: gray iron (graphite flakes), ductile iron (graphite spheroids), malleable iron (irregular temper-carbon nodules), white iron (carbon as cementite, no graphite), and compacted graphite iron (short, blunt worm-shaped graphite). Each family has a characteristic property profile and a home territory of applications. Choosing the wrong family is the most expensive cast iron mistake, because no amount of grade tuning lets gray iron survive a tensile shock load or lets ductile iron damp vibration like gray iron. The table below summarizes the five.

Gray iron is the original and still the most-produced cast iron. Its interconnected graphite flakes are excellent at damping vibration and conducting heat, which is why machine tool bases and disc brake rotors are gray iron, but the sharp flake tips act as internal cracks, so the material has essentially zero ductility and modest tensile strength. Gray iron is the cheapest, most machinable, and most thermally conductive of the family, and it is graded by tensile strength alone under ASTM A48 and EN 1561.

Ductile iron, also called nodular or spheroidal-graphite (SG) iron, is produced by adding roughly 0.03 to 0.05 percent magnesium (sometimes with cerium) to the melt, which forces graphite to grow as isolated spheres. Because spheres do not concentrate stress, ductile iron combines castability with steel-like tensile strength and real elongation, from 18 percent in ferritic grades down to 2 percent in high-strength pearlitic grades. It has displaced steel forgings in crankshafts and gray iron in pressure pipe, and is the standard body material for many a gate valve, butterfly valve, and centrifugal pump casing, making it the default modern engineering cast iron.

Malleable iron is made by casting white iron first, then annealing it for many hours so the cementite decomposes into clusters of irregular temper-carbon. It predates ductile iron and delivers similar toughness, but the long heat treatment limits section size and raises cost, so it has been largely superseded by ductile iron except in established small parts such as pipe fittings (ASTM A47, A220) and electrical hardware. White iron contains no graphite at all; rapid cooling or alloying with chromium locks carbon as a continuous hard cementite network, giving extreme hardness and abrasion resistance but extreme brittleness. It is used only where wear is the dominant duty, such as slurry pump parts, crusher liners, and grinding media, often as high-chromium white iron.

Compacted graphite iron (CGI), also called vermicular iron, has graphite in short, thick, blunt worm shapes that interconnect less than gray flakes but more than ductile spheres. The result is a material roughly 75 percent stronger and stiffer than gray iron while retaining most of gray iron's thermal conductivity and damping. CGI is the enabling material for modern high-pressure diesel engine blocks and heads, where it lets designers thin walls and raise cylinder pressure beyond what gray iron can survive, though its tougher graphite makes it markedly harder to machine.

Microstructure and Chemistry

Cast iron properties are set by two independent structural variables: the shape of the graphite (covered in Chapter 2) and the structure of the surrounding metallic matrix. The matrix can be ferrite (soft, ductile, low strength), pearlite (a lamellar ferrite-cementite mix that is harder and stronger), or a mixture, and after special heat treatment it can be martensite or ausferrite. A useful mental model: graphite shape sets the ceiling on toughness, matrix structure sets strength and hardness underneath that ceiling. A ferritic ductile iron is tough but soft, a pearlitic ductile iron is strong but less ductile, and an austempered ductile iron is both strong and tough.

Chemistry steers both variables. Carbon (2.5 to 4.0 percent) provides the graphite and lowers the melting point. Silicon (1 to 3 percent) is the master graphitizer that decides whether carbon precipitates as graphite (gray, ductile) or stays as cementite (white); it also strengthens ferrite. Manganese and sulfur trade off: sulfur promotes flake graphite and machinability in gray iron but must be tied up by manganese as MnS, whereas ductile iron requires very low sulfur so magnesium treatment can work. Phosphorus improves fluidity but forms the brittle steadite phase, so it is held low in structural grades. Magnesium (residual ~0.04 percent) is the spheroidizer for ductile iron; chromium stabilizes carbides for white iron and wear grades. The table below maps the three principal families to typical chemistry.

Two processing levers translate chemistry into structure. Inoculation adds a late ferrosilicon-based agent to the ladle that seeds graphite nucleation, refining graphite and suppressing the brittle white-iron chill in thin sections. Section thickness sets the cooling rate: thin walls cool fast, giving fine graphite and more pearlite (higher strength and hardness), while thick walls cool slowly, coarsening graphite and raising ferrite (lower strength). This section sensitivity is unique to cast iron among common engineering metals and is why a separately cast 30 mm test bar can over-report the strength inside a heavy casting. Critical parts are therefore qualified on coupons cut from the actual casting, not only on standard bars.

Carbon equivalent, CE = %C + (%Si + %P)/3, is the foundry's single most useful number. A CE near the 4.3 percent eutectic gives the best fluidity and the lowest melting point but the lowest strength; a hypoeutectic CE around 3.8 to 4.1 percent raises strength at the cost of castability. Foundries tune CE, inoculation, and pouring temperature together to hit a target grade in a given section.

Grade Standards: ASTM, EN, ISO

Cast iron grades are specified through a handful of standards whose naming logic differs by region. American ASTM standards name gray iron by tensile class in ksi and ductile iron by a tensile-yield-elongation triplet. European EN standards and international ISO standards name both by tensile strength in MPa. Getting the controlling standard and grade right on the drawing is what makes a casting orderable and certifiable. The table below gives the principal cast iron standards.

ASTM A48 grades gray iron purely by minimum tensile strength on a separately cast bar, with no chemistry or ductility requirement. Class 20 is 138 MPa (20 ksi), Class 30 is 207 MPa, Class 40 is 276 MPa, and Class 60 is 414 MPa. Hardness rises with class, from about 110 to 140 BHN at Class 20 to 260 to 350 BHN at Class 60, while machinability and damping fall. Low classes (20 to 35) are favored for machine tool castings and brake rotors, high classes (40 to 60) for hydraulic and high-load parts.

ASTM A536 grades ductile iron by the tensile-yield-elongation triplet in ksi. The five grades and their approximate properties are tabulated below. Grade 60-40-18 is fully ferritic and usually annealed for maximum toughness; 65-45-12 and 80-55-06 are normally supplied as-cast; 100-70-03 and 120-90-02 are pearlitic or quench-and-tempered for strength and wear. The metric equivalents under EN 1563 (EN-GJS-400-18 through EN-GJS-700-2) follow the same property ladder using MPa.

Beyond these, ASTM A897 austempered ductile iron covers heat-treated grades from roughly 750 MPa (Grade 110/70/11) to 1,600 MPa, doubling the strength of as-cast ductile iron and adding about 50 percent more fatigue strength. Malleable iron under ASTM A47 (ferritic, e.g. 32510 meaning 32 ksi yield and 10 percent elongation) and A220 (pearlitic) covers small tough fittings. For buried pressure pipe, ISO 2531, EN 545, and AWWA C151 govern centrifugally cast ductile iron pipe with their own wall-thickness class systems (C-class or K-class) plus cement-mortar lining and zinc or bitumen coating requirements; these pipe standards are not interchangeable with the casting standards above.

Key Specification Parameters

A complete cast iron specification goes well beyond a single strength number. Eight parameters drive most selection and qualification decisions: tensile strength, yield strength and elongation, hardness, the physical properties (density, modulus, thermal conductivity), wear and damping behavior, section sensitivity, and the inspection requirements. Each is explained below, with representative values for the common families.

Tensile, yield, and elongation are the headline mechanical numbers and the basis of every grade designation. The critical subtlety is that gray and white iron have no meaningful yield point or elongation, so they are specified by tensile strength only, whereas ductile, malleable, ADI, and CGI carry a full strength-ductility specification. Never assume a gray iron casting can absorb a tensile overload, its useful strength is in compression, where it reaches roughly 570 to 1,290 MPa, three to four times its tensile value.

Hardness in Brinell (BHN) tracks matrix structure and is the fastest shop-floor proxy for strength and wear resistance. Typical ranges are 110 to 200 BHN for gray iron, 130 to 300 BHN for ductile iron, 140 to 350 BHN for high-class gray and pearlitic ductile, 350 to 700 BHN for white and high-chromium irons, and up to 500 BHN for hardened ADI. Specify a hardness band on wear parts and verify it on the finished surface, because heat treatment and section effects shift it.

Physical properties are where cast iron families diverge sharply from steel and from each other. The table below gives representative values; note that gray iron's high thermal conductivity and damping, and its low elastic modulus, are the reasons it is irreplaceable in brake rotors and machine frames, while ductile iron trades some of that away for strength and a higher, steel-like modulus.

Wear resistance and damping are application-defining for two families: white and high-chromium iron win on abrasion (crusher liners, slurry pumps, grinding media), while gray iron's flake graphite gives the best vibration damping of any engineering metal, which is why it underpins precision machine tool beds and large compressor frames. Section sensitivity, the variation of properties with wall thickness, must be specified for heavy or thin-walled parts; the standards key their test bars to specific reference sections (ASTM A48 uses a 30 mm bar) precisely because thick castings test below the bar and thin sections can chill to brittle white iron.

Inspection and soundness requirements complete the spec. Common callouts include per-heat tensile and hardness certification, nodularity percentage for ductile iron (typically 80 to 90 percent minimum on a metallographic section), magnetic-particle or dye-penetrant surface inspection, radiographic or ultrasonic volumetric inspection for pressure-containing parts, and pressure or leak testing for valve and pump bodies. For safety-critical castings, specify coupons cut from production parts rather than separately cast bars, because that is the only way to confirm the structure inside the actual section.

Selection Decision Factors

To turn the metallurgy of the previous chapters into a buyable specification, follow the decision sequence below. Most cast iron selection errors come not from one wrong value but from skipping a level, for example fixing on a grade before deciding the family, or naming a grade with no controlling standard. These steps work as a fixed RFQ template.

1. Loading mode first, then family: If the part sees tensile, bending, or impact loads, start with ductile iron, ADI, or malleable iron. If it sees mainly compression with a need for damping or thermal conductivity (machine bases, brake rotors), gray iron is right. If abrasion dominates and toughness is irrelevant, white or high-chromium iron wins. Family is the highest-leverage decision and cannot be fixed later by grade.
2. Controlling standard and grade: Name one governing standard (ASTM A48, A536, A897, EN 1561, EN 1563, or a pipe standard) and one grade within it. Never write "cast iron" or a bare class without its standard, because Class 40 means different things to different foundries and codes.
3. Section thickness and weight: Declare the controlling wall thickness and overall casting weight. Heavy sections cool slowly and lose strength, thin sections risk chill; both change the achievable grade. Ask the foundry to confirm the grade is attainable in your governing section, not only on a 30 mm bar.
4. Machinability and post-processing: Gray iron and ferritic ductile iron machine easily; CGI, pearlitic ductile, ADI, and white iron machine slowly and wear tooling. If the part needs extensive machining, factor that into both grade choice and cost.
5. Heat treatment and coatings: Decide whether the casting is as-cast, annealed (ferritic ductile, malleable), normalized, or austempered (ADI). For corrosive or buried service (pipe, valves), specify lining and coating: cement-mortar lining, zinc plus bitumen, epoxy, or polyethylene sleeve per EN 545 / AWWA C151.
6. Inspection level: Match non-destructive testing to consequence of failure. Decorative or low-stress parts need only dimensional and visual checks; pressure-containing or safety-critical parts need per-heat tensile, nodularity, MT/PT, and radiographic or ultrasonic inspection, plus production-coupon qualification.
7. Standards and certification: For pressure pipe and water contact, require the relevant ISO 2531, EN 545, EN 598, or AWWA C151 certification plus potable-water approval (NSF/ANSI 61 where applicable). For automotive and structural castings, require IATF 16949 or ISO 9001 foundry accreditation and a material test report per heat.
8. Total cost of ownership: Compare casting price plus machining plus inspection plus field life. A cheaper gray iron part that cracks under an unexpected tensile load, or a ductile iron that needed ADI for fatigue life, costs far more in downtime than the grade upgrade would have. Carbon equivalent and inoculation control, invisible on the drawing, drive scrap rate and consistency, so foundry capability is part of the cost equation.

One frequently overlooked dimension is foundry serviceability and qualification. Cast iron is a foundry product, not a branded mill stock, so the supplier's metallurgy is the product. Confirm in-house spectrometer and metallographic capability, per-heat certification, proven section-sensitive process control, and a track record on parts of similar weight and grade. For ductile iron pressure pipe, established producers such as Saint-Gobain PAM, US Pipe, American Cast Iron Pipe Company, McWane, and Electrosteel certify to ISO 2531, EN 545, or AWWA C151. For engineered castings, qualify the foundry on accreditation and demonstrated capability rather than on a catalog name, and require coupons cut from production castings for any safety-critical part.