Cast iron is an iron-carbon-silicon alloy with 2-4% carbon and 1-3% silicon, the chemistry that makes it pour at lower temperatures than steel and form graphite structures during solidification [S1][S2]. Five families dominate general fabrication: gray, white, malleable, ductile (nodular), and compacted graphite iron, each named for the shape carbon takes in the matrix [S8][S9].
Selection is governed by the dominant load mode, casting section thickness, machinability budget, and damping requirement; chemistry alone does not pick the grade, section size and cooling rate do [S2][S4]. For a working reference, see the cast iron selection guide used across the foundry and CNC-machining supply chain.
ASTM A48 Gray Iron: Compressive Strength and Vibration Damping
Gray iron's graphite flakes give it compressive strength roughly 3-5x its tensile strength, with a Class 20 grade specifying a minimum 20,000 psi tensile strength in a defined test-bar section [S2]. The flake structure also damps vibration, which is why machine tool beds, engine blocks, and pump housings are almost always gray rather than ductile [S2][S5]. Gray iron has no distinct yield point, so it fails in a brittle, non-warning mode and is unsuitable for parts that must plastically deform before rupture [S2].
For general fabrication, gray iron is specified by the two-digit ASTM A48 class (20, 25, 30, 35, 40, 45, 50, 55, 60) tied to minimum tensile strength in 1,000 psi units, with a matching test bar cross-section that mirrors the heaviest section of the finished part [S2]. Smaller sections cool faster and develop higher strength, so a Class 30 bar poured 1 inch thick will run higher than Class 30 poured 3 inches thick, which is why section size is part of the spec, not an afterthought [S2]. Gray iron machines cleanly because the graphite flakes act as a built-in chip-breaker and lubricant, extending tool life versus most carbon steels of equal hardness [S4].
Ductile (Nodular) Iron ASTM A536: When You Need Tensile and Elongation
Ductile iron, also called nodular or spheroidal graphite iron, replaces gray iron's flakes with spheroidal graphite nodules produced by adding magnesium to the melt, which lifts tensile strength and elongation into steel territory while retaining castability [S4][S8]. Common ASTM A536 grades are 60-40-18, 65-45-12, and 80-55-06, with the digits reading tensile (ksi), yield (ksi), and elongation (%) [S2]. It is the default pick for steering knuckles, differential housings, valve bodies, and any cast component that sees dynamic or impact loading, and it is the grade most heavy-equipment wear parts were switched to from gray iron from the 1960s onward [S3].
Ductile iron is more expensive per pound than gray and is less forgiving of section variation, so pour thick-to-thin transitions with generous radii, and budget for the heat-treat option (ferritic, pearlitic, or quenched-and-tempered) that the ASTM A536 grade name implies [S2][S4]. The trade-off against gray is roughly +20-40% material cost for a step-change in impact resistance, elongation, and fatigue life, which is why the pressure transmitter and industrial valve industries default to ductile iron bodies for anything rated above Class 150 or for low-temperature service.
White, Malleable, and Compacted Graphite Iron: Specialty Branches
White iron solidifies with carbon bound as iron carbide instead of graphite, which makes it extremely hard and abrasion-resistant but un-machinable with standard tooling, so it is reserved for crusher liners, slurry pump volutes, and grinding mill balls where wear life beats machinability [S2][S4]. Malleable iron starts as white iron, then is heat-treated for tens of hours to convert the carbide to temper graphite rosettes, giving a ferritic matrix with useful ductility and shock resistance at thinner sections where ductile iron would be overkill [S4][S8].
Compacted graphite iron (CGI) sits between gray and ductile, with short, thick interconnecting graphite flakes that retain much of gray's damping while delivering roughly 75% higher tensile strength and 40% higher stiffness, and it is the material of choice for large diesel engine blocks and bed plates where neither pure gray nor ductile is the right tool [S2][S8]. In a working foundry quote, expect CGI to carry a 10-30% premium over ductile and to need more controlled cooling and inoculation than gray, so do not spec it casually on a first-article job.
Selection Criteria Mapped to Load, Section, and Machining
The five families line up against four shop-floor decision criteria as follows: (1) dominant load mode, where gray wins for static compression, ductile for tension or impact, white for abrasion, malleable for thin-section shock, CGI for combined thermal-mechanical; (2) vibration damping, where gray leads, CGI is roughly half that, ductile and malleable are noticeably lower, and white is negligible [S2][S4]; (3) machinability, where gray is the easiest, ductile and malleable are close behind, CGI is markedly worse, and white is generally ground-only [S4]; (4) tensile strength envelope, where Class 20 gray starts around 20 ksi, ductile ASTM A536 60-40-18 hits 60 ksi, and heat-treated pearlitic ductile grades reach 100+ ksi [S2].
When the application is marine rather than general fabrication, grade and corrosion limits shift substantially, and the cast iron selection for marine engineering map is the better reference.
Foundry Process, Section Sensitivity, and Machining Allowance
Sand casting, in either green-sand or no-bake (chemically bonded) form, is the default process for general fabrication because it tolerates parts from ounces to several tons and accepts both gray and ductile chemistries without retooling the line [S5]. No-bake sand casting is favored where dimensional accuracy, surface finish, and structural integrity matter, since the bonded mold holds shape better through pour and solidification than green sand, and it is the route most U.S. jobbing foundries use for defense, heavy-equipment, and flow meter body castings [S5].
Two fabrication traps to plan around: section sensitivity means the same ASTM A48 Class 30 grade poured 0.5 inch and 4 inch thick will deliver different actual strengths, so freeze the test bar section and the heaviest casting section together in the spec; and machining allowance on rough castings typically runs 1.5-3 mm (0.06-0.12 in) per face for sand cast iron, more for large castings, so leave uniform stock and identify datum faces before the part hits the CNC [S2][S4]. For downstream pressure sensor and PLC integration work, hold tighter stock on mounting faces so the final skim cut lands within 0.1 mm flatness.
Limits, Failure Modes, and When Not to Pick Cast Iron
Cast iron fails in tension before it fails in compression, with gray showing essentially zero plastic deformation and ductile reaching 2-18% elongation depending on grade [S2][S4]. It also corrodes in untreated atmospheric exposure, so outdoor fabrications need paint, epoxy, or a corrosion-resistant alloy insert, and it loses strength above roughly 230-260 C in continuous service because the pearlite phase starts to break down [S1][S2]. Cast iron is not weldable in the same way steel is: gray iron can be repaired by warm pre-heat welding for non-critical fills, but ductile and white iron are routinely considered non-repairable by fusion welding because the heat-affected zone hardens and cracks, which is why many shops now cut A36 steel weldments as a substitute for legacy cast parts that are no longer available [S3][S4].
Two signals to track before the next spec revision: ASTM and ISO continue to harmonize graphite morphology descriptors under ISO 945-1, so confirm your foundry's optical micrograph report format on first article; and gray iron's damping advantage is being challenged by polymer-concrete and mineral-cast machine tool bases in precision equipment, which is worth a side-by-side review if vibration is the dominant design driver. For a related material-selection read on non-metallic alternatives in the same general-fab envelope, the rubber molded part sourcing guide covers elastomer choices that often sit alongside cast iron in pump and valve assemblies.