Cast iron is not a single alloy but a family of Fe-C-Si grades separated by graphite morphology, matrix structure, and solidification behaviour; the five working classifications used in 2026 specifications are grey, white, malleable, ductile (nodular), and compacted graphite iron (CGI) [S1].
Selecting among them is a function-graphite-shape-and-matrix decision, not a single-property pick: grey iron delivers vibration damping and machinability, white iron delivers wear, malleable iron bridges the gap after heat treatment, ductile iron delivers tensile strength, and CGI delivers a strength-to-damping ratio used in mid-to-large diesel and stamping tooling [S1]. The relevant international reference points for graphite morphology in 2026 are ISO 1083 for spheroidal graphite cast irons and BS ISO 16112 for compacted (vermicular) graphite cast irons [S1].
Grey, White and Malleable: The Three Foundational Cast Irons
Grey cast iron is defined by flake graphite in a pearlitic or ferritic matrix and is the most-produced grade by tonnage for machine bases, pump housings, and municipal castings [S2]. Its damping index and machinability come directly from the flake morphology, which also creates stress-raising notches that bound its tensile strength.
White cast iron forms when carbon is retained as iron carbide (Fe3C, cementite) rather than graphite during solidification, producing a hard, abrasion-resistant but brittle structure used in mill liners and cement-grinding balls [S2]. It is the metallurgical starting point for malleable iron: a long-cycle heat-treatment (typically exceeding 24 h, often multi-stage) converts the carbide to irregular graphite clusters, giving a tougher, more ductile product historically used for pipe fittings and small hardware [S2].
Ductile (Nodular) Iron: ISO 1083 and the Nodularity Gate
Ductile iron is produced by adding magnesium or cerium to the melt so that graphite precipitates as near-spheroidal nodules rather than flakes, lifting tensile strength into the 350-700 MPa range depending on matrix tempering [S1]. The classification under ISO 1083 grades the material on tensile strength, elongation, and matrix; the spheroidal-graphite reference (ISO 1083) sits alongside the qualitative "nodularity" and "nodule count" definitions in ISO 16112 part 5.1 [S1].
Below the nodularity gate, the part reverts toward CGI behaviour; above 80% with the right inoculant practice, ductile iron substitutes for cast steel in many valve, pump, and industrial valve body applications [S1].
Compacted Graphite Iron (CGI): BS ISO 16112 and the Vermicular Sweet Spot
Compacted graphite iron is described under BS ISO 16112 as a graphite structure that is "vermicular" — short, thick, interconnected flakes with rounded edges, often labelled Type III in visual classification — and is intermediate between flake and nodular morphologies [S1]. It is typically produced with magnesium additions lower than for ductile iron plus a titanium or rare-earth-containing inoculant to suppress full spheroidisation.
The payoff is a 75-85% strength rise over grey iron at the cost of only a 25-40% drop in thermal conductivity and damping, which is why CGI is the workhorse material for mid- and heavy-duty diesel engine blocks, cylinder heads, and high-tonnage stamping-press frames in 2026 [S1]. Specifying CGI means calling out both the ISO 16112 vermicular morphology target and a minimum tensile/yield ratio, since the property blend collapses if the melt drifts back towards flake graphite [S1].
Common Defects That Drive Reclassification in the Foundry
Eight iron-casting defects dominate the MAGMASOFT® iron casting-defect case-study set: shrinkage porosity, gas porosity, inclusions, sand defects, cold shuts, misruns, hot tears, and cracks [S2]. Each defect is a direct signal of either chemistry drift, cooling-rate error, or mould-rigidity shortfall, and any one of them can push a casting from its nominal grade (grey, ductile, CGI) into a borderline or rejected category [S2].
Shrink porosity and gas porosity are the two defects most often mistaken for "as-cast" iron behaviour; both are process-correctable through riser placement and melt-degassing schedules rather than re-alloying, which is the practical line between reclassification and scrap in 2026 foundry economics [S2]. Cold shuts, misruns, and hot tears tie to pour temperature and mould yield, and the MAGMA six-step methodology links each defect to a process parameter the foundry engineer can adjust without re-melting the heat [S2].
Selection Criteria: Matching Grade to Service Demands
Five decision gates separate the five grades: required tensile strength, required damping or thermal conductivity, machinability need, wear/abrasion exposure, and cost-per-kg versus lead-time. Grey iron wins on damping and machinability at the lowest cost; white iron wins on wear but is unmachinable without grinding; malleable iron is the legacy fallback for thin-section fittings. [S1]
Ductile iron wins on strength and impact, with grades under ISO 1083 covering 350-700 MPa tensile for general engineering and higher-Si grades for corrosion service; it is the default material for pressure sensor bodies, flow meter housings, and industrial valve bonnets where pressure ratings exceed Class 150. CGI wins where the load is cyclic and the section is thick (engine blocks, stamping slides); malleable and white are largely restricted to legacy replacement and wear-service niches in 2026 production [S1].
Standards, Sourcing and the 2026 Specification Stack
The minimum standards stack a 2026 cast-iron spec should call out: ISO 1083 for ductile (nodular) iron grade designation and nodularity acceptance, BS ISO 16112 for CGI classification, plus the relevant ASTM A48 / A536 / A842 / A220 cross-references for North American procurement [S1]. For corrosion or sour-service iron castings, NACE MR0175 hardness caps apply; for elevated-temperature service, the matrix stability window of the chosen grade must be checked against the operating envelope.
Foundry process simulation (the MAGMASOFT® six-step workflow is the visible benchmark) is now standard practice for predicting shrinkage, gas, and hot-tear risk before pattern release, and it is the main lever foundries use to keep defect-driven reclassification below 2% on ductile and CGI heats [S2]. For buyers evaluating cast iron procurement, the next trackable signals are the foundry's published ISO 1083 nodularity control chart, the MAGMA or equivalent process-simulation report per new pattern, and the heat-lot traceability sheet showing actual Mg and Si residuals.
For related coverage, see Vacuum Die Casting TCO: Pumps, Leak Rate and Downtime Drive 5-Year Spend.