Cast iron selection is driven by four concrete gates: grade family (gray ASTM A48 class 20–60, ductile ASTM A536 60-40-18 through 120-90-02, white, malleable, compacted graphite), minimum tensile/yield/elongation band, casting-process compatibility (sand, shell, investment, centrifugal, continuous), and final machining tolerance, with FOB-China grate unit pricing reported in the US$0.10–10.00 band at 100-piece MOQ on 2026-05-15 [S1].
Buyers who skip the grade-vs-process match typically pay twice — once in the foundry PO and once in-field on cracked housings, leaked seats or warped grates. The 2026 sourcing environment is dominated by Chinese OEM/ODM shops offering both raw casting and finished cookware or grate lines, with audits (ISO 9001:2015, LFGB, FDA, SGS, Prop-65) carried as the default paper trail rather than the exception [S1][S2].
Grade Family and the Mechanical Band That Drives Everything Else
Gray iron ASTM A48 is the default for vibration-damping housings, pump bodies, valve bodies and brake components; classes 20 (≈138 MPa tensile) through 60 (≈414 MPa tensile) bracket roughly 90% of industrial and architectural casting work, with class 30 being the most common baseline for general machinery [S1]. Ductile (nodular) iron ASTM A536 60-40-18 (≈414 MPa tensile, 18% elongation) through 120-90-02 (≈828 MPa tensile, 2% elongation) replaces forged steel when impact and fatigue resistance matter, and is the correct pick for pressure-sensitive valve bodies and pump casings in duty cycles above 20 bar.
White iron and high-chrome white iron (ASTM A532) sit on the wear side of the spectrum and are specified for slurry-handling liners, grinding balls and chute liners where abrasive resistance dominates over ductility. Malleable iron ASTM A47 (ferritic 32510 ≈345 MPa tensile, 10% elongation) bridges gray and ductile where thin sections and moderate ductility are required. Compacted graphite iron (CGI) ASTM A842 sits between gray and ductile, with ≈450 MPa tensile and roughly 1.5–4% elongation; it is the OEM pick for heavy-duty diesel engine blocks and high-specific-power industrial valve bodies.
Cookware-grade cast iron is a separate branch: it is typically non-alloyed gray iron poured into sand molds and seasoned by the user, with OEM/ODM suppliers like Geovein exporting frying pans, skillets, woks, Dutch ovens and casseroles under LFGB, FDA, SGS and Prop-65 documentation per their published 2026 capability statement [S2]. Selecting a cookware supplier against an industrial casting supplier is a category mistake — the spec gates (food-contact migration testing vs. ASTM A48 class) do not overlap.
Casting Process vs. Geometry, Tolerance and Order Quantity
Sand casting (green sand, resin sand) is the lowest-cost route for runs of 100 pieces and up, and is the process behind the US$0.10–10.00 cast iron grate price band on Made-in-China (2026-05-15), with typical as-cast tolerances of ±1.5 mm on small grates and ±3.0 mm on parts above 500 mm [S1]. Shell molding tightens that to ±0.3–0.8 mm on small parts and is the go-to for batch runs between 500 and 20,000 pieces where surface finish (Ra 3.2–6.3 µm) matters.
Investment (lost-wax) cast iron is reserved for thin-wall or complex-geometry parts where draft angles and parting lines would otherwise force post-machining costs to dominate; typical tolerance window ±0.1–0.3 mm. Centrifugal casting is the correct pick for cylindrical wear parts (pipes, sleeves, rolls) and gives a fine-grained working surface with minimal inclusions. Continuous casting is reserved for bar and section stock feeding machine shops. Die casting of true cast iron is uncommon — the metal freezes against the die wall and cracks the die; the products labelled "cast iron" on Made-in-China under the "Aluminum Cast Iron" cross-reference are typically aluminum-alloy components with cast-iron tooling, not iron castings [S5].
For any quote, the process decision should be locked before grade is finalised: a class 40 gray iron sand casting at 1,000 pieces will run in a different foundry than a class 60 ductile iron shell-molded part at 5,000 pieces, and the tooling amortisation curve (typical USD 3,000–15,000 for sand patterns, USD 15,000–80,000 for shell or investment tooling) is what sets the break-even MOQ, not the piece price.
Spec Gates: Tensile, Hardness, Ductility and Machinability
Buyer-side spec sheets should pin at least three numbers: minimum tensile strength (MPa or psi), minimum elongation (%) for ductile/CGI/malleable, and Brinell hardness range. ASTM A48 class 30 = 30,000 psi (≈207 MPa) tensile with no specified elongation; ASTM A536 65-45-12 = 65,000 psi (≈448 MPa) tensile with 12% elongation minimum; ASTM A842 grade 400 = 400 MPa tensile with 1.5% elongation minimum. Hardness for as-cast gray iron typically falls in the 170–260 HB band; for ductile 60-40-18 in the 140–190 HB band; for high-chrome white iron above 500 HB [S1].
Machinability drops sharply with hardness: a gray iron at 180 HB can be cut at 80–120 m/min with coated carbide; a ductile 120-90-02 at 240 HB should drop to 40–60 m/min, and high-chrome white iron above 500 HB typically requires cubic boron nitride or grinding. If a spec calls for both tight as-cast tolerance (±0.2 mm) and high hardness, plan for grinding as a secondary operation — do not push the foundry to hold the as-cast number, they will not, and the inspection clause will fail the lot.
For cookware and grate imports into the EU/US, the parallel set of spec gates is food-contact or product-safety compliance (LFGB, FDA, Prop-65, SGS migration test reports) rather than mechanical properties, and these should be attached to the PO as conditional release documents, not requested after the container has sailed [S2].
Who Cast Iron Is For — and Where It Is the Wrong Material
Cast iron is the correct pick for: pump and flow-meter housings where vibration damping matters; valve bodies in low-to-medium pressure water, steam and chemical service; gear blanks and flywheels; brake drums and discs; grate and frame assemblies; cookware; and architectural castings. It is the wrong pick for: thin-wall structural sections below 3 mm, applications with impact loads above 100 J at -20 °C (where austempered ductile or steel dominates), and any service where the casting weight-to-strength ratio drives logistics cost above the material saving. [S1]
Cast iron is also a poor pick where field welding is the primary repair method — although austempered ductile and certain grades are weldable, the standard gray iron family is not field-repairable without pre-heat and controlled cooling, and a failed weld is more dangerous than the original crack. Specify pressure sensor and instrument bracket castings in ductile or steel if the field service plan is "weld it back".
For PLC cabinet bases, motor feet and small instrument housings, sheet steel is often cheaper than cast iron at MOQs above 5,000 pieces once the stamping and welding line is amortised — run the cost comparison before locking the casting supplier.
Comparison Table: Cast Iron Families at the Four Decision Gates
Gray A48 vs. Ductile A536 vs. CGI A842 vs. High-Chrome White A532 on the four decision axes: [S2]
Tensile/elongation band: Gray 138–414 MPa / <1% — Ductile 414–828 MPa / 2–18% — CGI ≈400–550 MPa / 1.5–4% — White/A532 ≈250–650 MPa / <1% with hardness 450–650 HB.
Typical service: Gray for housings, pump bodies, grates — Ductile for valve bodies, pressure-bearing casings — CGI for engine blocks and high-power cylinder heads — White for slurry, chute, mill liner service.
Casting process: Gray = sand/shell — Ductile = sand/shell with Mg treatment — CGI = controlled cooling sand — White = sand with alloying and chill control.
Sourcing price band: 2026-05-15 FOB-China grate FOB US$0.10–10.00/pc at 100 pc MOQ [S1]; structural gray A48 class 30 housings typically sit in the US$1.50–4.00/kg band at 1-tonne MOQ; ductile A536 60-40-18 valve bodies typically US$2.20–5.50/kg at 1-tonne MOQ.
Standards, Documentation and the Audit Trail Buyers Should Demand
Buyer-side paperwork should include the ASTM/SAE/EN standard code referenced on the drawing (A48, A536, A842, A532, A47, EN-GJL, EN-GJS), the heat-lot-traced mill cert with chemistry (C, Si, Mn, S, P plus Mg for ductile, Cr/Mo for white), and the mechanical test report (tensile, elongation, hardness).
For Chinese OEM/ODM suppliers, the minimum audit baseline is ISO 9001:2015 (general quality system) plus product-specific marks — LFGB and FDA for cookware, SGS migration test reports, Prop-65 for California-bound shipments, and CE/PED where pressure-bearing. Shandong and Zhejiang province suppliers dominate the cast iron grate listings on Made-in-China, with Diamond/Audited Member status on the platform acting as a first-pass filter rather than a guarantee [S1].
For cookware specifically, Geovein publishes LFGB/FDA/SGS/Prop-65 coverage and a one-stop OEM/ODM service covering frying pans, skillets, woks, Dutch ovens and casseroles, with a stated dedicated focus on cast iron cookware manufacturing per their 2026 company page [S2]. This is the documentation profile to demand from any cookware supplier; it is also worth noting that household care instructions (seasoning oil, salt scrub, low-heat drying per the Today.com 2024-03-14 cast-iron maintenance guide, and the wikiHow burnt-pan recovery method published 2025-03-25) are downstream of the casting spec and do not affect the inbound material decision.
Failure Modes and the Field Signals That Travel Back to the Spec
The most common field failure on cast iron housings is cracking along the parting line, caused by either an under-strength grade (class 20 specified where class 30 was needed) or a riser placement that left a shrinkage void in a stress-concentration zone. Second most common is brinelling/flattening of machined wear surfaces, a sign the hardness band was set too low for the duty cycle. Third is corrosion-driven leaktightness loss on pressure transmitter process connections — typically a material selection miss (gray iron in a chloride-bearing wet service) rather than a foundry error. [S3]
For cast iron grates, thermal-shock fracture from rapid cool-down (water-quench on a hot grate) is the dominant field failure and is not a material defect — it is a use-case mismatch. The grate spec should state either "preheat to 200 °C before water contact" or specify a ductile iron grade (e.g. A536 60-40-18) if thermal cycling is expected.
For cookware, the failure signature is seasoning loss and rust pitting, both of which are owner-maintenance issues rather than casting defects, and the standard remediation (scrub, re-oil, oven-cure) is well documented [S3][S4]. A cookware casting with surface porosity that holds oil and stains is a casting defect and should be rejected at incoming inspection under a visible-porosity sampling plan (ASTM E505 level 2 or equivalent).
Trackable Signals for the Next 6 Months
Two signals are worth watching into late 2026: (1) the spread between FOB China grate pricing and FOB China ductile iron valve-body pricing — if the grate band holds at US$0.10–10.00/pc while ductile body quotes climb above US$6.00/kg, expect capacity shift away from ductile as the more profitable line absorbs foundry hours; (2) the publication of additional ASTM A842 CGI grade updates as engine-block electrification reshapes the demand mix, which would push some foundry capacity from CGI back to gray. [S4]
For buyers in adjacent categories, the Carbon Steel Suppliers 2026 OEM map and the Tool & Die Steel sizing piece cover the ferrous-metals comparison set that should sit next to a cast iron sourcing review, and the PVC-U Pipe selection guide covers the non-metallic alternative for low-pressure fluid service where cast iron is now being displaced.