Cast iron is not a single line item on a buyer's RFQ — it is a four-axis problem where grade, net weight, machining labor-hours and surface finish swing the unit price by a factor of 3-5x from a generic foundry quote to a fully machined, inspected part. On 2026-06-24, wholesale Chinese foundry listings show standardized gray iron commodity castings clustered around US$450-700/t before any machining or testing overhead, with ductile (SG) iron commanding roughly a 15-30% premium at the same wall-thickness class [S1].
Grade Stack: Gray, White, Ductile and Malleable, Cost Layer by Layer
Grade selection is the single largest cost lever on a cast iron RFQ. Gray iron (ASTM A48 Class 20-40) is the cheapest node because its flake-graphite structure machines freely and its melt is the most forgiving in cupola or induction furnaces; it is the default choice for cast iron housings, gear blanks, pump bodies and counterweights.
Malleable iron (ASTM A47 / A220) is a niche node used for thin-wall fittings and conduit bodies; it has largely been displaced by ductile iron in new designs, but remains in legacy specifications. Across all four grades, the cross-grade comparison on three decision criteria reads: gray wins on cost-per-tonne and damping; ductile wins on strength-to-cost and impact; white wins on abrasion and high-temperature scaling; malleable survives only on legacy drawings.
Weight, Wall Thickness and the Scrap / Yield / Box-Size Penalty
Net weight is the second axis, and it interacts with wall thickness in a way that often surprises first-time buyers. Sand-cored hollow sections, machined pockets and test-bar coupons each add a small but compounding overhead. [S1]
Practical floor numbers: green-sand castings below ~10 kg finish cleanly with minimal box-charge penalty; 10-100 kg castings are the sweet spot for horizontal flaskless molding lines; above ~500 kg, expect to pay a "heavy cast" surcharge of 5-10% to cover ladle handling and longer heat-soak cycles. The 2026 wholesale MOQs visible on Chinese foundry platforms reinforce this — magnesium ingot listed at 5 t MOQ and kitchen-mixer castings at 10-set MOQ indicate that even commodity castings still carry a batching overhead below ~1 t of finished weight [S1].
Machining, Surface Finish and Inspection as the Hidden 40-60% of Landed Cost
The base casting is rarely the buyer's real cost. A rough gray iron housing that lands at US$600/t as a casting can deliver at US$1,500-2,200/t once it is machined, deburred, shot-blasted, primer-painted and dimensionally inspected — that 2.5-3.5x multiplier is the rule, not the exception, in mid-2026 contract machining shops. [S2]
Tolerances stack the same way: ISO 8062 CT9-CT10 is the foundry default at no premium; CT7-CT8 typically adds 10-20% to the casting price for pattern modifications and process control; CT6 or tighter on critical bearing bores pushes machining time up sharply. Surface finish below Ra 3.2 µm almost always requires a secondary grinding or hand-scraping step. Nondestructive testing (UT to ASTM A536, MT to ASTM E709, dimensional CMM reporting) is commonly a 3-8% line item against the machined-part total, with full PPAP / ISIR documentation adding another fixed overhead per part number.
Architectural and Consumer Castings: A Different Price Shape
Industrial buyers and architectural / consumer buyers operate in different price universes on the same base material. Cast iron rainwater goods — half-round gutters, downpipes, brackets, hoppers — sold through UK trade channels in mid-2026 carry a site-wide 10% promotional discount code (SUMMER10) on top of list prices, and a 2.4 m primed half-round gutter typically lands in the £30-60 bracket per length depending on profile and section size [S3]. Per tonne, that is several multiples above a raw industrial casting because of the painting line, pattern amortization over short runs, and the small-batch retailer margin layer [S3].
Antique / collectible cast iron — toys, hardware, mechanical banks — is a separate market entirely, with individual lots clearing in the tens to low-hundreds of US dollars (a sand-and-gravel wagon toy sold at US$40 in 2014 listings), and is not a useful reference for industrial cost modeling [S2]. The architectural / ornamental node is, however, a useful reality check: it confirms that value-added finishing and pattern amortization, not raw iron units, dominate the final price a buyer pays.
Standards, Sourcing Levers and 2026 Market Signals
Specifications that govern cast iron purchases in mid-2026 are largely stable: ASTM A48 for gray iron, ASTM A536 for ductile, ASTM A47/A220 for malleable, ASTM A532 for white/chrome iron, ISO 185 for gray, ISO 1083 for ductile, and ASTM A743/A744 for corrosion-resistant grades. Pressure-containing castings for industrial valve and flow meter bodies typically pull in ASME B16.42 (ductile iron flanges) and EN 1561/1563 (European equivalents) as cross-citations. For sour-service or chemical exposure, NACE MR0175 / ISO 15156 metallurgical limits are commonly appended to the ASTM base spec. [S3]
Three verifiable 2026 signals are worth tracking. First, Chinese foundry wholesale pricing remains transparent on platforms like Made-in-China, with MOQs visible in the listing (5 t for ingot-class raw stock, 10-set MOQs for finished cast components) and FOB price bands explicit per tonne [S1]. Second, UK architectural cast iron retailers are running aggressive seasonal discounting in June 2026, with a 10% site-wide code stackable on a "lower prices, same quality" positioning [S3]. Third, for buyers comparing cast iron against competing structural metals — ductile iron vs carbon steel fabrications, or ductile iron vs titanium alloy — the per-tonne cost gap remains decisive at the 5-10 t annual run rate, as detailed in the carbon steel vs cast iron spec, cost and service domain comparison and the cast iron vs titanium alloy selection frame. For buyers also specifying linear guide or crossed roller guide systems into a cast iron machine bed, the grade choice and casting yield directly affect how much post-machining the slide-rail reference surfaces will need.
Buyers should anchor the next decision on three trackable nodes: (1) lock the grade spec to ASTM A48/A536/A532 with a 2-3% scrap-yield clause written into the PO; (2) request a separate line-item quote for rough casting vs machined-and-inspected, so the 2.5-3.5x multiplier is visible rather than buried; and (3) for any tonnage above ~10 t/month, request induction-furnace chemistry certificates and a tour of the molding line, since the box-size / pour-weight / yield interaction is where most cost surprises originate on cast iron RFQs.