Cast Iron Manufacturers & Suppliers

Gray iron's graphite solidifies as interconnected flakes that act as stress concentrators under tensile loading — its tensile strength is typically 20-50 ksi, but compressive strength can reach 100 ksi or more. The graphite flakes also give gray iron its characteristic damping and machinability. Ductile iron (also called nodular or spheroidal graphite iron) uses a magnesium addition that causes graphite to solidify as discrete spheres rather than interconnected flakes. Spheres don't propagate cracks the way flakes do, giving ductile iron tensile strengths of 60-100 ksi and meaningful elongation — properties impossible in gray iron. Choose gray iron for vibration-damping structural castings (machine tool beds, engine blocks) and ductile iron for dynamically loaded components (crankshafts, axle housings, hydraulic manifolds) where impact resistance matters.

ASTM A48 grades gray iron castings by the minimum tensile strength of separately cast test bars in units of ksi — Class 20 means 20 ksi minimum tensile, Class 40 means 40 ksi minimum tensile. Strength correlates with the iron's carbon equivalent and cooling rate in the casting: higher strength classes require more controlled iron chemistry (lower carbon equivalent, more alloying) and often faster solidification rates, both of which promote finer pearlite and more refined graphite. Class 40 is commonly specified for machinery components, cylinder liners, and hydraulic valve bodies where a baseline mechanical property guarantee is required by engineering drawings or procurement specifications. Simply calling out 'gray iron' without an ASTM class leaves the casting house free to pour whatever grade is convenient, which can result in parts with lower-than-expected properties.

Cast iron machines in a fundamentally different manner from steel because its free graphite flakes act as chip breakers, producing short, brittle chips that break cleanly rather than the long stringy chips common in steel machining. This makes cast iron one of the cleaner-cutting materials in production environments, but the abrasive silica and carbide phases in the matrix accelerate flank wear on carbide inserts. Speeds and feeds for gray iron are typically 400-600 SFM with carbide inserts, and dry machining is common — coolant can cause thermal shock cracking in some gray iron grades. The scale (casting skin) on unmachined surfaces is significantly harder than the sub-surface material and should be machined through in a single aggressive first pass to get below it before taking finish cuts.