Cast Iron
CNC Machining

Cast iron is a family of iron-carbon alloys with carbon content greater than 2% (typically 2.5–4.5%), which causes the excess carbon to precipitate as graphite or iron carbide during solidification. This microstructure gives cast iron its defining characteristics: excellent compressive strength, outstanding vibration damping, inherent lubricity from graphite, and very good machinability despite its hardness. Cast iron is widely used for machine tool bases, engine blocks, pump housings, brake components, and heavy-duty structural parts where rigidity, wear resistance, and dimensional stability are required.

Gray cast iron is the most widely used cast iron grade, characterized by graphite precipitating as flake graphite within a pearlitic or ferritic iron matrix. Common grades include ASTM A48 Class 20–40, ISO EN-GJL-150 to EN-GJL-300, and GB HT150–HT350. Gray cast iron offers outstanding machinability, excellent vibration damping, good compressive strength, and low cost. It is used for engine blocks, cylinder heads, machine tool bases, brake drums, pump bodies, and gearbox housings. Its tensile strength is moderate (150–350 MPa) but compressive strength is 3–5 times higher.

Ductile cast iron is produced by adding magnesium to the melt, which causes graphite to solidify as spherical nodules rather than flakes. This dramatically improves tensile strength (400–900 MPa), elongation (up to 18%), and impact resistance compared to gray cast iron — giving properties approaching those of steel while retaining excellent machinability. Common grades include ASTM A536 Grade 65-45-12, 80-55-06, and EN-GJS-400-18 to EN-GJS-700-2. Ductile iron is used for crankshafts, gears, differential housings, hydraulic valve bodies, and structural components requiring both the machinability of cast iron and higher mechanical strength.

Alloy cast iron incorporates additions of chromium, nickel, molybdenum, or copper to produce properties beyond those of standard gray or ductile iron. High-chromium white iron (15–30% Cr) achieves HRC 55–65 for extreme abrasion resistance. Ni-Hard provides excellent wear resistance with improved toughness. Austenitic Ni-Resist iron offers corrosion resistance and dimensional stability at elevated temperatures up to 800°C. Alloy cast irons are specified when standard cast irons fail due to abrasive wear, elevated temperature, or corrosive service — they are significantly harder to machine and require CBN tooling.

Cast iron machining presents several specific challenges: (1) Hard skin — the as-cast surface contains scale, sand inclusions, and a hard chilled layer that rapidly destroys tooling if the first pass does not cut below it. (2) Abrasive graphite — graphite flakes accelerate flank wear on carbide tools. (3) Dry or near-dry machining — cast iron is often machined dry or with MQL as flood coolant can cause thermal shock. (4) Porosity — internal voids can cause dimensional surprises when machining thin walls. (5) Alloy iron hardness variation — high-chromium white iron can vary significantly in hardness across a single casting.

Standard CNC machining tolerance for gray and ductile cast iron is ±0.02 mm for turned features and ±0.05 mm for milled features. For precision bearing bores and sealing faces, we hold ±0.010–0.015 mm on gray and ductile iron. Alloy cast iron typically achieves ±0.02–0.03 mm on machined features due to hardness variability. Note that castings themselves have inherent dimensional variation — datum selection and fixturing strategy must account for casting variability before machining begins. CMM reports are provided for all precision cast iron components.

Cast iron has limited inherent corrosion resistance and the graphite-iron matrix corrodes in humid or wet environments. Surface treatment depends on application: machine tool bases and internal components are often supplied with rust-preventive oil only. Parts in moderate environments receive black oxide, phosphate coating, or zinc plating. Pump bodies and valve housings in wet service receive epoxy paint or powder coating. Note that gray cast iron should not be electroplated with acid processes without proper pre-treatment, as the graphite flake network can trap hydrogen and cause blistering.

Cast iron welding is possible but requires specialized procedures. Gray cast iron must be preheated to 300–600°C before welding to prevent heat-affected zone cracking. Nickel-based or cast iron welding rods are used. Ductile iron is more weldable than gray iron but still requires preheat. High-chromium alloy irons are generally considered unweldable. For heat treatment: gray cast iron can be stress-relieved (550–600°C) to reduce residual casting stresses before precision machining. Ductile iron can be annealed to soften, or normalized and quenched and tempered to increase hardness.