Glossary

Cast iron is a group of iron-carbon alloys containing more than 2% carbon, along with varying amounts of silicon, manganese, and trace impurities such as sulfur and phosphorus. It is produced by melting pig iron (a crude form of iron extracted from a blast furnace) and then casting the molten metal into molds, rather than shaping it through forging or rolling. The high carbon content gives cast iron its characteristic hardness, wear resistance, and excellent fluidity when molten, but it also makes the material brittle compared to steel.

The structure of cast iron depends on how carbon solidifies in the alloy. Carbon can appear either as graphite flakes, nodules, or iron carbides (cementite), and these different forms define the main types of cast iron:

- Gray cast iron: The most common type, in which carbon forms as graphite flakes. These flakes interrupt the metal matrix, giving gray iron its characteristic dull gray fracture surface. It has excellent machinability, vibration damping, and compressive strength, making it ideal for engine blocks, machinery bases, and pipe fittings.

- Ductile (nodular) cast iron: Also called spheroidal graphite iron, this type is treated with magnesium or cerium to make the graphite form into spherical nodules instead of flakes. This structure significantly improves tensile strength and ductility, allowing it to behave more like steel while retaining cast iron’s wear resistance.

- White cast iron: Here, carbon remains combined as iron carbide (Fe₃C), giving the metal a white fracture surface. It is extremely hard and wear-resistant but very brittle, often used for crusher liners, grinding balls, and abrasion-resistant surfaces.

- Malleable cast iron: Produced by heat-treating white cast iron, which decomposes the carbides into small graphite particles. The result is a more ductile and tough material suitable for pipe fittings, brackets, and automotive parts.

Cast iron has a lower melting point (around 1150–1200°C) than pure iron, which makes it easy to pour into intricate molds—a major reason it’s favored for complex or detailed castings. However, its brittleness means it cannot absorb much tensile stress or bending without fracturing.

Because of its combination of strength, castability, vibration damping, and heat retention, cast iron remains one of the most widely used materials in engineering. It’s found in everything from machine tools and cookware to engine components, heavy-duty pipes, and architectural structures—a testament to its versatility despite being one of the oldest known engineering materials.

A cast iron fastener is a piece of hardware whose main body is made from cast iron—most commonly gray iron, ductile (nodular) iron, or malleable iron—formed by pouring molten iron into a mold and then machining features such as threads. Unlike the vast majority of modern bolts and screws, which are wrought (rolled) steel, cast iron fasteners rely on the cast microstructure of iron. This makes them suitable for parts where the geometry is complex and casting is economical (clamps, hangers, brackets, decorative hardware, certain rail clips, and some thumb/wing-style nuts), but it also means they are rarely used for high-tension, safety-critical bolting.

Performance depends on the iron type. Gray iron (ASTM A48) contains flake graphite that gives excellent compressive strength, vibration damping, and machinability, but it is brittle in tension and notch-sensitive, so it is generally poor for highly stressed threaded fasteners. Ductile iron (ASTM A536; grades such as 60-40-18, 65-45-12, 80-55-06) has spheroidal graphite that markedly improves tensile strength and elongation; it can handle moderate service loads and is common for cast components that include tapped holes or integral studs, though it still does not match the fatigue and impact resistance of quenched-and-tempered alloy steel bolts. Malleable iron (ASTM A197/A197M) is heat-treated white iron that attains reasonable ductility and has long been used in pipe fittings, beam and conduit clamps, and other hardware with threads, but it is likewise intended for relatively low to moderate stresses.

Manufacturing and design constraints follow from the material. Because cast iron cannot be cold-formed, the part’s shape is created in the mold and then finished by machining. Threads in cast iron are cut or chased, not roll-formed, and coarse pitches with generous root radii are preferred to reduce stress concentration. Good practice is to keep the iron component primarily in compression or shear, avoid shock and cyclic tension, provide generous fillets and section transitions, and size bosses and wall thicknesses to support threads. Where higher preload is required, the better solution is usually to cast the body from iron (for shape and cost) and use separate steel fasteners for the threaded elements, or to install steel inserts (helicoils/solid inserts) in tapped cast iron to increase thread strength and wear resistance.

In application, cast iron fasteners and fastener-like hardware appear in building hardware, pipe and conduit supports, rail and heavy-equipment castings, vintage machinery, and architectural pieces where casting enables complex geometry or an “as-cast” aesthetic. They are commonly zinc-plated, galvanized, painted, or powder-coated to mitigate corrosion, which is broadly similar to carbon steel. They are not appropriate substitutes for standardized structural or pressure-boundary bolting; for those uses, wrought steel fasteners to standards such as ASTM/SAE/ISO bolting specifications are required, with the cast iron component serving only as the clamped part or housing.

It is also worth distinguishing “cast iron fastener” from “fastening into cast iron.” When you are joining to a cast iron substrate (engine blocks, housings, machinery bases), the recommended practice is to use steel screws/bolts with adequate washer area to spread load, apply appropriate torque (often lower than for steel-on-steel to avoid cracking), use anti-seize to protect threads, and consider thread inserts for repeated service or higher loads. In short, cast iron can be used to make certain fastener bodies and fastener-like components when casting advantages outweigh mechanical limits, but for high-preload, fatigue- or impact-critical joints, the threaded fastener itself should be steel while the cast iron remains the part being clamped.