Iron Foundries: Types, Properties, and Applications
Iron Foundries: Composition and Properties
Foundries work with ferrous alloys, characterized by a carbon content greater than 2.06% and approximately 3% silicon. Key alloying elements include:
- Graphitizing Agents (Silicon): Promote the formation of graphite within the cast iron matrix.
- Magnesium (Mg): Added alongside silicon, magnesium encourages the formation of spheroidal graphite particles.
- Anti-graphitizing/Carbide-Forming Agents (Titanium, Chromium, Vanadium): These elements promote the formation of carbides instead of graphite.
Foundries produce materials with higher mechanical strength and hardness, but lower toughness and ductility. These are often not directly machinable.
White Cast Iron
White cast iron has no direct industrial application but serves as a base material for other castings. Its composition includes cementite (Fe3C), formed according to the metastable Fe-C diagram. It features low carbon content and is produced through moderately slow cooling. White cast iron is extremely hard due to the excess cementite, but also very brittle, with brittleness increasing with carbon content. Alloyed with nickel (Ni-Hard), it exhibits increased abrasion resistance, surpassing even gray cast iron in this regard. After solidification, it undergoes thermal treatment to modify its structure. Ledeburite is a key constituent, initially composed of austenite and cementite, transforming into pearlite and cementite.
Gray Cast Iron
Gray cast iron contains carbon in the free state as flake graphite. It’s the least expensive to produce, making it more economical than white cast iron. It exhibits good castability due to its near-eutectic composition. Shrinkage during solidification is minimal, and its fragility depends on the size of the graphite flakes (larger flakes lead to increased fragility). Alloying with nickel and chromium enhances resistance to sulfuric and nitric acids, as well as high-temperature corrosion. Mechanical strength depends on the amount, size, and distribution of graphite. It has low tensile elongation due to the notch effect of graphite flakes. Graphite’s softness compared to the matrix results in different behavior under stress; higher graphite content leads to lower strength. It shows good compressive strength because the notch effect is less significant. Corrosion resistance is better than steel, and elongation at break is practically zero. It has excellent vibration absorption capacity and better thermal conductivity than steel. Toughness is superior to white cast iron but inferior to steel. It also exhibits good wear resistance.
Always present:
- Silicon (0.6-3%, stabilizer): Lower carbon content requires more silicon to ensure flake graphite formation.
- Phosphorus (<0.17%, castability): Aids in graphite formation and improves castability, but reduces toughness and increases fragility due to the formation of a cementite-containing compound.
Graphite’s crystal structure is hexagonal. The carbon equivalent (Ce) is calculated as: Ce = Ct + 1/3 (Si + P).
Malleable and Nodular Cast Iron
Malleable and nodular cast irons contain carbon in the form of graphite nodules. They are produced by annealing white cast iron, resulting in good toughness, malleability, low brittleness, and greater deformation capacity. These are used for parts that cannot be obtained through other methods. The treatment is lengthy and expensive. Starting with a white cast iron piece, annealing is performed to create either European or American malleable cast iron.
European Malleable Cast Iron
Starting from white cast iron, it’s heated to 900-1050°C in an oxidizing atmosphere furnace. This process can take about a day. At this temperature, the alloy consists of austenite and cementite. The goal is to separate carbon from the cementite. Slow cooling (5-10°C per hour, lasting about 2 days) decarbonizes the piece. This method can be applied to thick pieces.
American Malleable Cast Iron
Starting from white cast iron, a controlled atmosphere furnace prevents carbon loss. Slow heating to 800-950°C is followed by slow cooling to 760°C. Subsequent cooling determines the final product: pearlitic cast iron (fast cooling) or ferritic cast iron (slow cooling).
Ductile (Nodular) Cast Iron
In ductile cast iron, carbon forms spheroidal graphite. This is achieved through inoculation, adding magnesium and/or cerium. This reduces the notch effect of graphite, leading to improved mechanical properties, ductility, and tensile strength. The material must be very pure, consisting almost exclusively of iron, carbon, silicon, and a limited percentage of phosphorus. Parts have compositions very close to the eutectic point to minimize the solidification interval, as inoculants have a very short lifespan.