Grey cast irons

• Microstructural characteristics of grey cast irons
• Mechanical characteristics of grey cast irons
• Characteristics of use of grey cast irons
• Applications of grey cast irons

Grey cast iron is the most widespread type of cast iron and is produced by melting cast iron and steel scrap. The carbon content varies between 2.5% and 4%; the silicon, with a content between 1% and 3%, on the other hand, guarantees graphitising characteristics.

Grey cast irons are microstructurally characterised by the presence of lamellar-structured graphite.

The formation of graphite during eutectic solidification is ensured by an adequate silicon content and proper inoculation.
Once the graphite has nucleated, the accretion phase also begins, which completes the solidification process. Graphite accretion can typically occur in two directions: lateral with respect to the hexagonal graphite matrix crystalline planes (blue arrow in following image), or perpendicular (red arrow). The lamellar morphology of the graphite is derived from lateral accretion, which is thermodynamically facilitated (requires less energy to occur), and which results in a series of successive branchings.

The lamellar graphite in grey cast irons is typically surrounded by a pearlitic matrix.

Now let’s take a look at the mechanical characteristics of grey cast irons. The lamellar morphology of graphite tends to penalise the mechanical behaviour of grey cast irons, in particular in relation to ductility and impact resistance. According to the standard (UNI-EN 1561), the minimum breaking stress of a grey cast iron can vary between 200 and 300 MPa, with elongations at fracture of 1-2%.

The mechanical characteristics described suggest use of grey cast irons in which the tensile stresses are not too high. It should be noted, however, that the presence of lamellar graphite favours good thermal conductivity and vibration damping characteristics.

In general, grey cast irons exhibit high fluidity characteristics, particularly in the presence of phosphorus as an alloying element. This allows thin-walled castings that are particularly suited to applications requiring efficient heat exchange.
Typical examples of applications of grey cast irons include: crankcases, manifolds, flanges and valves, drill components, machine tool heads, pumps, ingot moulds, compressor parts, boiler and radiator parts and pumps. The main contexts of use are therefore the mechanical, textiles, tractor, railway and transport, steel, boiler and heating system industries.