CAST IRON QUALITY CONTROL
THE MAIN CAST IRONS
WHITE CAST IRONS
The carbon is in the form of Fe3C carbide with a pearlitic matrix. Its name comes from the fact that when broken, it has a shiny white metallic appearance. White cast iron is a cast iron alloyed with manganese. The presence of chromium and molybdenum also favours the creation of white cast iron. This type of cast iron is resistant to wear and tear but is fragile to impacts.
GREY CAST IRONS
Carbon appears in the form of (spheroidal or lamellar) graphite. These cast irons are rich in silicon; this alloy addition favours the formation of graphite. It also improves corrosion resistance.
=> Lamellar graphite cast irons are brittle due to the geometry of the graphite which has a notch effect. Tensile strength is not optimal but it is suitable for compression work applications or if wear resistance is required.
=> Spheroidal graphite cast irons are cast irons in which the cooling has been slowed down so that the carbon crystallises in the form of a sphere. The geometry of the graphite improves machinability and the mechanical characteristics are close to those of steel. It produces a ductile, malleable cast iron. The addition of magnesium in the cast iron reduces the presence of sulphur. Thus, the graphite is formed spherically and not in lamellae.
It is possible to create spheroidal graphite cast iron from white cast iron if subjected to heat treatment.
In alloyed cast irons:
• chromium increases the mechanical characteristics
• molybdenum improves impact resistance
• phosphorus gives the cast iron better castability.
The chemical composition of the mixture is one of the parameters that will determine the type of cast iron obtained.
Cooling speed influences the formation of one or other of the cast irons.
• If the cooling is quick, it favours the formation of cementite, thus producing a white cast iron.
• However, if cooling is slower, the carbon has time to collect as graphite, producing grey cast iron.
CAST IRON DESIGNATIONS
Their designation always starts with EN-GJ (G corresponds to a cast metal and J for iron).
Another letter corresponding to the structure of the graphite follows this beginning of the designation:
L for Lamellar
S for Spheroidal
M for annealed graphite (malleable)
V for vermicular
Y for special structure
N for no graphite
Generally, this is followed by either the required minimum tensile strength and minimum elongation in %, or the same designation as a high-alloy steel
EN-GJS-400-15: Lamellar graphite cast iron, strength R min 400 MPa and elongation A 15%.
EN-GJN-X 300 Cr Ni Si 9-5-2: Graphite-free cast iron (white cast iron) with 3% carbon, 9% chromium, 5% nickel and 2% silicon
=> They are available in a wide range of cast iron grades to meet all applications according to the properties of impact resistance, wear resistance and good castability.
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