This is a microstructure of a ductile cast iron with a carbon content of 3.6% and a silicon content of 2.6% by weight.
The microstructure above has two main constituents. The round dark grey spots are nodules of graphite and the background or matrix of the alloy is pearlite with some ferrite present. Pearlite is a fine mixture of ferrite and iron carbide.
Technical Factors for Ductile Iron Castings
A high cooling rate and a low carbon equivalent favours the formation of white cast iron whereas a low cooling rate or a high carbon equivalent promotes grey cast iron. During solidification, the major proportion of the carbon precipitates in the form of graphite or cementite. When solidification is just complete, the precipitated phase is embedded in a matrix of austenite which has an equilibrium carbon concentration of about 2 wt%. On further cooling, the carbon concentration of the austenite decreases as more cementite or graphite precipitates from solid solution.
For conventional cast irons, the austenite then decomposes into pearlite at the eutectoid temperature. However, in grey cast irons, if the cooling rate through the eutectoid temperature is sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon being deposited on the already existing graphite. To convert the flakes of graphite to spheres, a nodulariser such as magnesium or cerium is used. This is added to the melt just prior to casting and is used to remove as much of the sulphur in the melt as possible. Sulphur is a surface active material and tends to come out of solution at surface boundaries, in this case at the graphite/matrix boundary. It also reduces surface tension allowing the graphite to form irregular shapes, such as flakes, if the sulphur is present in large enough quantities. By reducing the sulphur levels (usually to below 0.003%) with magnesium, which forms magnesium sulphide when the melt is treated with Ferro Silicon Magnesium, the surface tension increases on the graphite at the graphite/matrix boundary "pulling" the graphite into a more regular shape, in this case - spheres.
If cast iron is subject to a compressive load the stress points at the end of the graphite flakes are not particularly detrimental and flake graphite cast iron is excellent under compressive load, although its use is more limited in situations where it is subject to bending or shock loading as these stress points cause a brittle failure at stresses above the tensile strength of the grade used. If a material is required that needs to withstand bending, tensional or shock loading then a ductile cast iron may be more suitable as the spheres remove these stress points and give a material with mechanical properties more in line with a cast mild steel.
Ductile iron can be alloyed with small amounts of copper to produce increasingly stronger irons as the alloy levels are increased. This is achieved by controlling the amount of ferrite and pearlite in the iron matrix. Ferrite is much softer that pearlite so an alloy is used, along with lower levels of carbon and silicon, which will promote a pearlitic structure. These alloyed ductile irons have applications where a higher tensile strength or hardness is required, although it should be noted that as the tensile strength increases the ductility decreases. These alloys also produce an iron that can respond to heat treatment, particularly flame or induction hardening, although this is uncommon. The only heat treatment routinely used is either stress relieving or annealing.
With the addition of nickel, molybdenum and copper an alloy can be produced that can be austempered. This produces a range of Austempered Ductile Irons (or ADI) which have high strength and hardness and can also work harden. They are covered by a separate production standard, BS ISO 17804:2005
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