Grey iron is an alloy of iron, carbon and silicon which refers to a range of cast irons which solidify with a eutectic. Consequently these alloys should be considered ternary Fe-C-Si alloys. Despite this, the principles of cast iron solidification are understood from the binary iron-carbon phase diagram, where the eutectic point lies at 1154°C and 4.3 wt% carbon. A few of the terms used to describe cast iron and its constituents are:-
This is a microstructure of a grey cast iron with a carbon content of 3.2% and a silicon content of 2.2% by weight
The microstructure above has two main constituents. The long grey regions are flakes of graphite and the background or matrix of the alloy is pearlite. Pearlite is a fine mixture of ferrite and iron carbide.
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.
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 these have properties more in line with a cast mild steel.
Grey iron can be alloyed with small amounts of copper, molybdenum, vanadium or chrome 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 alloys are used, along with lower levels of carbon and silicon, which will promote a pearlitic structure. These alloyed grey irons have applications where a higher tensile strength or hardness is required. These alloys also produce an iron that can respond to heat treatment although this is uncommon. The only heat treatment routinely used is either stress relieving or annealing.
With the addition of nickel up to 30% and chrome at smaller levels a range of austenitic grey irons can be produced which have improved properties at elevated temperatures and in aggressive atmospheres and environments. Additions primarily of chrome up to 28% with smaller amounts of nickel and copper produces a range of white iron grades suitable for wear applications. Both these alloyed irons are covered by separate ISO standards, ISO 2892 for the austenitics and BS ISO 21988 for the chrome irons.
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