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. The term grey is used because of the
colour of the fracture surface if a piece is broken. If molten cast
iron is allowed to cool normally the carbon comes out of solution
and forms flakes of graphite which run through the ferrite/pearlite
matrix, hence the alternative term, flake graphite iron. These
flakes are at the microscopic level, the ends of which form stress
points in the cast iron.
If cast iron is subject to a compressive load these stress points
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. Grey
cast iron came into its own during the industrial revolution when
improved production methods and understanding of the metallurgy led
to more a reliable material that could be cast into an increasing
number of different shapes and section thicknesses. The Victorian
era saw an explosion in the use of cast iron and that has continued
to this day due to the combination of the ability to cast complex
shapes at low cost compared to other production methods and
materials.
As well as grey iron castings, our customer base also covers ductile
and alloyed irons for the engineering, decorative, architectural and
artistic sectors and we can supply castings from one off up to small
to medium batch production using Alkali Phenolic resin bonded sands.
Our workforce of highly skilled moulders can work from complex loose
pattern equipment, particularly where low volumes are required,
whilst our moulding line is suited for batch production. Our long
trading history, coupled with an investment programme that has
enabled us to keep up with modern production methods and
environmental legislation has meant that we have been involved with
many projects requiring ductile iron castings and continue to be so.
All our grey iron castings are produced to the current ISO material
specification, ISO 185 2005, along with any further certification
which a customer may require. Our diverse customer base has also
given us experience in a wide range of applications for grey iron
castings , including pumps and valves, forges, foundries and rolling
mills, automotive and aerospace, a wide range of OEMs, quarries and
mines, railways and rolling stock, local authorities and artists and
sculptors. We also have long term trading relationships with local
pattern makers, machine shops and surface finishers which enable us
to quote for the complete supply of the finished casting.
We produce our grey iron from a melt of pig iron and steel scrap,
with small additions of carbon, ferro silicon, ferro manganese,
ferro sulphide and a zirconium based ferro silicon innoculant. All
these materials have a known analysis and together give a reliable
analysis for every melt. 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.
One area of potential confusion is the spelling of the word "grey".
This can be spelt either as "grey" or as "gray" although gray tends
to be an American usage.
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