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.
Grey Iron Castings & Grey Cast Iron Principles
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.
At Durham Foundry 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. These materials have a known analysis giving a reliable result for every melt. If cast iron is subject to a compressive load these stress points are not particularly detrimental and flake graphite cast iron is excellent under compression, although its use is more limited where it is subject to bending or shock loading.
Grey Cast Iron Production
At Durham Foundry we have been producing grey iron castings for over a hundred years. 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.
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.
