In grey cast iron the free graphite is in the form of flakes. The matrix may be ferritic for the lower strengths to pearlitic for higher strength.

Properties
The strength of a grey cast iron varies with the section size of the casting due to solidification effects: This is referred to as "section sensitivity".

Also, the strength in compression is three to four times the tensile strength because of the planes of weakness created by the graphite flakes. The material tends to be brittle compared to steel, but is extremely stiff and deflects little before fracture. This imparts the characteristic damping qualities of grey cast iron.

The graphite flakes also have a lubricating effect giving the material advantages in sliding wear applications. Grey cast iron has better casting characteristics than steel allowing finer detail and complex shapes. It also machines more easily and quickly. A disadvantage is that it cannot be readily welded.

Applications
Classic applications for grey cast iron are engine cylinder heads and blocks utilising the thin and complex cast sections for water cooling passages and the damping characteristics for quietness.

Gear boxes and gears are made in grey cast iron for quietness and the wear properties.

Machine tool slideways use the wear characteristics.

General engineering castings benefit from the ease of casting, the comparatively simple pattern equipment and the shorter lead times to produce castings.

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