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Grey Iron

Structure

Click here for the Metallurgy of Cast Iron

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.


GREY IRON : to BS EN 1561 : 1997 Equivalents Equivalents
Grade Minimum Tensile Strength
N/mm2
Maximum Tensile Strength
N/mm2
Typical Hardness
HB 10/300
(usual range)
BS 1452
1990
N/mm2
BS 1452
1961
EN-GJL-150 150 250 150-183 180 12
EN-GJL-200 200 300 159-194 220 14
EN-GJL-250 250 350 180-222 250 17
EN-GJL-300 300 400 200-280 300 20