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Mechanical Properties of Gray Iron - Hardness
Hardness is the most commonly determined property of metal because it is a simple test and many of the useful properties of metal are directly related to its hardness. Within a class or type of gray iron, hardness is a good indicator of its engineering properties, but this relation is not useful between types of gray iron because differences in graphite structure have more of an effect on tensile properties than on hardness. Specifying the hardness at a designated place on each casting is an excellent method of establishing consistency of castings in production quantities where the type of iron being used has been established as satisfactory for the application. Compression strength does correlate very well with hardness for all types of iron because hardness is essentially a compression test. Hardness usually gives a good indication of tool life in machining, however, the presence of free carbides in the microstructure will reduce the machinability much more than it increases the hardness.
The typical hardness ranges available in gray iron are listed in Table 3. The Brinell hardness test is used for all irons because the Brinell test impression is large enough to average the hardness of the constituents in the microstructure. Rockwell hardness B or C scale tests can be used satisfactorily on machined surfaces where the supporting surface is also machined. Several Rockwell tests should be made and averaged, but extreme values should be discarded because of inordinate influence by a graphite flake or a hard constituent.
A conversion chart between Rockwell and Brinell hardness values can be used accurately for steel but deviations from this relation for steel occur with gray irons. This deviation increases with high carbon equivalent irons. The amount of flake graphite present influences the two tests differently. This is evident from a comparison of microhardness test results on the matrix of gray irons compared to standard Rockwell C values on the same irons. The microhardness impressions do not include the graphite flakes that are present under the Rockwell C hardness indenter.
For this reason the hardness of gray iron should not be compared directly to the hardness of other metals for an indication of properties such as machinability or wear resistance. However, some effective hardness conversions can be made between selective types of hardnesses.
The hardness is affected by the processing of the gray iron as well as the composition because these factors influence the microstructure. Hardness is also related to other properties of gray iron as described in the following sections.
Table 3. Hardness Ranges for Gray Irons
Type of Gray Iron
Matrix Microstructure around Flake Graphite
||Pearlite and Ferrite
|Heat Treat Hardened
|Hardened and Tempered
|Chilled (white iron)
||Pearlite and Carbides
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