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Hardness is the property of a material that enables it to
resist plastic deformation, penetration,
indentation, and scratching. Therefore, hardness is important from an
engineering standpoint because resistance to
wear by either friction or erosion by steam, oil, and water generally increases with hardness.
Hardness tests serve an
important need in industry even though they do not measure a unique
quality that can be termed hardness. The tests are
empirical, based on experiments and observation, rather than fundamental theory. Its chief value is as an inspection
device, able todetect certain differences
in material when they arise even though these differences may be
undefinable. For example, two lots of material that
have the same hardness may or may not be
alike, but if their hardness is different, the materials certainly are not
alike.
Several methods have
been developed for hardness testing. Those most often used are Brinell,
Rockwell, Vickers, Tukon, Sclerscope, and the files
test. The first four are based on indentationtests and the fifth on the rebound height of a
diamond-tipped metallic hammer. The file test
establishes the characteristics of how well a file
takes a bite on the material.
As a result of many
tests, comparisons have been prepared using formulas, tables, and graphs
that show the relationships between the results of
various hardness tests of specific alloys. There
is, however, no exact mathematical relation between
any two of the methods. For this reason, the
result of one type of hardness test converted to readings of another type should
carry the notation " converted from " (for
example "352 Brinell converted from Rockwell
C-38").
Another convenient
conversion is that of Brinell hardness to ultimate tensile strength. For
quenched and tempered steel, the tensile strength (psi)
is about 500 times the Brinell hardness
number (provided the strength is not over 200,000 psi).
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