Properties of Metals
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 to
detect 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 indentation
tests 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
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).
Nickel is an important alloying element. In concentrations of less than 5%, nickel will raise the
toughness and ductility of steel without raising the hardness. It will not raise the hardness when
added in these small quantities because it does not form carbides, solid compounds with carbon.
Chromium in steel forms a carbide that hardens the metal. The chromium atoms may also
occupy locations in the crystal lattice, which will have the effect of increasing hardness without
affecting ductility. The addition of nickel intensifies the effects of chromium, producing a steel
with increased hardness and ductility.
Copper is quite similar to nickel in its effects on steel. Copper does not form a carbide, but
increases hardness by retarding dislocation movement.