|
An important feature of the Rockwell hardness test procedure is the use of
the preliminary force as part of the testing cycle. Application of the preliminary
force acts to push the indenter through minor surface imperfections and to
crush residual foreign particles present on the test surface. By establishing a
reference beneath the surface prior to making the first depth measurement, it
allows testing of materials with slight surface flaws while maintaining much of
the test accuracy. Still, as a general rule, the better a test surface is prepared,
the more likely the measurement will represent the true Rockwell hardness
value of a material.
For the best results, the test surface and the surface in contact with the
support anvil should be smooth, flat, and free of oxides, foreign matter, and
lubricants. The test surface should be prepared in a manner that will not alter
the properties of the test material such as by overheating or cold-working.
The test surface should be representative of the material under test. For that
reason, surface effects, such as carburization or decarburization, should be
removed prior to testing, unless the purpose of the test is to measure these
surface features. Similarly, other types of coatings, such as paint, galvanizing,
etc., should also be removed prior to testing.
The degree of surface roughness that can be tolerated depends on the
force levels to be applied. A finish ground surface is usually sufficient for the
Rockwell C scale and for the Rockwell ball scales that apply a force of at least
980.7 N (100 kgf). In general, lighter test forces require better surface finishes.
For the superficial scales that use a total force of 147.1 N (15 kgf), a polished
surface is usually required
Back
|
