Figure 15-24.--Principle of the Rockwell hardness test.
The Rockwell superficial tester differs from the
normal Rockwell tester in the amount of load applied to
slightly.)
perform the test and in the kind of scale used to interpret
With the Rockwell tester, a deadweight, acting
the results. While the major loads on the normal tester
through a series of levers, is used to press the ball or
are 100 and 150 kg, the major loads on the superficial
cone into the surface of the metal to be tested. Then, the
tester are 15, 30, and 45 kg. One division on the dial
depth of penetration is measured. The softer the metal
gauge of the normal tester represents a vertical
being tested, the deeper the penetration will be under a
displacement of the indenter of 0.002 millimeter (mm).
given load. The average depth of penetration on samples
One division of the dial gauge of the superficial tester
of very soft steel is only about 0.008 inch. The hardness
represents a vertical displacement of the indenter of
0.001 mm. Hardness scales for the Rockwell superficial
is indicated on a dial, calibrated in the Rockwell B and
tester are the N and T scales. The N scale is used for
the Rockwell C hardness scales. The harder the metal,
materials that, if they were thicker, would usually be
tested with the normal tester using the C scale. The T
are usually tested with the spheroconical penetrator,
scale is comparable to the B scale used with the normal
with hardness numbers being read from the Rockwell
tester. In other respects, the normal and superficial
C scale. The steel ball is used for nonferrous metals, and
Rockwell testers are much alike.
the results are read on the B scale.
Once you have prepared your sample for testing,
With most indenter-type hardness tests, the metal
refer to your testers operator manual for step-by-step
being tested must be sufficiently thick to avoid bulging
operating instructions.
or marking the opposite side. The specimen thickness
should be at least 10 times the depth of penetration. It
FILE HARDNESS TEST
is also essential that the surface of the specimen be flat
and clean. When hardness tests are necessary on thin
The simplest method of testing a material's
material, a superficial Rockwell tester should be used.
hardness is the file hardness test. This test cannot be
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