STEEL - Materials for Civil and Construction Engineers - page 123

Force

(a)

Force

(b)

Force

FIGURE 3.21 Schematic fixtures
for semi-guided bend test (ASTM
E290). Copyright ASTM. Reprinted with
permission.

(c)

magnitude and rate of application. The depth, or the size, of the indentation
is measured and related to a hardness index number. Hard materials result
in small impressions, corresponding to high hardness numbers. Hardness
measurements depend on test conditions and are, therefore, relative. Corre-
lations and tables are available to convert the hardness measurements from
one test to another and to approximate the tensile strength of the material
(ASTM A370).
One of the methods commonly used to measure hardness of steel and
other metals is the Rockwell hardness test (ASTM E18). In this test the depth
of penetration of a diamond cone, or a steel ball, into the specimen is deter-
mined under fixed conditions (Figure 3.22). A preliminary load of 10 kg is
applied first, followed by an additional load. The Rockwell number, which
is proportional to the difference in penetration between the preliminary and
total loads, is read from the machine by means of a dial, digital display, point-
er, or other device. Two scales are frequently used, namely, B and C. Scale B
uses a 1.588 mm (1/16 in.) steel ball indenter and a total load of 100 kg,
while scale C uses a diamond spheroconical indenter with a 120 ° angle and

a total load of 150 kg.
To test very thin steel or thin surface layers, the Rockwell superficial
hardness test is used. The procedure is the same as the Rockwell hardness
test except that smaller preliminary and total loads are used. The Rockwell
hardness number is reported as a number, followed by the symbol HR, and
another symbol representing the indenter and forces used. For example, 68
HRC indicates a Rockwell hardness number of 68 on Rockwell C scale.