Page 220 - Materials Science and Engineering An Introduction

192 • Chapter 6 / Mechanical Properties of Metals

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Indenters include spherical and hardened steel balls having diameters of , , , and in.
1 1
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16 8 4
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(1.588, 3.175, 6.350, and 12.70 mm, respectively), as well as a conical diamond (Brale)
indenter, which is used for the hardest materials.
With this system, a hardness number is determined by the difference in depth of
penetration resulting from the application of an initial minor load followed by a larger
major load; utilization of a minor load enhances test accuracy. On the basis of the mag-
nitude of both major and minor loads, there are two types of tests: Rockwell and su-
perficial Rockwell. For the Rockwell test, the minor load is 10 kg, whereas major loads
are 60, 100, and 150 kg. Each scale is represented by a letter of the alphabet; several are
listed with the corresponding indenter and load in Tables 6.5 and 6.6a. For superficial
tests, 3 kg is the minor load; 15, 30, and 45 kg are the possible major load values. These
scales are identified by a 15, 30, or 45 (according to load), followed by N, T, W, X, or
Y, depending on the indenter. Superficial tests are frequently performed on thin speci-
mens. Table 6.6b presents several superficial scales.
When specifying Rockwell and superficial hardnesses, both hardness number and
scale symbol must be indicated. The scale is designated by the symbol HR followed
by the appropriate scale identification. For example, 80 HRB represents a Rockwell
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hardness of 80 on the B scale, and 60 HR30W indicates a superficial hardness of 60 on
the 30W scale.
For each scale, hardnesses may range up to 130; however, as hardness values rise above
100 or drop below 20 on any scale, they become inaccurate; and because the scales have
some overlap, in such a situation it is best to utilize the next-harder or next-softer scale.
Inaccuracies also result if the test specimen is too thin, if an indentation is made
too near a specimen edge, or if two indentations are made too close to one another.
Specimen thickness should be at least 10 times the indentation depth, whereas al-
lowance should be made for at least three indentation diameters between the center
of one indentation and the specimen edge, or to the center of a second indentation.
Furthermore, testing of specimens stacked one on top of another is not recom-
mended. Also, accuracy is dependent on the indentation being made into a smooth
flat surface.
The modern apparatus for making Rockwell hardness measurements is automated
and very simple to use; hardness is read directly, and each measurement requires only a
few seconds. This apparatus also permits a variation in the time of load application. This
variable must also be considered in interpreting hardness data.

Brinell Hardness Tests 15
In Brinell tests, as in Rockwell measurements, a hard, spherical indenter is forced into
the surface of the metal to be tested. The diameter of the hardened steel (or tungsten
carbide) indenter is 10.00 mm (0.394 in.). Standard loads range between 500 and 3000
kg in 500-kg increments; during a test, the load is maintained constant for a specified
time (between 10 and 30 s). Harder materials require greater applied loads. The Brinell
hardness number, HB, is a function of both the magnitude of the load and the diameter
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of the resulting indentation (see Table 6.5). This diameter is measured with a special
low-power microscope using a scale that is etched on the eyepiece. The measured di-
ameter is then converted to the appropriate HB number using a chart; only one scale is
employed with this technique.

14 Rockwell scales are also frequently designated by an R with the appropriate scale letter as a subscript, for example,
R C denotes the Rockwell C scale.
15 ASTM Standard E10, “Standard Test Method for Brinell Hardness of Metallic Materials.”
16 The Brinell hardness number is also represented by BHN.