Page 162 - Mechanical Behavior of Materials

Section 4.7 Hardness Tests 163

Table 4.8 Commonly Used Rockwell Hardness Scales

Symbol, HRX Penetrator Diameter Force
X = if Ball, mm (in) kg Typical Application
A Diamond point 60 Tool materials
D Diamond point 100 Cast irons, sheet steels
C Diamond point 150 Steels, hard cast irons,
Ti alloys
B 1.588 100 Soft steels, Cu and Al
(0.0625) alloys
E 3.175 100 Al and Mg alloys, other soft
(0.125) metals; reinforced polymers
M 6.35 100 Very soft metals; high-
(0.250) modulus polymers
R 12.70 60 Very soft metals; low-
(0.500) modulus polymers

scales using ball indenters (B, E, M, R, etc., scales). The hardness numbers are designated HRX,
where X indicates the scale involved, such as 60 HRC for 60 points on the C scale. Note that a
Rockwell hardness number is meaningless unless the scale is specified. In practice, the hardness
numbers are read directly from a dial on the hardness tester, rather than being calculated.

4.7.4 Hardness Correlations and Conversions
The deformations caused by a hardness indenter are of similar magnitude to those occurring at the
ultimate tensile strength in a tension test. However, an important difference is that the material
cannot freely flow outward, so that a complex triaxial state of stress exists under the indenter.
Nevertheless, empirical correlations can be established between hardness and tensile properties,
primarily the ultimate tensile strength σ u . For example, for low- and medium-strength carbon and
alloy steels, σ u can be estimated from Brinell hardness as

σ u = 3.45(HB) MPa, σ u = 0.50(HB) ksi (4.32)

2
where HB is assumed to be in units of kg/mm . Note that we may also express hardness in units of
2
MPa by applying the conversion factor 1 kg/mm = 9.807 MPa. If the same units (such as MPa) are
used for both HB and σ u , Eq. 4.32 becomes σ u = 0.35(HB).
Observe that Eq. 4.32 approximates the curve shown in Fig. 4.33. However, there is consid-
erable scatter in actual data, so this relationship should be considered to provide rough estimates
only. For other classes of material, the empirical constant will differ, and the relationship may
even become nonlinear. Similarly, the relationship will change for a different type of hardness test.
Rockwell hardness correlates well with σ u and with other types of hardness test, but the relationships
are usually nonlinear. This situation results from the unique indentation-depth basis of this test. For