ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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Rockwell diamond indenters are used mainly for testing hard materials such as hardened steels and cemented

carbides. “Hard materials” are those with hardness greater than 100 HRB and greater than 83.1 HR30T (see the

section “Rockwell Scales” in this article for further explanation).

The Rockwell diamond indenter is a spheroconical shape with a 120° cone and a spherical tip radius of 200 μm

(Fig. 2a). Older standard indenters in the United States had a nominal radius closer to 192 μm, which was

within ASTM specifications (200 ± 10 μm); standard indenters in the rest of the world have been closer to 200

μm. While not out of tolerance, the old U.S. standard indenter is at the low end of the specification. This led to

a change in tip radius closer to 200 μm used in the rest of the world.

Fig. 2 Rockwell indenter. (a) Diamond-cone Brale indenter (shown at about 2×). (b) Comparison of old

and new U.S. diamond indenters. The angle of the new indenter remains at 120°, but has a larger radius

closer to the average ASTM specified value of 200 μm; the old indenter has a radius of 192 μm. The

indenter with the larger radius has a greater resistance to penetration of the surface.

A comparison of the old (192 μm) U.S. standard diamond indenter and the current (200 μm tip) U.S. indenter is

shown in Fig. 2(b). The larger radius increases the resistance of the indenter to penetration into the surface of

the testpiece. At higher Rockwell C hardness, most of the indenter travel is along the radius; whereas at the

lower hardnesses, more indenter travel is along the angle. This is why the hardness shift from old to new has

been most significant in the 63 HRC range and not the 25 HRC range.

Rockwell Scales

Rockwell hardness values are expressed as a combination of hardness number and a scale symbol representing

the indenter and the minor and major loads. The Rockwell hardness is expressed by the symbol HR and the

scale designation. For example, 64.0 HRC represents the Rockwell hardness number of 64.0 on the Rockwell C

scale; 81.3 HR30N represents the Rockwell superficial hardness number of 81.3 on the Rockwell 30N scale.

Because of the changeover to carbide balls, the designation for the ball scales will require the use of an S or W

to indicate the ball used. For example, a HRB scale reading of 80.0 obtained using a steel ball would be labeled

80.0 HRBS, while the same result using a carbide ball would be designated 80.0 HRBW.

There are 30 different scales, defined by the combination of the indenter and the minor and major loads (Tables

2 and 3). In many instances, Rockwell hardness tolerances are specified or are indicated on drawings. At times,

however, the Rockwell scale must be selected to suit a given set of circumstances.

Table 2 Rockwell standard hardness

Scale

symbol

Indenter Major

load, kgf

Typical applications

A Diamond (two

scales—carbide and

steel)

Cemented carbides, thin steel, and shallow case-hardened

steel

in. (1.588 mm)

ball

100

Copper alloys, soft steels, aluminum alloys, malleable iron

C Diamond 150

Steel, hard cast irons, pearlitic malleable iron, titanium,

deep case-hardened steel, and other materials harder

than 100 HRB

D Diamond 100

Thin steel and medium case-hardened steel and pearlitic

malleable iron

⅛ in. (3.175 mm)

ball

100

Cast iron, aluminum and magnesium alloys, bearing

metals

in. (1.588 mm)

ball

Annealed copper alloys, thin soft sheet metals

in. (1.588 mm)

ball

150

Phosphor bronze, beryllium copper, malleable irons.

Upper limit 92 HRG to avoid possible flattening of ball

⅛ in. (3.175 mm)

ball

Aluminum, zinc, lead

⅛ in. (3.175 mm)

ball

150

Bearing metals and other very soft or thin materials. Use