The standard Rockwell method employs only one conical diamond indenter having a 120° angle and 0.2 mm radius on the diamond point (see figure 2) and several hard metal ball penetrators, whose diameter is always given in inches: 1/16” 1/8” ¼” ½”
Preload is constant: 10 kgf (98.1 N).
Test loads (preload + additional load) can be: 60, 100, 150 kgf (588.4, 980.7, 1471 N).
According to the current DIN norms, preloads and loads of the different test methods (Rockwell, Brinell, Vickers) have to be given only in N (Newton). However, for practical reasons the original indication in kgf is still used. The unit of measurement of the standard Rockwell method corresponds to 0.002 mm penetration. The hardness number must increase according to the hardness, while the difference of penetration between preload and load decreases as the hardness increases.
Therefore, the Rockwell hardness number is obtained subtracting from 100 (by diamond indenter) or from 130 (by any other ball penetrator) the difference of penetration expressed in units of 0.002 mm.
With a diamond indenter and a difference of penetration of 0.082 mm, the Rockwell number is given by: 100- (82÷2)= 59 Rockwell. Conversely, with a ball penetrator and the same difference of penetration, the Rockwell number is different: 130- (82÷2)= 89 Rockwell. In instruments having a dial indicator, that shows the penetrator’s displacements, the dial is generally divided into 100 graduations, so that a complete revolution corresponds to 0.2 mm. Dial indicators have two series of numbers: black numbers for testing with diamond indenter; red numbers for testing with ball penetrator. The zero setting must be done always on the black zero (130 red). In case of digital instruments, the test results are directly shown on a display at the end of the test cycle. Combining the different penetrators and loads, it is possible to obtain a wide range of hardness scales, as shown in table 1
When working with diamond indenter and load at 150 kgf, hardness will be called HRC, where H means Hardness, R the Rockwell method, C the scale in use. The hardness number is placed before this abbreviation, for example 60 HRC.