Kinds of Rockwell Hardness Scales

Initially, 15 scales were created using a 10 kg preliminary force; 60, 100 or 150 kg total test forces combined with diamond; 1⁄16, 1⁄8, 1⁄4 and 1⁄2-inch diameter ball indenters; and 0.2 millimeter per Rockwell point.

These forces are called the Regular Rockwell scales. The HRC, for hardened steels, and the HRB, for softer steels and brass, are the most common scales.

While the HRC scale uses a maximum of 0.2millimeter (100 Rockwell points) penetration, the HRB and other regular scales use a ball indenter because they test softer materials and have a maximum depth of 0.26 millimeter (130 Rockwell points).

As the Rockwell test gained popularity, the need to test thin and case-hardened parts became apparent. To accommodate these samples, the Superficial Rockwell scales were created.

The same indenters are used, but the preliminary force is reduced to 3 kg and the total forces are 15, 30 and 45 kg. In addition, because of the shallower indents the depth value of one Rockwell point was reduced to 0.001 millimeter. The same maximum depth of 0.1 millimeter is used for both diamond and ball indenters.

Testers can be designed and sold as either Regular or Superficial units. However, the most common units currently produced are called twin Rockwell testers because they can perform both Regular and Superficial test ranges.