We continue our discussion of hardness testing by focusing on Rockwell testing.
Rockwell Hardness Testing Tips
The Rockwell hardness test (Fig. 1) is the most widely used and versatile of the hardness tests. It measures the hardness of a metal by resistance to penetration. The depth of the impression is measured rather than the diametric area (as in the case of a Brinell test). The Rockwell test uses two loads, one applied directly after the other. The first load (known as the minor) of 10 kg is applied to the specimen to help seat the indenter and remove the effects of any surface irregularities during testing. In essence, the minor load creates a uniformly shaped surface for the major load to be applied to. The difference in the depth of the indentation between the minor and major loads provides the Rockwell hardness number.
Rockwell hardness measurement can be defined as macro-, micro- or even nano-scale according to the forces applied and displacements obtained. Typical indenters used during Rockwell tests include diamonds and steel or carbide balls. However, steel is being phased out of use. The type of indenter and the test load determine the hardness scale. Rockwell “B” and “C” scales are the most common, although many other scales can be used for specific test purposes. The hardness numbers themselves have no units. A higher number in each of the scales means the material is harder.
Rockwell hardness measurements are actually a conversion of the depth of the indentation left by the indenter. Each Rockwell point represents 0.00008 inches of depth. For a reading of 60 HRC, subtract the major and minor loads, or 100-60 x 0.00008 = 0.0032 inches. So, the depth of a 60 HRC reading is 0.0032 inches.
Rockwell Superficial Hardness Testing Tips
There is a second Rockwell test referred to as the Rockwell Superficial hardness test. This machine works the same as the standard Rockwell tester, but it is used to test thin strip, lightly carburized surfaces, small parts or parts that might collapse under the conditions of the regular test. The Superficial tester uses a reduced minor load (3 kg) and has the major load reduced to either 15 or 45 kg, depending on the indenter, which is the same one used for the common scales.
We continue our discussion next time with a list of tips and common errors encountered in Rockwell testing.
1. Stone, Alan and Daniel H. Herring, “Practical Considerations for Successful Hardness Testing,” Industrial Heating, April 2006.
2. Lysaght, Vincent E., and DeBellis, Anthony, Hardness Testing Handbook, American Chain and Cable Company, 1969.
3. Wilson Instruments Division, Instron Corporation, Norwood, MA (www.wilsoninstruments.com)
4. Herring, Daniel H., “Common Pitfalls in Hardness Testing,” Wire Forming Technology International, Spring 2012.
5. ASTM Specification Nos. E3, E10, E18, E103, E140 and E384 (www.astm.org).