By Specimen Thickness, How to choose rockwell hardness scales？

Scan QR

×

Scan QR Code

https://mikrosize.com/by-specimen-thickness-how-to-choose-rockwell-hardness-scales%ef%bc%9f/

The Rockwell hardness test specifies a minimum specimen thickness: for scales using a diamond indenter, it is 10 times the depth of the permanent indentation; for scales using a ball indenter, it is 15 times the depth of the permanent indentation. Standards require that no visible signs of deformation appear on the back of the specimen after testing.
The indentation depth for any Rockwell hardness test can be calculated using a simple formula. In practice, however, such calculations are unnecessary, as standard charts are available to conveniently determine these “minimum thickness values.” While these values are calculated based on 10:1 or 15:1 ratios within certain ranges, most are derived from experimental data collected using varying thicknesses of low-carbon steel and quenched-and-tempered strip steel.

For a specimen with a known hardness value, its thickness should fall above the curve. For example, a specimen with a hardness of 60 HRC should have a thickness greater than 0.8 mm. For a specimen with a known thickness, the corresponding hardness value can be identified on the chart; while any scale covering this hardness value is technically selectable, the scale with the higher test force is typically chosen to ensure high sensitivity and accuracy. For instance, a quenched steel specimen with a thickness of 0.5 mm corresponds to a Rockwell hardness of 75; the available scales are HRD and HRA. Since HRD employs a higher test force, it should be selected. A 0.5 mm thick quenched steel specimen can also be tested using a Superficial Rockwell hardness tester. In this case, the corresponding Superficial Rockwell hardness value is 50, and the available scales are HR30N and HR45N; the HR45N scale should be chosen. However, for 0.5 mm thick mild steel or brass, no standard Rockwell scale is suitable; only Superficial Rockwell scales can be used. The available options are HR30T and HR45T, with HR45T being the appropriate choice.
For example, the HRC scale should be selected for a specimen with a thickness of 0.8 mm and a hardness of 60 HRC, while the HRF scale should be selected for a specimen with a thickness of 0.8 mm and a hardness of approximately 60 HRB. The HR45N scale should be selected for a specimen with a thickness of 0.5 mm and a hardness of 60 HRC, whereas the HR15T scale should be selected for a specimen with a thickness of 0.5 mm and a hardness of 60 HRB.

The specimen is a hard steel strip with a thickness of 0.35 mm and a hardness of approximately 63 HRC.  a material with a hardness of 63 HRC requires a minimum thickness of 0.7 mm for accurate HRC hardness testing; therefore, the HRC scale is unsuitable for this material. If other scales are considered, the “Hardness Conversion Table for Ferrous Metals” indicates that 63 HRC corresponds to 73 HRD, 83 HRA, 70 HR45N, 80 HR30N, and 91 HR15N. Referring back to Table 2.1, the viable hardness scales are HR30N and HR15N; in this case, the HR30N scale—which utilizes a higher test force—should be selected.