Thirty different Rockwell tests allow for use of different combinations of indenter types, sizes and loading conditions. This flexibility contributes to the frequent use of Rockwell testing when characterizing sheet metal hardness. Depending on the specific alloy and thickness, the most-used scales are Rockwell B, C, N and T.  

While the Brinell scale is based on the area of the indentation, indentation depth defines the Rockwell scale. The test apparatus first applies an initial minor preload that defines a reference depth set at zero. Then, the test setup applies the major load to induce additional penetration. The incremental depth difference relative to the zero point determines the Rockwell hardness value. Each Rockwell point equals a penetration depth of 0.002 mm (2 microns, or 0.00008 in.)  

ASTM E18 and ISO 6508 describe the standard test method for Rockwell hardness testing. For the B and C scales, application of the initial minor load requires 10 kgf, with total test force ranging from 60 to 150 kgf. This contrasts with superficial hardness scales such as N and T, where the initial minor preload is 3 kgf and the total test force ranges from 15 to 45 kgf, making these appropriate only for thinner materials.

C and N scales require use of a conical-shaped diamond indenter, leading to their application on indentation-resistant materials such as tool steels or case-hardened surfaces. B and T scales use a carbide ball, making them the preferred scales when testing softer alloys such as low-carbon steels and aluminum.

Vickers testing, as with Brinell testing, requires measurement of the dimensions of the indentation. Where the Brinell test requires calculation of the deformed area, the Vickers test measures the diagonal surface length of the impression formed from a diamond square-based pyramid with a 136-deg. angle between the opposing faces. Polishing the test samples enables accurate measurement of the impression dimensions, and etching the samples facilitates measurement of specific metallurgical phases.

Vickers test procedures allow for use of a wide range of applied loads, from 0.001 to 120 kgf. Tests performed with loads below 1 kgf are known as microhardness tests. These relatively light loads help target measurements of specific microstructural components, as heavier loads leave a larger impression and lead to an averaging of all structures under the indenter. Vickers microhardness testing assists with characterizing case-hardened depths, dimensions of weld heat-affected zones, differences in microstructural phase hardness and metallurgical phase segregation, among other properties. ASTM E384 and ISO 6507 cover the standard test method for Vickers microhardness testing.

Converting Between Scales

Ideally, investigators should perform hardness tests using the appropriate scale for the material type and thickness. When possible, avoid converting between different hardness scales. The thickness of the test material must be at least 10 times greater than the expected indentation depth. Otherwise, the hardness of the support structure influences the test reading.

Conversion charts, specific to each metal alloy, have been determined experimentally over time. While these charts prove valid for the tested datasets, those products no longer may be widely used. Consider, for example, the simple formula relating ultimate tensile strength (UTS, measured in psi) to Brinell hardness number (BHN): UTS = 500 BHN. This formula traces its roots to a publication from 1926, when researchers selected from then-current alloys.

If you must convert from one scale to another, consult ASTM E140 and select the table that contains the specific metal alloy of interest. Different conversion factors exist for materials with varying elastic moduli or strain-hardening capacity. MF

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials, Quality Control

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