Hardness is a materials characteristic and not a fundamental physical property. Knowing the hardness of any metal or alloy is important since hardness value is in correlation to other properties such as materials size and thickness.
The Rockwell hardness test is a measurement based on the net increase in depth of impression as a load is applied. Hardness numbers have no units and are commonly given in the R, L, M, E, and K scales. The higher the number in each of the scales means the harder the material.
The Rockwell hardness test worldwide adoption likely resulted from the test method advantages it provides. It is fast, inexpensive, and relatively non-destructive, leaving only a small indentation in the material. The simplicity of operation provides the added advantage of not requiring a highly skilled operator to perform the test.
By way of correlation with other material properties, the Rockwell hardness test provides important information about metallic materials to include things like tensile strength, wear resistance, and ductility.
A preliminary test force — pre or minor load test — is applied to a sample using a diamond indenter. This load represents the zero reference point-break through the surface, reducing the effects of surface finish.
After the pre-load, a major load is applied to reach the total required test load. This force is held for a predetermined amount of time (dwell time) to allow for elastic recovery. The major load is then released and the final position is measured against the position derived from the pre-load: the indentation depth variance between the pre-load value and major load value. This distance is then converted to a hardness number.
Preliminary — pre-loads range from 3 kgf (used in the Superficial Rockwell scale) to 10 kgf (used in the Regular Rockwell scale) to 200 kgs (used as a macro scale and not part of ASTM E-18; see ASTM E-1842). Total test forces range from 15 kgf to 150 kgf (superficial and regular) to 500 to 3000 kgf (macrohardness).
Test Method Illustration
A = Depth reached by indenter after application of preload (minor load)
B = Position of indenter during major load
C = Final position reached by indenter after elastic recovery of sample material
D = Distance measurement taken representing difference between preload and major load position
A variety of indenters — conical diamond with a round tip for harder metals to ball indenters — range in diameter from 1/16” to ½” for softer materials.
When selecting a Rockwell scale, a general guide is to select the scale that specifies the largest load and the smallest indenter possible without exceeding defined operation conditions and accounting for conditions that may influence the test result. These conditions include test specimens that are below the minimum thickness for the depth of indentation — a test impression that falls too close to the edge of the specimen or another impression, or testing on cylindrical specimens.
The test axis should be within 2° of perpendicular to assure precise loading. There should be no deflection of the test sample or tester during the loading application from conditions such as dirt under the test specimen or on the elevating screw. It is important to keep the surface finish clean and for heat treatment to be removed.
Sheet metal can be too thin and too soft for testing on a particular Rockwell scale without exceeding minimum thickness requirements and potentially indenting the test anvil. In this case, a diamond anvil can be used to provide a consistent influence of results.
Another special case in testing cold rolled sheet metal is that work hardening can create a gradient of hardness throughout the sample so that any test is measuring the average of the hardness over the depth of indentation effect. In this case, any Rockwell test result is going to be subject to doubt.
There is a history of testing using a particular scale on a particular material that operators are used to and able to functionally interpret.
