The History and Principles of the Rockwell Hardness Test

The Rockwell hardness test?measures hardness in the simplest way possible, by pressing an indenter into the surface of the material with a specific load and then measuring how far the indenter can penetrate. Generally, the indenter is made of either a steel ball or a diamond.

In February 1919, Hugh Rockwell, and Stanley Rockwell, were awarded a patent for the Rockwell hardness testing device, an apparatus designed to measure the resistance of metallic materials to an applied force. Though they were unrelated, both men worked for the New Departure Company in Connecticut and were respectively an automobile enthusiast and a metallurgist.

The Rockwell hardness tester was developed out of a need for a machine that could quickly and easily measure the effects of heat treatment on bearing raceways, as establishing hardness provided information on the durability, strength and flexibility of the material being tested. A commonly used method of measuring hardness involved attempting to indent the material being tested by pressing a sharp object into its surface using increasing amounts of force. The two engineers augmented the traditional test method by employing a conical diamond for the indention device and basing the test results on the depth of indenter penetration. Important advantages of the Rockwell test were the much smaller test area needed to obtain an accurate measurement and the fact that the test could be performed on both flat and curved surfaces. In 1924, Stanley Rockwell began the commercial production of Rockwell hardness testers with Charles Wilson, an instrument manufacturer based in Connecticut.

Rockwell hardness remains the most efficient and extensively used hardness test and is recognized as one of the 20th century’s metallurgical innovations. The Rockwell test method is used on all metals, except in circumstances where the test metal structure or surface conditions introduces too much variation, for example, the indentations would be too large for the application or where the sample size or shape would prohibit its application.

The Rockwell method measures the permanent depth of indentation produced by a force on an indenter. First, a preliminary test force, commonly referred to as preload or minor load, is applied to the test sample using a diamond or ball indenter. This preload breaks through the surface to reduce the effects of the sample surface finish. After maintaining the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.?

After the preload, an additional force, called the major load, is added to achieve the total required test force. This force is held for a predetermined amount of time or dwell time, to allow for elastic recovery. This major load is then released, returning to the preliminary load. ?After holding the preliminary test force for a specified dwell time, the final depth of indentation is measured. The Rockwell hardness value is derived from the difference in the baseline and final depth measurements. This distance is converted to a hardness number and the preliminary test force is removed and the indenter is retracted from the test sample.

A = Depth reached by the indenter after application of the preload or minor load?

B = Position of indenter during total load, minor plus major loads ?

C = Final position reached by indenter after elastic recovery of test sample material ?

D = Distance measurement taken representing the difference between preload and major load position. This distance is used to calculate the Rockwell Hardness Number.

A variety of indenters may be used, for example, conical diamond with a round tip for harder metals to ball indenters with diameters ranging from 1/16” to 1/2” for softer materials.

?When selecting the appropriate Rockwell hardness, a general guide is to adopt the scale that specifies the largest load and the largest 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 is too close to the edge of the specimen or another impression or testing on cylindrical specimens. In addition, the test axis should be within two degrees of perpendicular to ensure precise loading and there should be no deflection of the test sample or test machine during the loading application from conditions such as dirt under the test specimen or on the elevating screw column. It is also important to keep the surface finish clean.

Conversion charts are available that allow conversion from one method of hardness to another, but it must be remembered that these conversion charts cannot precisely convert from one to another.

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