ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2025 31 Understanding the different hard- ness testing types and systems is useful to determine an optimal solution. Hardness testing functionality has evolved and now users can dial into world-class caliber instrumentation that more closely aligns with their applications. HARDNESS TESTING DEFINED Most indentation hardness tests measure the deformation that occurs when a material is penetrated with an indenter (Fig. 1). One of the most common indentation hardness tests used today is the Rockwell hardness test. Two levels of force are applied to the indenter at specified rates and dwell TECHNICAL SPOTLIGHT HARDNESS TESTERS: KNOW THE OPTIONS FOR BEST RESULTS Learn how to select the ideal hardness testing method for specific applications and utilize new efficiencies such as automation, digital systems, and data mobility. with the test material, and a preliminary force (i.e., the minor load) is applied to the indenter. The preliminary force is usually held constant for a set period, known as the dwell time, after which the depth of indentation is measured. After the measurement is made, an additional amount of force is applied at a set rate to increase the applied force to the total force level (i.e., the major load). The total force is held constant for a set time and the additional force is removed, returning to the preliminary force level. After holding the preliminary force constant for another set time, the depth of indentation is measured a second time, followed by the removal of the indenter from the test material. To calculate the Rockwell hardness number, the measured difference between the first and second indentation depth measurements, “h,” is used. If using an older Rockwell hardness system, the operator may have to manually control most or all of the test procedure steps. A majority of new machines automatically perform the entire test. Also, when leaving a mark or indentation is not an option, non- destructive ultrasonic technology can be used. DIGITAL ROCKWELL HARDNESS TESTERS When testing the hardness of carbon steel, alloy steel, cast iron, nonferrous metals, or engineering plastics, digital Rockwell benchtop hardness testers times when performing a Rockwell hardness test. This is different than the Brinell and Vickers hardness tests, where the size of the indentation is measured after the indentation process (Table 1). The Rockwell hardness of a material is based on the difference in the depth of the indenter at two specific times during the testing cycle. The value of hardness is calculated using a formula that was derived to yield a number falling within an arbitrarily defined range of numbers known as a Rockwell hardness scale. Regardless of the Rockwell scale or indenter being used, the overall Rockwell test procedure is the same. The indenter is brought into contact Fig. 1 — The majority of indentation hardness tests measure the deformation that occurs when the material being tested is penetrated with an indenter.
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