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Rockwell hardness testing is arguably the most common tool used by the heat-treating industry to measure the success or failure of a heat-treatment process. It is one of the seemingly easiest tests to perform on the shop floor, in the quality-control department or in the metallurgical laboratory but arguably one of the hardest tests to perform properly. Let's learn more.
What is hardness?
A common definition of hardness is the measure of the resistance of a material to an applied force and involves the use of an indenter of fixed geometry under static load. However, hardness can also refer to stiffness (temper resistance) or to resistance to scratching, abrasion or cutting. It can be thought of as the ability of the material to resist permanent deformation (i.e., to be bent, broken or have its shape changed) in service. The greater the hardness, the greater its resistance. It is important to recognize that the hardness of a material is highly dependent on its microstructure, however, which in turn is influenced by the heat-treatment process.
Hardness is not a fundamental material property but rather a composite value. As such, it is an indication that all is well, not a guarantee that it is so. It is of great interest because hardness can be directly related to the expected strength of the material, which would otherwise require destructive testing to measure. The location of the hardness indentations, away from working surfaces, is often an important consideration if hardness testing is to be considered a nondestructive test.
Hardness measuring methods fall into three general categories, depending upon the manner in which the tests are conducted: scratch hardness, indentation hardness (Fig. 1) and dynamic rebound hardness (Fig. 2). Rockwell hardness testing machines can be either benchtop or portable devices.
Rockwell Testing Tips
Many different factors can affect the results of a hardness test. In most instances, incorrect or misleading readings are the result of poor testing practices that violate simple yet basic testing rules. This leads to good parts being discarded or scrapped or, worse yet, bad parts being accepted and used. By...