The hardness tester is the instrument used to measure the hardness of materials; it is used especially in the testing of metals.
In general, the hardness of a material is measured by detecting the degree of penetration into the material by a tip (indenter) under a load defined according to the material and the type of test: in conjunction with these factors, the hardness can be measured by measuring the depth of penetration or the size of the impression left by the indenter.
The instrument is often associated with the use of test specimens to verify the proper functioning of the hardness tester.
The measurement of hardness by means of a hardness tester is not destructive; however, material changes caused by the indenter at the test point lead to the possibility of false measurement if repeated in the same position or close to it.
The development of electronics
Over time, the development of electronics has led to a significant refinement of hardness control, introducing greater precision in reading as well as the possibility of storage, processing and display of the results: these functions are related to the sole processing of data - while the mechanical fundamentals that constitute the test remain the same.
Different procedures, load sizes and scales are used in hardness measurement, depending on the type and hardness of the materials to be tested.
The Rockwell measurement principle
The Rockwell process - which measures the depth of penetration - involves the use of diamond indenters for harder materials and carbide or steel for softer materials. The reading of the Rockwell test is straightforward, not contemplating the optical measurement required by other procedures. In the face of a fast reading and less subject to individual evaluation, it offers a more limited load range, excluding very low or very high loads.
The Brinell measurement principle
The Brinell process consists in pressing a hard steel ball (of variable diameter) on the flat and polished surface with a known load for a defined time and measure by optical means (microscope or projector) the diameter of the impression. The formula expressing the hardness value is is based on the relationship between the load and the values in mm of the ball and impression diameters.