Rockwell hardness test, approved in 1919, is based on the measure of the net indentation depth increase when a load is applied. The test is performed by pressing an indenter with a pre-load, which cause an initial penetration, set the zero point on the tester and then full-load application. Removing the full-load, the result value is referred to the depth while the pre-load is still applied. Different type of indenters are used : 120° diamond spheroconical, 3.175mm and 1.588mm spheres. This test is widely used to its speed, resolution, reliability and small indentation area. Superficial Rockwell tests uses low loads on thin samples and, according to the indenter, can be performed from ceramic to metal sheets. The hardness values are expressed with dimensionless numbers, most common used scales are the C and B. There are several alternative scales A,B,C,D,E,F,G and others. Superficial Rockwell scales are expressed as 15, 30 and 45 followed by N, T, W, X and Y according to the used indenter.
The oldest hardness test in common use is frequently adopted to measure the hardness of forgings and castings. The test is performed by pressing a steel, or tungsten ball indenter on a well prepared sample surface under specific load. The resulting indentation is measured by scaled eyepiece or through specific analysing software and converted in the HB scale. Tipically a 10, 5, 2.5 or 1mm diameter sphere indenter with kgf to 3000kgf load is used for the test, smaller spheres and low loads are used for softer materials tests. For harder materials the tungsten sphere is used. The Brinell hardness is expressed as HB and must be followed by the test conditions. These standards are two HBS when steel ball indenters are used, or HBW when tungsten indenter is used. These units are followed by XX/YYYY where XX is the ball diameter inmm and YYYY is the applied load in kgf.
Developed in 1921, as an alternative to the Brinell Hardness test, is one common method to measure the hardness of materials specifically for thin sections and small parts. The test is done by pressing a square-based pyramid diamond indenter on a well prepared sample surface under specific load. The resulting indentation is measured by scaled eyepiece or through specific analysing software and converted in the HV scale. The Vickers method can be divided in two ranges, which uses the same indenter, according to the applied load: Macro and Micro range. Vickers hardness numbers are always expressed as xxHVyy where xx is the hardness number, HV is the hardness scale and yy the applied load. Sometimes after the load value is indicated the loading time if different from 10 to 15 seconds. Knoop Hardness Developed in 1939, Knoop Hardness Test is used to measure the hardness of brittle materials or thin sheets. Knoop test can replace a Vickers test and use a pyramidal diamond indenter. It is expressed as HK.
Universal hardness Testers are multi functions instruments for the determination of Rockwell, Brinell and Vickers hardness. As all-in-one equipment allows the user to concentrate the measurements, the calibrations and the maintenance in one single unit. Most used where all harness have to be tested with specific ranges. Always applying the international standards ASTM, ISO and JIS this solution grants performance and versatility.