ASTM E384-16

5.1 Hardness tests have been found to be very useful for materials evaluation, quality control of manufacturing processes and research and development efforts. Hardness, although empirical in nature, can be correlated to tensile strength for many metals and alloys, and is also an indicator of machinability, wear resistance, toughness and ductility.

5.2 Microindentation tests are utilized to evaluate and quantify hardness variations that occur over a small distance. These variations may be intentional, such as produced by localized surface hardening, for example, from shot blasting, cold drawing, flame hardening, induction hardening, etc., or from processes such as carburization, nitriding, carbonitriding, etc.; or, they may be unintentional variations due to problems, such as decarburization, localized softening in service, or from compositional/microstructural segregation problems. Low test forces also extend hardness testing to materials too thin or too small for macroindentation tests. Microindentation tests permit hardness testing of specific phases or constituents and regions or gradients too small for evaluation by macroindentation tests.

5.3 Because microindentation hardness tests will reveal hardness variations that commonly exist within most materials, a single test value may not be representative of the bulk hardness. Vickers tests at 1000 gf can be utilized for determination of the bulk hardness, but, as for any hardness test, it is recommended that a number of indents are made and the average and standard deviation are calculated, as needed or as required.

5.4 Microindentation hardness testing is generally performed to quantify variations in hardness that occur over small distances. To determine these differences requires a very small physical indentation. Testers that create indents at very low test forces must be carefully constructed to accurately apply the test forces exactly at the desired location and must have a high-quality optical system to precisely measure the diagonal (or diagonals) of the small indents. Test forces in the upper range of the force range defined in 1.2 may be used to evaluate bulk hardness. In general, the Vickers indenter is better suited for determining bulk (average) properties as Vickers hardness is not altered by the choice of the test force, from 25 to 1000 gf, because the indent geometry is constant as a function of indent depth. The Knoop indentation, however, is not geometrically identical as a function of depth and there will be variations in Knoop hardness, particularly at test forces <200 gf, over the force range defined in 1.2 (and above this range); consequently, Knoop hardness is not normally used to define bulk hardness, except at 500 gf where E140 gives conversions to other test scales, and Knoop tests should not be performed at test forces above 1000 gf. The majority of Knoop tests of case hardness variations are conducted at forces from 100 to 500 gf. If the test is being conducted to meet a specified bulk hardness value, such as HRC, then most such tests will be conducted with Knoop at a 500 gf load. Because of the large difference between the long and short Knoop diagonals, the Knoop indenter is often better suited for determining variations of hardness over very small distances compared to the Vickers indenter. Vickers and Knoop tests at forces ≤25 gf are susceptible to imprecision due to the difficulty in measuring extremely small indents (<20 µm) by light microscopy with high precision and reproducibility. Tests made at forces ≤25 gf should be considered to be qualitative in nature. Likewise, test forces that create indents <20 µm in length should be avoided whenever possible and should be considered to be qualitative in nature. The success of the specimen preparation procedure in removing preparation-induced damage can, and will, influence test results; this problem becomes more critical as the test force decreases.