Machinery's Handbook, 31st Edition
504 Hardness Testing Knoop Hardness Numbers.— The Knoop hardness test is applicable to extremely thin metal, plated surfaces, exceptionally hard and brittle materials, very shallow carburized or nitrided surfaces, or whenever the applied load must be kept below 3600 grams. The Knoop indentor is a diamond ground to an elongated pyramidal form and it produces an indentation having long and short diagonals with a ratio of approximately 7 to 1. The longitudinal angle of the indentor is 172 degrees, 30 minutes, and the transverse angle 130 degrees. The Tukon Tester in which the Knoop indentor is used is fully automatic under electronic control. The Knoop hardness number equals the load in kilograms di- vided by the projected area of indentation in square millimeters. The indentation number corresponding to the long diagonal and for a given load may be determined from a table computed for a theoretically perfect indentor. The load, which may be varied from 25 to 3600 grams, is applied for a definite period and always normal to the surface tested. Lapped plane surfaces free from scratches are required. Monotron Hardness Indicator.— With this instrument, a diamond-ball impressor point 3 ⁄ 4 mm in diameter is forced into the material to a depth of 9 ⁄ 5000 inch; the pressure required to produce this constant impression indicates the hardness. One of two dials shows the pressure in kilograms and pounds, and the other shows the depth of the impression in mil limeters and inches. Readings in Brinell Hardness Numbers may be obtained by means of a scale designated as M -1. Keep’s Test.— With this apparatus, a standard steel drill makes a definite number of rev - olutions while it is pressed with standard force against the specimen to be tested. The hardness is automatically recorded on a diagram on which a dead soft material gives a horizontal line, and a material as hard as the drill itself gives a vertical line, intermediate hardness being represented by the corresponding angle between 0 and 90 degrees. Comparison of Hardness Scales.— Table 5, Table 6, and Table 7 show comparisons of various hardness scales. All such tables are based on the assumption that the metal tested is homogeneous to a depth several times that of the indentation. To the extent that the metal being tested is not homogeneous, errors are introduced because different loads and different shapes of penetrators meet the resistance of metal of varying hardness, de - pending on the depth of indentation. Another source of error is introduced in comparing the hardness of different materials as measured on different hardness scales. This error arises from the fact that in any hardness test, metal that is severely cold-worked actually supports the penetrator, and different metals, different alloys, and different analyses of the same type of alloy have different cold-working properties. In spite of the possible inaccuracies introduced by such factors, it is of considerable value to be able to compare hardness values in a general way. The data shown in Table 5 are based on extensive tests on carbon and alloy steels mostly in the heat-treated condition but have been found to be reliable on constructional alloy steels and tool steels in the as-forged, annealed, normalized, quenched, and tempered conditions, providing they are homogeneous. These hardness comparisons are not as accurate for special alloys such as high manganese steel, 18-8 stainless steel and other austenitic steels, nickel-base alloys, constructional alloy steels, and nickel-base alloys in the cold-worked condition. The data shown in Table 6 are for hardness measurements of unhardened steel, steel of soft temper, gray and malleable cast iron, and most nonferrous metals. Again these hard ness comparisons are not as accurate for annealed metals of high Rockwell B hardness, such as austenitic stainless steel, nickel and high nickel alloys, and cold-worked metals of low Rockwell B scale hardness, such as aluminum and the softer alloys. Table 7 is intended to provided a general numerical comparison of the various Rockwell hardness scales; however, it should not be used for conversion between the various scales.
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