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Hardness TestingCategory: ALLOYS AND THEIR EFFECT UPON STEEL The word hardness is used to express various properties of metals, and is measured in as many different ways. Scratch hardness is used by the geologist, who has constructed Moh's scale as follows: Talc has a hardness of 1 Rock Salt has a hardness of 2 Calcite has a hardness of 3 Fluorite has a hardness of 4 Apatite has a hardness of 5 Feldspar has a hardness of 6 Quartz has a hardness of 7 Topaz has a hardness of 8 Corundum has a hardness of 9 Diamond has a hardness of 10 A mineral will scratch all those above it in the series, and will be scratched by those below. A weighted diamond cone drawn slowly over a surface will leave a path the width of which (measured by a microscope) varies inversely as the scratch hardness. Cutting hardness is measured by a standardized drilling machine, and has a limited application in machine-shop practice. Rebounding hardness is commonly measured by the Shore scleroscope, illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter, 3/4 in. in length, and weighing about 1/12 oz. is dropped a distance of 10 in. upon the test piece. The height of rebound in arbitrary units represents the hardness numeral. Should the hammer have a hard flat surface and drop on steel so hard that no impression were made, it would rebound about 90 per cent of the fall. The point, however, consists of a slightly spherical, blunt diamond nose 0.02 in. in diameter, which will indent the steel to a certain extent. The work required to make the indentation is taken from the energy of the falling body; the rebound will absorb the balance, and the hammer will now rise from the same steel a distance equal to about 75 per cent of the fall. A permanent impression is left upon the test piece because the impact will develop a force of several hundred thousand pounds per square inch under the tiny diamond-pointed hammer head, stressing the test piece at this point of contact much beyond its ultimate strength. The rebound is thus dependent upon the indentation hardness, for the reason that the less the indentation, the more energy will reappear in the rebound; also, the less the indentation, the harder the material. Consequently, the harder the material, the more the rebound. Indentation hardness is a measure of a material's resistance to penetration and deformation. The standard testing machine is the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter, is forced into the test piece with a pressure of 3,000 kg. (3-1/3 tons). The resulting indentation is then measured. While under load, the steel ball in a Brinell machine naturally flattens somewhat. The indentation left behind in the test piece is a duplicate of the surface which made it, and is usually regarded as being the segment of a sphere of somewhat larger radius than the ball. The radius of curvature of this spherical indentation will vary slightly with the load and the depth of indentation. The Brinell hardness numeral is the quotient found by dividing the test pressure in kilograms by the spherical area of the indentation. The denominator, as before, will vary according to the size of the sphere, the hardness of the sphere and the load. These items have been standardized, and the following table has been constructed so that if the diameter of the identation produced by a load of 3,000 kg. be measured the hardness numeral is found directly. TABLE FOR BRINELL BALL TEST ------------------------------------------------------------------------ Diameter of Ball Hardness Number Diameter of Ball Hardness Number Impression, mm. for a Load of Impression, mm. for a Load of 3,000 kg. 3,000 kg. --------------------------------------------------------------------- 2.0 946 4.5 179 2.1 857 4.6 170 2.2 782 4 7 163 2.3 713 4.8 156 2.4 652 4.9 149 2.5 600 5.0 143 2.6 555 5.1 137 2.7 512 5.2 131 2.8 477 5.3 126 2.9 444 5.4 121 3.0 418 5.5 116 3.1 387 5.6 112 3.2 364 5.7 107 3.3 340 5.8 103 3.4 321 5.9 99 3.5 302 6.0 95 3.6 286 6.1 92 3.7 269 6.2 89 3.8 255 6.3 86 3.9 241 6.4 83 4.0 228 6.5 80 4.1 217 6.6 77 4.2 207 6.7 74 4.3 196 6.8 71.5 4.4 187 6.9 69 ------------------------------------------------------------------------ Next: Nickel Previous: Fatigue Tests
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