Machinery's Handbook, 31st Edition
Hardness Testing 503 Rockwell Hardness Test.— The Rockwell hardness tester is essentially a machine that measures hardness by determining the depth of penetration of a penetrator into the speci men under certain fixed test conditions. The penetrator may be either a steel ball or a diamond spheroconical penetrator. The hardness number is related to the depth of in- dentation, and the number is higher the harder the material. A minor load of 10 kg is first applied, causing an initial penetration; the dial is set at zero on the black-figure scale, and the major load is applied. This major load is customarily 60 or 100 kg when a steel ball is used as a penetrator, but other loads may be used when necessary. The ball penetrator is 1 ⁄ 16 inch in diameter normally, but other penetrators of larger diameter, such as 1 ⁄ 8 inch, may be employed for soft metals. When a diamond spheroconical penetrator is employed, the load usually is 150 kg. Experience decides the best combination of load and penetrator for use. After the major load is applied and removed, according to standard procedure, the reading is taken while the minor load is still applied. The Rockwell Hardness Scales.— The various Rockwell scales and their applications are shown in the following table. The type of penetrator and load used with each are shown in Table 5 and Table 6 , which give comparative hardness values for different hardness scales. Scale Testing Application A For tungsten carbide and other extremely hard materials. Also for thin, hard sheets. B For materials of medium hardness such as low- and medium-carbon steels in the annealed condition. C For materials harder than 100 RB on the Rockwell B scale. D Where a somewhat lighter load is desired than on the Rockwell C scale, as on case-hardened pieces. E For very soft materials such as bearing metals. F Same as the Rockwell E scale but using a 1 ⁄ 16 -inch ball. G For metals harder than tested on the Rockwell B scale. H & K For softer metals. 15-N; 30-N; 45-N Where a shallow impression or a small area is desired. For hardened steel and hard alloys. 15-T; 30-T; 45-T Where a shallow impression or a small area is desired for materials softer than hardened steel. Shore’s Scleroscope.— The scleroscope is an instrument that measures the hardness of the work in terms of elasticity. A diamond-tipped hammer is allowed to drop from a known height on the metal to be tested. As this hammer strikes the metal, it rebounds, and the harder the metal, the greater the rebound. The extreme height of the rebound is recorded, and an average of a number of readings taken on a single piece will give a good indication of the hardness of the work. The surface smoothness of the work affects the reading of the instrument. The readings are also affected by the contour and mass of the work and the depth of the case, and, in carburized work, the soft core of light-depth carbu- rizing, pack-hardening, or cyanide hardening, absorbing the force of the hammer fall and decreasing the rebound. The hammer weighs about 40 grains, the height of the rebound of hardened steel is in the neighborhood of 100 on the scale, or about 6 1 ⁄ 4 inches (158.8 mm), and the total fall is about 10 inches or 255 millimeters. Vickers Hardness Test.— The Vickers test is similar in principle to the Brinell Hard- ness test. The standard Vickers penetrator is a square-based diamond pyramid having an included point angle of 136 degrees. The numerical value of the hardness number equals the applied load in kilograms divided by the area of the pyramidal impression. A smooth, firmly supported, flat surface is required. The load, which is usually applied for 30 sec - onds, may be 5, 10, 20, 30, 50, or 120 kilograms. The 50-kilogram load is the most usual. The hardness number is based upon the diagonal length of the square impression. The Vickers test is considered to be very accurate and may be applied to thin sheets as well as to larger sections with proper load regulation.
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