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HARDNESS TESTING (TYPES)

Brinell Hardness Test: The Brinell test for determining the hardness of metallic materials consists in applying a known load to the surface of the material to be tested through a hardened steel ball of known diameter. The diameter of the resulting permanent impression in the metal is measured and the Brinell Hardness Number (BHN) is then calculated for a formula.

The Rockwell hardness tester is essentially a machine that measures hardness by determining the depth of penetration of a penetrator into the specimen under certain fixed conditions of test. The penetrator may be either a steel ball or a diamond sphero-conical penetrator. The hardness number is related to the depth of indentation and the higher number, the harder the material. A minor load of 10 kg is first applied which causes an initial penetration; the dial is set at zero on the black-figured scale. And the major load is applied.

This major load is customarily 60 kg. Or 100 kg. When a steel ball is used as a penetrator but other loads may be used when found necessary. The ball penetrator is 1/16 inch in diameter normally; but other penetators of larger diameter, such as 1/8 inch, may be employed for soft metals. When a diamond sphero-conical 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.

Shore's Scleroscope: The scleroscope is an instrument which 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 the mass of the work and the depth of the case, in carburized work, the soft core of light-depth carburizing, pack-hardening, or cyanide hardening, absorbing the force of the hammer fall and the decreasing the rebound. The hammer weights about 40 grains, the height of the rebound or hardened steel is in the neighborhood of 100 on the scale, or about 6-1/4 inches, while the total fall is about 10 inches or 255 millimeters.

Vickers Hardness Test: The Vickers test is similar in principle to the Brinell 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 usually is applied for 30 seconds, may either be 5, 10, 20, 30, 50 or 120 kilograms. The 50-kiliogram load is usually employed. The hardness number is based upon the diagonal length of the square impression. The Vickers test, which is considered very accurate may, with proper load regulation, be applied to thin sheets as well as to larger sections.

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 an 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 load in kilograms divided by the projected area of indentation is square millimeters. The indentation number corresponding to the long diagonal and for a given load, may be determined from a table computed for a theoretical perfect indentor. The load, which may be varied from 25 to 2500 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 and 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 millimeters and inches. Readings in Brinell numbers may be obtained by means of a scale designated as M-1.

Keep's Test: With this apparatus a standard steel drill is caused to make a definite number of revolutions, 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, while 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.

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Last modified: October 02, 2001