HARDNESS TESTING. Hardness in the widest sense is that property of any material which causes it to resist deforma tion by external forces. It is necessary to know the exact degree of hardness of a metal so that, by using a metal corresponding to a given analysis and subjecting it to heat treatment, it can be brought to its maximum strength. It is therefore desirable to have some exact method of determining hardness so that a stand ard quality of the product may be obtained. Tests are made to determine the uniformity of the grade of the metal; the tensile and other strengths in relation to its hardness ; the frictional and antifrictional qualities in accordance with its hardness; the elim ination and checking of flows in defective materials ; the exact ness of temper (see TEMPERING) in heat treatment ; and the homogeneity of the metal. Fig. 1 shows the various methods used in the determination of the hardness of a material.
The scratch test as proposed by Mohs is one of the oldest methods of testing, and is used by geologists to determine to some degree the hardness of rock and other similar substances. The scratch test, perfected by Martens, depends upon the width of scratch produced by dragging across the surface of the mate rial a diamond point under a definite load. The average width of the scratches, measured by microscope, is checked against a chart with calibrated numbers on it. A hard surface gives a nar row scratch, and a soft one a wide scratch.
About 'goo, J. A. Brinnell brought out a method of testing which consists in measuring the indentation produced by forcing in the material a hardened steel ball of definite size, under a steady pressure. The Brinnell machine operates on the hydraulic principle and is provided with a dead-weight roller valve for maintaining the desired load on the plunger, at one end of which is located the ball that is used to make the indentation. This consists of a spherical piston, accurately fitted to a cylinder which is connected to a pressure chamber. This piston carries a cross bar and weight. The maximum pressure is limited by the weights, and when the desired pressure is reached the piston rises in the cylinder. The load remains constant as long as the piston chamber is full of oil, and the oil is put under a pressure by a hand-operated plunger pump. The pressure varies from 5o to 3,00o kg., according to the part or kind of material being tested. The area of the impression is obtained with a measuring micro scope, with a magnification of ten, which permits the reading in hundredths of millimetres; this measurement is checked against a chart which gives the degree of hardness.
The scleroscope, invented by A. F. Shore, is another important method of testing hardness. This instrument operates on the rebound principle, and has a diamond pointed cylindrical ham mer (weighing less than an ounce) encased in a vertical glass tube which guides the hammer. This tube has a scale giving the different degrees of hardness, with the numbers starting from the bottom upward, as the high numbers up the scale represent the harder metals. The hammer is brought to the top of the tube by a vacuum, and then, being released, falls on the part to be tested. The height of the rebound is noted on the scale as well as the point where the hammer stops momentarily before it falls downward again, and the degree of hardness is thus determined.
Fig. 2 shows the Rockwell hardness-testing machine. This method of testing is of the latest type, and has gained consider able headway in nearly all lines of industries in the United States and other parts of the world. The results obtained by this method are quick and accurate ; little experience is required by the operator. It is well adapted to mass production methods of manufacturing, on account of the quickness to which a test can be made, averaging about 5 sec. per test. The principle consists of impressing a hardened steel or diamond point into the surface to be tested, and measuring the depth of the penetration—the resistance of the metal indicat ing the degree of hardness. The test points and weights used are of such small size that no damage is done to the piece tested. The averaging depth of the pene tration in the softest of steel is only about o.008 inch. Because of the accuracy required, a positive mechanical means of depth measurement is used. This consists of the dial, shown in fig.
which is calibrated with this mechanism so that a reading can be made quickly.