SUBZERO TREATMENT OF STEEL Machinery's Handbook, 31st Edition
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than 2 1 ⁄ 2 hours at 1050°F (566°C), there will not be sufficient precipitation of carbides at the tempering temperature to allow complete transformation of the retained austenite on cooling, whereas more than 3 hours causes some loss in room temperature hardness, hot hardness, strength, and toughness. f) Repeat subzero treatment, step Item d). g) Repeat the tempering operation, Item e). Note: The time for the second tempering operation is sometimes reduced to about 1 ⁄ 2 the time required for the first tempering. 2) Single Subzero Treatment: This treatment is the same as procedure Item 1) except that a second subzero cooling is omitted; hence, the cycle consists of hardening, subzero cool ing, and double tempering. Procedure Item 3), which follows, also has one subzero cooling period in the cycle, but this follows the first tempering operation. 3) Tempering Followed by Subzero Treatment: This treatment is for tools having irregular sections, sharp corners, or edges where cracks might develop if the hardening operation were followed immediately by subzero cooling. a) Preheat and heat for hardening. b) Preheat and heat for hardening. c) Quench as described under procedure Item 1). d) Temper to required hardness. e) Cool to subzero temperature − 100 to − 120°F ( − 73 to − 84°C) and then allow the tool to return to room temperature. f) Repeat tempering operation. Testing the Hardness of Metals Brinell Hardness Test.— The Brinell test for determining the hardness of metallic mate rials 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 impres sion in the metal is measured and the Brinell Hardness Number (BHN) * is then calculated from the following formula in which D = diameter of ball in millimeters, d = measured diameter at the rim of the impression in millimeters, and P = applied load in kilograms. BHN D D D d P 2 − − ^ h If the steel ball were not deformed under the applied load and if the impression were truly spherical, then the preceding formula would be a general one, and any combination of applied load and size of ball could be used. The impression, however, is not quite a spherical surface because there must always be some deformation of the steel ball and some recovery of form of the metal in the impression. Hence, for a standard Brinell Hardness test, the size and characteristics of the ball and the magnitude of the applied load must be standardized. In the standard test, a ball 10 millimeters in diameter and a load of 3000, 1500, or 500 kilograms is used. It is desirable, although not mandatory, that the test load be of such magnitude that the diameter of the impression be in the range of 2.50 to 4.75 millimeters. The following test loads and approximate Brinell Hardness Numbers for this range of impression diameters are: 3000 kg, 160 to 600 BHN; 1500 kg, 80 to 300 BHN; 500 kg, 26 to 100 BHN. In making a Brinell Hardness test, the load should be applied steadily and without a jerk for at least 15 seconds for iron and steel, and at least 30 seconds in testing other metals. A minimum period of 2 minutes, for example, has been recommended for magnesium and magnesium alloys. (For the softer metals, loads of 250, 125, or 100 kg are sometimes used.) surface area of indentation in sq. mm. loadon indenting tool in kilograms 2 2 π = = According to the American Society for Testing and Materials Standard E10-66, a steel ball may be used on material having a BHN not over 450, a Hultgren ball on material not over 500, or a carbide ball on material not over 630. The Brinell Hardness test is not recommended for material having a BHN over 630. * Brinell Hardness Numbers also may be identified with the letters HB, HBS for hardness measured with a steel indenter, and HBW for hardness measured with a carbide indenter.
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