Machinery's Handbook, 31st Edition
SUBZERO TREATMENT OF STEEL 501 changes as large as 0.0025 inch (0.064 mm) in medium size sections. A practical remedy is to apply the subzero treatment before the final grinding or other machining operation. Subzero Treatment of Carburized Parts to Improve Physical Properties.— The subzero treatment has been applied to carburized machine parts. For example, the amount of retained austenite in carburized gears may be sufficient to reduce the life of the gears. In one compo - nent, the Rockwell hardness was increased from 55 to 65 RC without loss of impact resistance qualities; in fact, impact and fatigue resistance may be increased in some examples. Application of Subzero Treatments to High-Speed Steel.— The subzero treatment has been applied to such tools as milling cutters, hobs, taps, broaches, and drills. It is applicable to different classes of high-speed steels, such as the 18-4-1 tungsten, 18-4-14 cobalt, and the molybdenum high-speed steels. This cold treatment is applied preferably in conjunction with the heat treatment, both being combined in a continuous cycle of oper- ations. The general procedure is either to harden the steel, cool it to a subzero temperature, and then temper; or, especially if there is more than one tempering operation, the first tem - pering may precede subzero cooling. The cooling and tempering cycle may be repeated two or more times. The number and order of the operations, or the complete cycle, may be varied to suit the class of work and to minimize the danger of cracking, particularly if the tool has large or irregular sections, sharp corners or edges, or a high cobalt content. A sub- zero treatment of some kind with a final tempering operation for stress relief is intended to increase strength and toughness without much loss in hardness; consequently, if there is greater strength at a given hardness, tools subjected to subzero treatment can operate with a higher degree of hardness than those heat treated in the ordinary manner, or, if preferred, greater toughness can be obtained by tempering to the original degree of hardness. Order of Cooling and Tempering Periods for High-Speed Steel.— The order or cycle for the cooling and tempering periods has not been standardized. The methods that follow have been applied to high-speed steel tools. They are given as examples of procedure and are subject to possible changes due to subsequent developments. The usual ranges of preheating and hardening temperatures are given, but, for a particular steel, the recommended temperatures should be obtained from the manufacturer. 1) Double Subzero Treatment: (For rugged simple tool forms without irregular sections, sharp corners or edges where cracks might develop during the subzero treatment). a) Preheat between 1400 and 1600°F (760 and 871°C). Double preheating is preferable, the first preheating ranging from 700 to 1000°F (371 to 538°C). b) Heat to the hardening temperature. Note: Tests indicate that the effect of subzero treatment on high-speed steel may be influenced decidedly by the hardening temperature. If this temperature is near the lower part of the range, the results are unsatisfactory. Effective temperatures for ordinary high-speed steels appear to range from 2300 to 2350°F (1260 to 1288°C). c) Quench in oil, salt, lead, or air, down to a workpiece temperature of 150 − 200°F (66 − 93°C). Note: One method is to quench in oil; a second method is to quench in oil to about 200 − 225°F (93 − 107°C) and then air cool; a third method is to quench in salt bath at 1050 − 1100°F (566 − 593°C) and then air cool. d) Cool in refrigerating unit to temperature of − 100 to − 120°F ( − 73 to − 84°C) right after quenching . Note: Tests have shown that a delay of one hour has a detrimental effect, and in 10 hours the efficiency of the subzero treatment is reduced 50 percent. This is because the austenite becomes more and more stabilized when the subzero treatment is delayed; consequently, the austenite is more difficult to transform into martensite.) The refrigerating period usually varies from 2 to 6 hours, depending on the size of the tool. Remove the tool from the refrigerating unit and allow it to return to room temperature. e) Temper to required hardness for a period of 2 1 ⁄ 2 to 3 hours. The tempering temperature usually varies from a minimum of 1000 to 1100°F (538 to 593°C) for ordinary high-speed steels. Tests indicate that if this first tempering is less
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