Machinery's Handbook, 31st Edition
500 SUBZERO TREATMENT OF STEEL involving temperature and dimensional changes; but if this cushioning effect is removed, particularly at very low temperatures as in subzero treatments, there may be danger of cracking, especially with tools having large or irregular sections and sharp corners offering relatively low resistance to stresses. This effect is one reason why subzero treatments may differ in regard to the cooling and tempering cycle. Stabilizing Dimensions of Gages or Precision Parts by Subzero Cooling.— Transformation of austenite into martensite is accompanied by an increase in volume; consequently, the transformation of austenite, that may occur naturally over a period of months or years, tends to change the dimensions and form of steel parts, and such changes may be serious in the case of precision gages, close-fitting machine parts, etc. To pre - vent such changes, the subzero treatment has proved effective. Gage blocks, for example, may be stabilized by hardening followed by repeated cycles of chilling and tempering, to transform a large percentage of the austenite into martensite. Order of Operations for Stabilizing Precision Gages: If precision gages and sine- bars are heat treated in the ordinary manner and then finished without some stabilizing treatment, dimensional changes and warpage are liable to occur. Subzero cooling provides a practical and fairly rapid method of obtaining the necessary stabilization by transforming the austenite into martensite. In stabilization treatments of this kind, tempering is the final operation. One series of treatments that has been recommended after hardening and rough- grinding is as follows: a) Cool to − 120°F ( − 84°C). This cooling period may require from 1 to 6 hours, depend ing on the size and form of the gage. b) Place gage in boiling water for 2 hours (oil or salt bath may also be used). Note: Steps Item a) and Item b) may be repeated from two to six times, depending on the size and form of the gage. These repeated cycles will eventually transform practically all the austenite into martensite. Two or three cooling and drawing operations usually are sufficient for such work as thread gages and gage blocks. c) Follow with regular tempering or drawing operation and finish gage by lapping. Series of Stabilizing Treatments for Chromium Steel: The following series of treatments has proved successful in stabilizing precision gage blocks made from SAE 52100 chro mium steel. a) Preheat to 600°F (316°C), then heat to 1575°F (857°C) for a period of four minutes. b) Quench in oil at 85°F (29°C). Uniform quenching is essential. c) Temper at 275°F (135°C) for one hour. d) Cool in tempering furnace to room temperature. e) Continue cooling in atmosphere of industrial refrigerator for 6 hours with tempera ture of atmosphere at − 120°F ( − 84°C). f) Allow gage blocks to return to room temperature and again temper. Note: The complete treatment consists of six subzero cooling periods, each followed by a tempering operation. The transformation to martensite is believed to be complete even after the fifth cooling period. The hardness is about 66 RC (Rockwell C scale). Transformation is checked by magnetic tests based upon the magnetism of martensite and the nonmagnetic qualities of austenite. Stabilizing Dimensions of Close-Fitting Machine Parts.— Subzero treatment will always cause an increase in size. Machine parts subjected to repeated and perhaps drastic changes in temperature, as in aircraft, may eventually cause trouble due to growth or warpage as the austenite gradually changes to martensite. In some instances, the sizes of close-fitting moving parts have increased sufficiently to cause seizure. Such treatment, for example, may be applied to precision bearings made from SAE 52100 or alloy carburizing steels for stabilizing or aging them. Time aging of 52100 steels after hardening has been found to cause
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