The Case-Hardening, Annealing and Tem pering of Steel.
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Annealing.
During the processes of rolling and forging, and its subsequent machining, steel is subjected to various strains; these strains produce serious distortion unless they are relieved by the softening, or annealing, of the material. Annealing consists of slowly and evenly heating the metal to a temperature of from 1,300 to 1,600 degrees Fahis, the heat varying with the class of steel, and of then allowing it to cool very slowly. When raised to the temperatures named, the molecules in the steel re-arrange themselves, and are relieved of all strain, and, if the steel is then allowed to cool down slowly, the equilibrium of the mole
cules will be maintained. The best way of annealing a piece is to allow it to cool down 8lowly in the same furnace, or oven, in which it has been heated, and, for that purpose, nothing could be better, or more convenient to use, than a gas-heated furnace because of the means of adjustment which are provided for the regulation of the temperature. If necessary, the gas supply maygradually he reduced, by automatic means, at any desired rate.
American engineers frequently make use of a quick method of annealing steel, and, although very satisfactory results have been obtained thereby, it is not to be recommendea for accurate work, but when urgently needed for machining operations, a piece of hard
steel may safely be partly softened by its means. The ',recess. consists of slowly heating the steel until it is at a dull red heat, and then alien ing it to cool in the open an until it will no longer char a piece of soft wood which is pressed against it; it may then be plunged inv., cold soapy or greasy water.
Tempering.
Tempering steel means reducing, or moderating, its degree of -hardness. When steel is hardened, it becomes brittle, and it is for that reason that parts of machinery that are liable to be submitted to shocks are made from low-earbon steel, which is then casehardened as already described, instead of being made of high-carbon steel ; the latter grade of steel hardens right through, a nil. conseli u en Fly, becomes brittle. The steel from which milling cutters, lathe tools, and other forms of cutting tools are made mast be uniformly hard right through and such parts are best made from one or other of the special to: l steels that are now obtainable. Such (eating tools, after hardening, must be tempered ; their sharp edges will otherwise quickly he chipped away and become useless. As tempering steel reduces its hardness, it also reduces its wearing qualities, but it is more satisfactory and economical to temper a tool than to run the risk of its breaking. possibly at a critical moment. In order to temper a piece of steel, it should be raised to a definite temperature—much below that from which it was quenched for hardening—and allowed to cool slowly. The quality of the temper varies with the temperature to which the hardened part has been raised, and is frequently identified by the colour of the oxide of iron that is formed on the surface of the work. The colour of this oxide varies, with a piece of average tool steel, from a pale straw colour (at about 420 degrees Fahrenheit), to a blackish blue (at about 640 degrees Fahrenheit). A temperature change of 20 degrees will cause a considerable difference in the quality of the tempered article, and for that reason, therefore, it is important that the greatest care should be exercised in order to secure uniform results. Probably the best method of ensuring uniform results is to heat the parts in a bath of oil, such as cottonseed oil, fish oil, etc., which bath IS kept at a uniform and suitable temperature by means of gas burners. Such a tempering bath was illustrated in last week's issue. The exact temperature of the bath may be ascertained by means of a thermometer— either of the mercury type, or one of the "saturated-vapour" thermometers which were described in our issue of the 30th December last ; there is then no need to rely on the judgment of the operator, so far as colour is
concerned. When tempering is effected in oil, the colour of the steel remains constant because, being protected by the oil from the influence of the atmosphere, no oxide is formed on the surface of the tempered parts. Hardened parts which have been ground and polished may, by this means, be tempered without resulting change of colour.
Conclusion.
Although the best-possible results, so far as they are affected by constant temperature are concerned, may be obtaMed with the use of gas-fired or unheated furnaces, great care should be exercised during every stage of the processes of hardening, case-harden
ing, annealing and tempering of steel parts. Many of the high-grade steels at present on the market reipiire very special treatment, and the existence ef definite temperature i•onclitions. The makers of slid' steels will always advise users as to the best methods of procedure, and the temperatures that will insure the best results. No one is more anxious than the steel milker that his material should prove satisfactory. and, therefore, any advive which he gives should be carefully observed and not lightly disregarded, as is so ,,rtton the case. One of the greatest mistakes that can be made by any steel worker is the attempt to secure a greater degree of hardness in self-hardening steels, (Hr a deeper
" case " in case-hardening steels, by the application of higher temperatures than those advocated by the makers of the steel. All such attempts invariably result in the burning of the material, and the effecting of a complete change in the structure of the metal—a change of a most-unsatisfactory nature. Some steel workers will argue that, when once burned, a piece of steel may be restored to its original condition by special treatment with certain chemicals. the composition of which chemicals they claim to be their secret ; it may safely be said, however, when once the steel is burned, the best course is to have another part made at once to replace it, and to " scrap " the spoiled piece.