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Watchmakers' Hand Book

Part II,
MATERIALS EMPLOYED IN HOROLOGY
Page 5

HARDENING

      84. It is well known that by the operation of hardening, which consists in heating a piece of steel to a red heat and immediately chilling it, the hardness is very materially increased.

      Hardening increases the dimensions of the object. A steel collar adjusted to fit a cylinder will slide on more easily after hardening.

      Rolled steel is more liable to be distorted in the hardening than metal which has been forged or hammer-hardened. As a general rule, when steel--especially cast steel--has been carefully annealed, cold-hammered and after working, heated to a blue temper and slowly cooled, it will not be distorted in the hardening, providing the heating is skillfully conducted, and if, at the moment of introducing the object vertically into the bath, the heat is evenly distributed throughout. Some practical men affirm that the mere presence of an oily layer on the surface of the water will check the tendency to distortion.

      A workman frequently pretends that he has some exceptionally good solution for hardening, of which a great mystery is often made; but it is very generally admitted by those who are well-informed that these so-called secrets are a delusion and in no sense superior to pure water. There is a certain amount of truth on both sides, and the former class are somewhat justified by experiments with the various solutions enumerated below. We may, however, lay down the three following conditions as essential to the successful conduct of the operation of hardening: (1) The steel must be carefully prepared and worked; metal that has been skillfully hammered cold or below a red heat, for instance, will harden better than when not so treated; but if hammered too much or carelessly, it will crack; (2) The method of heating should be such that the heat is evenly distributed throughout the object; if, on immersion, its temperature is not uniform, the degree of hardness will vary; (3) The skill of the workman must enable him to detect the exact degree of heat the variety of steel can withstand, and this must on no account be exceeded, for in that case the steel will lose tenacity, will be more or less burnt, etc. In the case of irregular shaped articles, considerable skill is often needed to ensure that the several parts of the mass shall be cooled at, approximately, the same rate.

      85. Methods of hardening. The baths used for hardening may be classed under three heads: Tough, Hard, and Glass-hard. It must be understood, however, that these classes may be made to merge more or less into one another, by varying the degree of temper.

      (88) The following receipts are drawn from various sources, and the reader is recommended to select the one which he finds on trial to be best adapted to his requirements, for, as Prof. Akerman has pointed out, there are very many conditions exceedingly difficult of calculation that influence hardening, and hence it follows that a workman accustomed to hardening often considers that only one method, which he has been in the habit of employing, can be used for a certain purpose, while another equally skillful workman can attain the same result by a method essentially different.

      I Tough. Tallow; tepid water; oil; resin; sealing-wax; lead; beeswax; a solution of 3 to 4 parts (by weight) of gum arabic in 100 parts of water; 1 part of soft soap in 100 parts of water; cold water with a layer of oil over it, the thickness of which varies with the degree of hardness; 10 parts mutton suet, 5 parts resin, 2 parts sal ammoniac, and 35 parts olive oil.

      II Hard. Cold water; water containing various salts, such as sal ammoniac and sea-salt; a solution of 5 parts sea-salt and 1 part sal ammoniac in 20 parts of water; 4 parts sulphuric acid, 50 parts sea-salt, 10 parts alcohol, and 1,000 parts water (all by weight); 4 parts sulphuric acid, 1 part nitric acid, 1 part pyroligneous acid in 1,000 parts water (to be used very cold).

      III Glass-hard. Mercury; nitric acid; opium; any cyanide.

      86. As a rule it is well to employ tallow for the hardening of small objects in which hardness without brittleness is needed. Oil renders the surface harder than the interior, and soapy water has the same effect. Saline solutions generally give the great hardness. Very minute drills may be hardened by simply whisking them about in te air after heating the blade to redness, and small objects may be hardened by pressing between two cold surfaces, as those of the hammer and anvil.

      If hardened in nitric acid, opium, or mercury, the hardness of steel is so great that it will easily cut glass. But such steel is brittle and all the more delicate according as the precise temperature necessary (which is not very high) has been exceeded. For it must always be borne in mind that steel which has been heated too highly has deteriorated in quality and will remain brittle.

      87. Precautions to be observed in hardening. In the case of delicate pieces it is necessary to avoid the use of the blow-pipe, as the current of air causes the surface to scale, and, as is well known, the metal being unevenly heated will be distorted in the hardening, and will not be uniformly hardened.

      It is better to enclose the article between two pieces of ignited charcoal, or in a metal tube, or to bring it in contact with a sufficiently hot piece of metal, etc. An excellent plan is to heat the article in a bath of hot lead, or of lead and tin in proportions dependent on the temperature required. The heating is thus exceedingly uniform, and, if operating in a dark room, the temperature can be accurately judged.

      When it is required to harden an object without discoloring the surface or destroying the polish, it may be placed in a tube, and completely surrounded with powdered wood charcoal, or, preferably, animal charcoal. The whole after being heated is plunged in water without the steel being in any way exposed to the air. The pwder must be heaped up as a precaution against access of air. On being taken from the water, the steel is at once placed in alcohol, and if at all dull it will generally be only necessary to rub the surface with a little rouge.

      It is essential that the animal charcoal be previously heated in order to expel moisture, as otherwise it would adhere to the surface and produce marks and uneven irregularity in the hardness.

      As a rule the object must be immersed in the cooling liquid vertically in the direction of its greatest length, and if of unequal thickness, the stout portion should touch the surface first, so that the metal may cool more uniformly. In hardening large masses of steel, various devices are resorted to in order to insure uniformity in the cooling, but space prevents us from entering more fully into this question.

      The vessel must be of such a depth that the object will not reach the bottom until quite cold. It is liable to distortion if introduced sideways, or if the vessel is too shallow.

      The method described above for protecting the surface from the action of the fire should be adopted when hardening delicate or complicated articles; but in the case of drills, for instance, a simple coating of one of the following preparations is sufficient.

      When an object is hardened in a saline solution, it is well to cover it with a paste composed of water, salt and flour (some use yeast and salt for this purpose), or a thin clay. This precaution prevents any blistering or oxidation of the surface. If it be desired to avoid oxidation, and, at the same time, to restore to the steel the carbon it has lost owing to the action of the fire, it must be rolled, while still wet, in another paste, containing powdred horn or leather, or some calcined animal matter. Delicate parts can also be protected by a layer of beeswax and olive oil made hot.

      In hardening small drills, very good results are obtained by enclosing the blade in a pellet formed of prussiate of potash, lard and castile soap, and cooling in beeswax, or the surface may be protected by a layer of soft soap.

      It will be well to warn the beginner that, if an object is not entirely immersed in the cooling liquid, it should never be held still, but rapidly moved up and down, as otherwise there is a liability to crack at that part which was level with the surface.

      As a watchmaker only uses steels of the best quality, he should, in hardening never exceed a cherry-red heat, and cherry-red comprises three distict tints (77); Incipient cherry-red, cherry-red, and clear cherry-red. The second of these should not be exceeded in hardening cast steel, and the third should be taken as an extreme limit in the case of shear steel.

      Ice-cold water should never be employed, but the extreme chill should be first taken off. Indeed, it is found that frosty weather interferes materially with the operation of hardening.

      Some workmen maintain that the hardening is done better if the water has been long used for the purpose without renewal.

Submitted by: Samuel Kirk (##)

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