Annealing.—A very important part of the novice's education is a knowledge pertaining to the annealing of metals. Unlike the artisan in wood, who works the materials as he finds them, the machinist can, and, in fact, with many of the substances, must prepare them so they can be handled or cut by the tools.
Annealing is one of the steps necessary with all cutting tools, and it is an absolute requirement with many metals for ordinary use, as well as for many other articles like glass. This is particularly true in the use of copper.
Toughness and Elasticity.—It means the putting of metals in such a condition that they will not only be less brittle, but also tougher and more elastic. Many substances, like glass, must be annealed before they can be put in condition for use, as this material when first turned out is so brittle that the slightest touch will shatter it, so that it must be toughened.
Malleable or wrought iron, if subjected to pressure, becomes brittle, and it is necessary to anneal it. Otherwise, if used, for instance, for boiler[p. 113] plates, from the rolled sheets, it would stand but little pressure.
The most immediate use the boy will have is the treatment of steel. He must learn the necessity of this process, and that of tempering, in all his cutting tools, and in the making of machinery where some parts are required to be constructed of very hard metal.
The Process.—To anneal steel it must be heated to a bright cherry red and then gradually cooled down. For this purpose a bed of fine charcoal, or iron filings and lime, is prepared, in which the article is embedded, and permitted to remain until it is cold.
There are many ways of doing the work, particularly in the use of substances which will the most readily give up their carbon to the tool. Yellow prussiate of potash is an excellent medium, and this is sprinkled over the cherry-heated article to be annealed. The process may be repeated several times.
Tempering.—This is the reverse of annealing as understood in the art. The word itself does not mean to "harden," but to put into some intermediate state. For instance, "tempered clay" means a clay which has been softened so it can be readily worked.
On the other hand, a tempered steel tool is put into a condition where it is hardened, but this hard[p. 114]ness is also accompanied by another quality, namely, toughness. For this reason, the word temper, and not hardness, is referred to. A lathe tool, if merely hardened, would be useless for that purpose.
Tempering Contrasted with Annealing.—It will be observed that in annealing three things are necessary: First, heating to a certain temperature; second, cooling slowly; third, the particular manner of cooling it.