The process of hardening and annealing brass is exactly the reverse of that used with steel. Brass is hardened when it is heated and allowed to cool slowly ; it is softened or annealed when heated and cooled suddenly. When annealing brass, care should be taken that it is evenly heated throughout and that it is evenly cooled.
Casehardening is the process of making a hardened steel case around a piece of iron. To do this the outer shell of the iron is converted into steel. The depth to which this conversion takes place varies from 1/64 to 1/32 inch. It depends upon the temperature to which the piece is subjected and the time during which the heat is continued. There arc two methods of doing this work ; the prussiate of potash and the box methods. The former is the more rapid; the latter the more reliable.
In casehardening with prussiate of potash, crush the material to a very fine powder. Then heat the steel to a red heat and apply the powdered chemical to the surface to be hardened The powder may be sprinkled on with a spoon and then rubbed over the surface with the back of the same. The iron must be hot enough so that the potash will melt, and run freely. If, during the process, the iron cools so that the potash is not kept hot, it must be put back into the tire and again brought to a red heat. It is then cooled in cold water. This process makes a very hard (MM: one that cannot be touched with a file. It is, however, expensive. Therefore, it is not used where a large number of pieces are to be hardened. Neither is it suited for surfaces that art) large or irregular in form. When hardening with prussiate of potash the heat decomposes it and the contained carbon unites with the iron. This forms a steel which in turn is hardened when it is suddenly cooled.
When a number of pieces or one single large piece is to be hardened, the box process should be used. This is a simpler and safer method. It is more easily executed and there is not the same danger of excessive warping as in the first described process. More or less warping, however, does occur whenever a piece of iron is case hardened.
In the box process of casehardening, the pieces are packed in a box with the hardening material. The box should be of wrought or cast iron, preferably the former- It should be of such size that all of the pieces may be packed without any of them touching the top, bottom or sides. In packing, first put a layer of the hardening material in the bottom. Next put in a layer of pieces to be hardened and pack the hardening material about and over them, ramming it down. Then put in another layer of pieces and pack in the same way. Continue until the box is filled. Fasten on the cover and fill all openings with fire clay so that the box is absolutely air tight. Now set the box and its contents in a furnace where it will be subjected to an even temperature. Raise it gradually to a bright .cherry red and keep it in that condition for twenty-four hours. At the end of that time, remove it from the fire, and upon opening, cool the contents in cold water.
When the pieces are taken from the water they will be found quite hard and slightly warped. They may be straightened by springing them in a press.
In this work care should be taken to place the largest articles at the bottom of the box. They should all be so packed that as they settle during the process, they will be so held that one piece does not bear unnecessarily upon another and thus cause a needless amount of warping. When cooling, put the pieces in edgewise so that the cooling may be uniform and the amount of warping made as small as possible.
Various materials are used to assist in hardening. The most common and the one that is the cleanest and least offensive is bone dust. There are a numher of compounds upon the market, which are all prepared and ready for use. These usually have bone dust as a base with other ingredients added. It is better and cheaper to buy these pronations than it is to attempt to make the mixture. Excellent results can be obtained from them. Where it is desired to do case hardening, and bone dust is not available, other materials may be used. Hoofs, old leather, salt and urine make an excellent casehardening mixture. To use these materials, cut the leather and hoofs into pieces of from ½ inch to 1 inch in largest dimensions. A satisfactory proportion will be 20 pounds of old leather, 15 pounds of hoofs, 4 pounds of salt, to which a gallon of urine may be added after the packing is completed.