Sometimes the steel is heated red hot, then held until the red entirely disappears. Then it is plunged into cold water. This process is known as water annealing and is a process used for tool steel when quick work is required. It softens the metal so that it can be filed and worked quite easily.
CASE HARDENING
We have learned that carbon gives the hardening quality to steel. Since there is little carbon in soft steel and none in wrought iron, they will not harden as carbon steel does when heated red hot and plunged into water. But there are many small articles which are best made of this soft steel and iron and which must be hardened in some way to make them useful. In order to supply the lacking carbon the metal is put through a process known as case hardening. There are two methods of case hardening. The first method is to heat a piece of soft steel or iron red hot and cover the part to be hardened with cyanide of potassium. The metal will absorb the carbon out of the cyanide and when cooled in water will have taken on a hardened surface. If this is repeated two or three times the hard surface deepens.
Any drug store will sell you some cyanide of potassium. It comes in cakes. The cakes are broken up into small pieces. Be careful to keep the hands as much as possible from contact with the cyanide.
SET SCREW
Set screws are made of soft metal, but the points must be hardened to resist the wear. This is done by heating the set screws red hot, then sticking the point into the cyanide until it cools off. Re-heat and repeat the work. During this time the iron is absorbing carbon. Heat again, plunge into cold water. The carbon forms a case on the end of the screw. Now you have a screw hard on the outside but of soft material in the inside. It is able to resist any shock that carbon steel itself could not stand.
The second method calls for the use of pulverized charcoal and bone. The principle is the same as in the first method. The iron articles to receive a case hardening are placed in a cast-iron box with a layer of charcoal above and a layer of bone dust below, alternating in this way until the box is filled to the top. The box is then placed in a heating furnace and heated to a temperature 1200° F. It is kept at this temperature from three to four hours. The articles all this time are absorbing charcoal from bone dust. The box is taken out of the furnace and plunged into a bath of cold water. From this cold bath it is plunged into a bath of boiling water, then taken out and dried. The pieces are oiled over slightly to bring out the colours. They have not only a hard exterior after this process, but they have a beautiful mottled surface of grays and blues, colours one often sees in parts of small guns, wrenches, etc.
Surgical instruments, guns, small wrenches, etc., are hardened in this way. The equipment for the second method is much more expensive than that for the first; however, the principles involved are the same in both cases.
BRAZING
Brazing is a process of joining two pieces of metal by the use of another metal, such as brass or copper. The use of solder has been explained in the article on soldering. This article will tell about brazing of wrought iron, cast-iron, and steel in the forge fire.