Steel from 1.4 to 1.5 per cent carbon should be used for tools for working chilled castings or locomotive wheel tires, lathe and planer tools, razors, or any tools required to cut hard materials. Forging should be done at a dull red heat. Welding can scarcely be accomplished with this grade of stock. Hardening should be done at a dark red heat.
87. Injuries.—One of the most common injuries to steel comes from carelessness in the heating for forging. It is one of the important operations, for unless the metal is uniformly heated, violent strains are liable to occur, and, when hardened, the steel will show these strains by cracking. These defects are known as fire cracks.
The smith should always have plenty of fuel surrounding the metal while it is in the fire so that the cold-air blast will not come in direct contact with the metal. The air should be heated by passing through a bed of hot coals before it strikes the steel. It is always necessary to heat steel thoroughly to make it plastic, being careful not to overheat or burn any part of the metal. If it is overheated or burned, it cannot be completely restored to its former state; the grain becomes coarse and the structure weak.
Never let steel lie in the fire to soak up heat after it is hot enough to work. If for any cause it cannot be worked when it is ready, it should be taken from the fire and left to cool, then reheated when it can be worked. By this precaution injury to the steel will be prevented.
If steel is heated so that the outer parts are hotter than the center, the metal will forge unevenly. The outer portion will be forged by the hammer blows, while the center remains almost in the original form. This will also cause an uneven grain, sure to produce cracks when the tool is hardened. Forging at too low a heat will injure the steel in the same manner as uneven heating.
After the steel has been properly heated, and forging has begun, the first blows should be struck rather heavily and followed by lighter ones as the heat vanishes. The forging should cease when the steel gets too cold, but it may be reheated as often as necessary to complete the work.
88. Annealing.—After the steel has been forged to the desired shape, it usually is necessary to do some finishing upon it before it can be hardened and tempered; in order to do this, it must be annealed or softened so that it can be machined or filed into shape. Annealing is the process of softening steel. It is done by heating the steel slowly to an even low red heat and placing it in an iron box containing unslaked lime or fine charcoal and leaving it there until perfectly cold. The object of this process is to retain the heat and prolong the cooling. The box is usually of cast iron, but sheet steel is equally good. It should be placed in a perfectly dry place and rest on bricks, if necessary, to avoid any dampness.
If an annealing box is not at hand, small steel forgings can be softened very satisfactorily by placing them between two boards, then completely covering all with dry ashes and leaving them there until entirely cold. Precaution should be taken here, also, to leave them in a dry place.
Another method, which is sometimes used, is called water annealing. Some mechanics claim to have had good results with it, while others condemn it entirely. By this method the article is heated to a dull red and allowed to cool partly, out of any direct current of air. When all redness has disappeared as it is held in a dark place, it is plunged into water and left there until perfectly cold.
The first method mentioned above is always the best; the second is nearly as good; and only when there is not sufficient time to allow the metal to cool slowly, should water annealing be attempted.