Practical forging and art smithing

by Thomas F. Googerty

Fig. 114.

In hardening the tool, it is heated 2½ inches of its length and 1½ inches is cooled in water to harden. The remaining heat gradually runs thruout the whole chisel and may be noted by the faint yellow color on the bright part of the tool traveling towards the cutting end. This faint yellow temper color, due to the heat and air, is followed with darker colors; if let run too much all of the hardness would be taken out of the tool. Four hundred and thirty degrees Fahrenheit would be about a light straw color, leaving the steel very hard. About 600° F. would be the darkest color, nearly black. This is as hot as steel can be made and still leave a trace of hardness. This temper is too soft for a chisel but about right for springs; therefore when the very dark purple temper color covers the whole bright part of the chisel the point is dipped in water. The chisel is then set in a dry place on the forge to cool slowly. The temper color must run to the end of the chisel very slowly. The reason for this is that if the temper color comes slow, the chisel is tempered farther back from the point. The temper colors on the surface of the bright steel are obtained by different degrees of heat, as it travels from the remaining heat left in the tool when the piece was hardened. The less heat allowed to travel toward the end of chisel, the paler the temper color and the harder the chisel; therefore, the faint yellow color indicates that the steel is very hard. The darker the temper color becomes the softer the tool.

The best chisels are those that are file proof. If, after hardening and tempering a chisel, it cannot be cut with a file, it is too hard and the temper must be run out more. If the grain of steel is very fine when broken the chisel had the proper heat when quenched, but if it looks coarse the tool was too hot when cooled and must be annealed, rehardened and tempered. A little judgment will enable one to determine the proper hardness for all tools of this character by noting these temper colors. The above explanation in a general way applies to the working of all carbon steel tools.

Spring Tempering.

There are many kinds of springs that are hardened and tempered. The methods of handling are about the same with all. As an example, a piece of spring steel 5 by 1 by ¹⁄₁₆ inches is to be tempered. In doing this, the piece is caught at one end with a pair of light tongs. The steel is heated to a dark red and dipped into a can of sperm oil, or equal parts of lard and tallow. When cool it is held over the fire until the surplus oil takes fire and blazes off. It is redipped in the oil, and the oil is burned three times in all. It is then partly cooled in the oil and set on the forge until cool, when it is ready for use. Steel is manufactured especially for springs. It is called spring steel. It is made in a different way from tool steel, by the open hearth process. It differs in quality and cannot be absolutely guaranteed. The steel is never free from all foreign elements which might be detrimental to its quality.

Tempering Thin Pieces of Steel.

In hardening thin pieces of steel such as knives, very thin milling cutters, etc., there is always difficulty in preventing warping after hardening. Two heavy surface plates, planed on one side, are used. On one of these plates equal parts of tallow and lard are spread ¼ inch thick. The knife is heated in a steam pipe with one end plugged and having fire under and over it. When an even red heat is reached, the knife is brought out and set on the oil and at the same time the top plate is set onto the knife until cool. This hardens the blade and keeps it from springing. The knife is brightened and the temper is drawn to a dark straw color by holding it on a hot iron.

Very small pieces of steel are packed into an iron pipe or box surrounded with charcoal. The whole is then heated red and the pieces are dumped out and cooled in water. To draw temper, they are put in an iron ladle filled with lard oil that is heated on the fire.