This may be accomplished in many ways according to the nature of the material. Nearly all metals can be hardened by rolling, hammering, stretching, bending, or by the addition of other metals. They can also be annealed or softened by heating to various temperatures and subsequent cooling. The longer steel is in cooling, within reason of course, the softer it becomes. What is known as spring brass, hard-rolled copper, and hard-rolled phosphor bronze, are hardened by rolling. Wrought iron and mild steel can be hardened by case hardening, i.e. the surface or skin is hardened and the interior left soft. This is a most useful combination in many instances. It is done as follows: The article to be case hardened is placed in an iron box containing substances rich in carbon, such as powdered charcoal, horns, bones, leather cuttings, etc.; the box is then luted up with clay and made airtight, put into a furnace and kept at a red heat until the objects have absorbed sufficient carbon. This may take from twelve hours upwards. The objects are then dumped into a tank of water which is kept agitated. Small objects may be case hardened by heating to a red heat, then rolling in yellow prussiate of potash, which fuses on the surface; on being reheated the mass is quenched in cold water. This operation has to be repeated until the necessary hardness is obtained. Cast iron can only be hardened by being chilled. Carbon Tool Steel can be hardened, tempered, and annealed, and these properties make it universally useful. It can be hardened by heating to a bright red heat and quenching in water, brine, oils, fat, wax, lead, and mercury. But the various kinds of tool steel vary as to the heat required to obtain the best results. Tempering is bringing the hardened steel to a temper or proper degree of hardness or toughness by reheating after hardening. The temper is judged by the colour of the film of oxide on the brightened surface. It is always necessary to brighten the tool after hardening. To anneal is to soften by heating and cooling slowly by burying in ashes or lime. The oxides seen in colours on the brightened surface of steel while it is being tempered are combinations of the oxygen of the air with the metal.

Non-Tempering Steel

This is simply carbon tool steel which is used for chisels and like tools; it is hardened by dipping right out in cold water from a cherry red, and as the name implies it does not have to be tempered.

Point hardening is a method sometimes adopted for hardening chisels and similar tools and is conducted as follows: The chisel is heated for about 2 in. from the cutting edge; the point is placed in water to a depth of three-quarters of an inch until cold; the chisel is then taken out and the point is rubbed with a piece of grit stone until bright; the heat from the remaining portion runs back to the point, and when the cutting edge is the right colour it is cooled right out. In oven tempering, ovens are used for tempering tools right through and in some cases is preferable. The ovens are very reliable and can be heated to any known degree of temperature. In small shops where expensive appliances are not available, tempering is done with a gas blowpipe or on a sand bath, which is very useful and simple. A metal box is filled with fine sand and placed on the fire or a gas stove. The tool is hardened and brightened and then laid in the hot sand and turned over and over until the correct temper is obtained. Another method is to heat a piece of iron and lay the tool on it. Lead, tin, and bismuth, alloys are also used. A bath of the molten metal is made and the articles to be tempered are immersed in it. The molten metal can be tested with a pyrometer and very accurate tempering can be accomplished. For very small tools a piece of pipe can be made hot and the article held inside until the right colour is obtained. Self-hardening or air-hardening steels owe their hardness to the presence in them of chromium, tungsten, or molybdenum. They are forged at a bright red heat, and to harden them the cutting edges of the tools are heated to a white glassy heat and cooled by a blast of cold air. This treatment would ruin ordinary carbon tool steel.

Tempering Scale.