To temper steel is to return it to a state of molecular equilibrium at atmospheric temperature by relieving any strains in the metal which have been caused by sudden quenching, and also to correct any exaggeration of certain properties which have been caused by the hardening process. In other words, the object of tempering is to reduce the brittleness in hardened steel, and to remove the internal strains caused by the sudden cooling in the quenching bath. The tempering process consists in heating the piece of work by one of various means to a certain temperature, and in permitting it to cool gradually.
The temperature to which a piece should be raised for tempering depends on several conditions: the use to which it is to be put; the condition in which it has been left by quenching; and the composition of the steel. The degree of heat to which the tool to be tempered is heated determines the degree of toughness it has attained - the higher the tempering heat, the less brittle, and also the less hard, the tool is. The maximum temperature desired need be maintained only long enough to be sure the piece is heated evenly. Steels which are reheated slightly after hardening give a freedom to the molecular change that lessens the molecular rigidity set up by the hardening process.
Tools are always left as hard as it is possible to leave them and still have them tough enough for the work for which they are intended. In reducing the brittleness of the steel, some of the hardness is of necessity taken out, and tempering is therefore sometimes spoken of as a reduction of the hardness, but it is in reality merely a reduction of the brittleness. After a tool or piece of steel has been hardened, some of the brittleness is taken out by a slight reheating to a low temperature. These temperatures vary from 200° Fahrenheit, to about 650° Fahrenheit, and are determined in various ways. The simplest and perhaps the most commonly used method is to polish the steel after it has been hardened and then to reheat the part to be tempered until the surface shows a certain color.
If any bright piece of iron or steel is heated, when a temperature of about 400° Fahrenheit is reached, the surface turns pale yellow. As the temperature is increased, this yellow grows darker until at about 500° Fahrenheit it is a decided brown. When 600° Fahrenheit is reached, a deep blue color shows on the surface. These colors are produced by a thin scale which is formed on the surface of the steel and are no indication whatever of hardness, merely showing to what heat steel or iron has been raised.
As the object of tempering is simply to reduce the brittleness and to remove the internal strains caused by the sudden cooling in quenching, the composition of a tempering bath is of little importance as compared with that of a quenching bath. Aside from the operator's convenience and the possible bad effects upon his health, the different baths used for this operation must be considered chiefly with regard to initial cost and lasting quality, and also effect on finish.
The two main points to be considered when using an oil tempering furnace are: (1) to have the heat uniform throughout; and (2) to leave the pieces to be tempered in the oil long enough to have attained the heat of the oil throughout when they are taken out. While oil is the most widely used medium for tempering tools in quantities, other means and methods are employed, especially by those who have tools to temper in small quantities when the expense of installing and running an oil ternpering furnace would not be warranted. First there is the old-style tool hardener's method of only partly cooling the tool when quenching, then quickly withdrawing it, polishing off the working surface, and letting the heat which remains in the tool produce the required temper as judged by the color. Heated sand is also used to produce the temper, likewise using the color scheme as a medium for judging the degree of hardness. Another common method is to heat the steel in a bath of red hot lead. The lead is heated in a pot or crucible, to the hardening heat of the steel. The top of the lead is covered with powdered charcoal or coal to prevent the formation of the slag or dross on top. When steel is heated in lead it must be perfectly clean, dry, and free from rust.
Fig. 192. B. C. and H. Company Thermometer.