A few secrets of the metallurgist simply told

by Gerald W. Hinkley

HEAT TREATMENT.

The heat treatment of tool steels is of the utmost importance. Tool makers of the old school proved their ability to accomplish certain desired results in the art of heat treatment without really fully understanding exactly how or why they were able to do so. Today, however, progressive manufacturers are using the results of research and such thorough scientific investigation that the process has become far more complicated and complex, and the results obtained are correspondingly more remarkable.

Chemically perfect steel may be easily and completely ruined during the process of melting, cogging, rolling, hammering, annealing, heat treating and tempering. It is the business of the steel manufacturer to carefully guard his product up through the process of annealing, but it usually falls to the tool maker to undertake the delicate operations of heat treatment and tempering.

HARDENING.

The application of heat alone to steel can very materially affect the condition of the structure of the metal, either with or without simultaneous mechanical treatment. Depending upon the degree of heat, the rate of heating and cooling and the duration of such treatment, this application may be decidedly beneficial or harmful as the case may be.

We now know that when steel is heated above the critical point, and is then allowed to rapidly cool, a very marked hardness in the metal is produced. The degree of hardness so attained will, in general, vary directly with (1) the percentage of carbon, (2) the rate of cooling, (3) and the temperature above the critical point from which the cooling takes place. When the steel comes from the rolling mill and from the finishing hammers it is in this hardened condition. Therefore, in order to render it soft and ductile enough to cut and work up into certain desired shapes, sizes and tools, it is necessary to subject the steel to the process of annealing. This operation is usually undertaken by the steel producer, under which circumstances he is able to control his product through this delicate procedure, and deliver the same to his customers in the best possible condition for their use.

ANNEALING.

Annealing has for its object: (1) Completely undoing the effect of hardening, leaving the steel soft and ductile (2) refining the grain, in which case the crystals are allowed to re-arrange and re-adjust themselves, usually growing to a rather large size (3) and removing strains and stresses caused by too rapid cooling. Such cooling strains are particularly likely to exist where the rate of cooling is different in different parts of the bar, but the process of annealing ought to remedy any such condition, leaving the steel soft, ductile and of refined and uniform crystalline structure throughout.

The process of annealing is easier to explain than it is to actually put into practice. The steel is first packed in lime, charcoal, fine dry ashes or sand, and then sealed in long air-tight tubes or boxes.

The whole receptacle is next slowly brought up to a dull red heat, of about 1500 degrees Fahrenheit.