It is necessary to heat steel not only to shape it, but to anneal and harden it. That heating for any of these purposes may be properly done, it is necessary to understand certain peculiarities, particularly of high-carbon steels (those containing .2% and over of carbon).

As steel rises in temperature it reaches a point, about 1400° F. (red heat), called the absorption point, at which it absorbs a perceptible amount of heat before its temperature again increases. Likewise, having been raised above the adsorption point and allowed to cool slowly, it reaches a temperature about 50° F. below the absorption point where it seems to give out more heat than is accounted for by loss of temperature. Its glow shows an increase of brightness if observed in a dark room. This is the recalescence point. Both of these points are known as critical points.

Heating to the absorption point brings the grain of steel to its finest texture. The higher the heating beyond this point and the lower the carbon, the larger the resulting crystals upon cooling. This may be seen, in many cases, by examining fractured specimens with the unaided eye. This enlargement of crystals reduces strength, hence the size of crystals is a visible sign indicating strength. The smallest crystals may be restored in high-carbon steel by heating it to the absorption point. It may then be cooled slowly or suddenly without change of grain.

Steel need never be heated above its absorption point except to have it amply hot for shaping in a single heat. If high-carbon steel is heated much above the absorption point, its strength is injured, its fracture looks dull, and it is said to be "burned." Such a condition may be impossible to remedy. However, low-carbon steel escapes injury at high heats in most instances, but becomes brittle if heated repeatedly below 1650° F. It is restored to its elasticity if heated above 1G50° F.

The crystalline structure of a piece of steel as affected by heat is determined by the five conditions as follows:

(1) Temperature, (2) duration of heating, (3) mass, (4) rapidity of cooling, and (5) whether or not steel cools without being rolled, hammered, or otherwise subjected to pressure or impact.

The presence of nickel, tungsten and other metals used to produce alloy steels have a marked effect upon the critical points of steel. These points are lowered, and the presence of the alloying metals seems to accomplish this by their influence upon the carbon which the steel contains.