A bar of this kind containing enough carbon to harden will harden thoroughly, and often appear to be sound and fine, but it is not sound and will not do good work; if it be brought up to a proper heat and forged to a point, it will almost certainly burst, showing that the integrity of the steel has been destroyed.
If a bar, or plate, or beam shows cracks on the surface or at the corners, with rough, torn surfaces, the steel has either been superficially burned or it is red-short. In either case it should be rejected, for the cracks, although small, will provide starting-points for ultimate fractures, whether it be tool-steel that is to be hardened, or structural steel that is to be strained without hardening. If the steel is to be machined, so that all of the cracks can be cut out, then in machinery-steel the removal of these surface defects might leave the finished piece sufficiently sound and good. If, however the steel is to be hardened, and the defects should be due to red-shortness, the piece would almost certainly break in the hardening; and if it were not red-short, then unless the cracks were cut away entirely, if the least trace of the crack is there, although it may not be visible, that trace will be sufficient to start a crack when the piece is hardened.
EFFECTS OF COOLING.
Increase of heat causes increase of softness up to the liquid condition.
Decrease of heat—cooling—increases hardness up to the hardness of glass.
As an invariable rule the rate of cooling fixes the degree of hardness to be had in the cold piece within the limits of obtainable hardness or softness.
Slow cooling retains softness, so that when annealing is to be done the slower the cooling the better. Cooling is always a hardening process, but when it is carried on slowly more softness, will be retained than when the cooling is quick.
Rapid cooling produces hardness, and the more nearly instantaneous it is the greater the hardness will be. This property of hardening is of such extreme importance that it will be treated fully in a separate chapter.
There is an apparent exception to this rule shown in the operation called water-annealing. It is common, when work is hurried, to heat a piece of steel carefully and uniformly up to the first color, that is, until it just begins to show color, and then to quench it in water.
This is called water-annealing; and many believe that because a piece so treated is left softer than it was before treatment, the water-cooling had something to do with it. The fact is that hammering and rolling are hardening processes. When the increment of heat due to the work is less than the decrement of heat due to radiation, the compacting of the grain increases hardness.