You remember that we said the divergence between the going up and the coming down pauses of the pyrometer needle was due to tardiness or lag? Among humans habitual tardiness is not considered a desirable trait, but it is undoubtedly through this very lag or tardiness that steel becomes so serviceable to us.
Heating and Cooling Curves of Steel with
.9 Per Cent of Carbon
This lag is peculiar in that it grows less the more slowly we heat or cool the steel, and, if the heating or cooling is done slowly enough, the lag disappears almost entirely, i.e., the pause of the pyrometer needle occurs at the same temperature on the upward as on the downward way. Conversely, the disagreement or split grows or widens the faster the temperature is raised or lowered.
Here is the vital point.
By extremely sudden cooling, such as quenching in water, the lag becomes so great that it never catches up at all and any structure with its consequent properties which was brought about in the steel by the higher temperature is thus frozen or fixed and made to “persist” after the steel has become cold.
It is just at this point, the “point of recalescence,” that steel changes from its soft and malleable, to its extremely hard and brittle condition. If it is quenched from temperatures above this point, it is extremely hard, if from temperatures below it, even those only a little below, it is soft and ductile. It is from just a little above this point, then, usually between 1350° F., and 1500° F., that the blacksmith hardens his tools by plunging them into cold water.
Heating and Cooling Curves of Steel with .46 Per Cent of Carbon