Immediately under the pieces being observed was a vessel of water into which the pieces could be dropped and quenched. After observing the heating and cooling until the eye was well trained, pieces were quenched at different heats and the results were noted. It was found that in the ascending heats no great hardness was produced until the recalescence heat was reached or passed slightly; and in the descending heat excessive hardening occurred at a little below the recalescent heat, although no such hardening occurred at that color during ascending heats. This apparent anomaly is due simply to lag. If, in ascending, the piece be held for a few moments at the recalescent point, no increase being allowed, and then it be quenched, it will harden thoroughly and be refined. If, in descending, the cooling be arrested at a little below the recalescence for a few moments, neither increase nor decrease being allowed, and then the piece be quenched, it will not harden any better than if it be quenched immediately upon reaching the same heat in ascending.
Time must be allowed for the changes to take place, and lag must be provided for.
These experiments show that refining and recalescence take place at the same temperature.
AS TO HARDNESS.
Prof. J. W. Langley showed by sp. gr. determinations that steel quenched from 212° F. in water at 60° F. showed the hardening effect of such quenching, the difference of temperature being only 152° F.
Prof. S. P. Langley, of the Smithsonian, proved the same to be true by delicate electrical tests, and these again were confirmed by Prof. J. W. Langley in the laboratory of the Case School of Sciences.
A piece of refined steel will rarely be hard enough to scratch glass. A piece of steel quenched from creamy heat will almost always scratch glass. The maximum hardness is produced by the highest heat, or when temperature minus cold is a maximum; the least hardness is found by quenching at the lowest heat above the cooling medium, or when temperature minus cold is a minimum—the time required to quench being a minimum in both cases.
What occurs between these limits? Is the curve of hardness a straight line, or an irregular line?
Let a piece of steel be heated as uniformly as possible from a creamy heat at one end to black at the other, and then be quenched.
Now take a newly broken hard file and draw its sharp corner gently and firmly over the piece, beginning at the black-heated end. The file will take hold, and as it is drawn along it will be felt that the piece becomes slightly harder as the file advances, until suddenly it will slip, and no amount of pressure will make it take hold above that point. The piece has become suddenly file hard.