Apparatus for Determining the Critical Points of Steel
But lo! Something must be wrong! The pyrometer needle does not now move forward but is standing still. Though we know that in that hot furnace the piece must be absorbing heat at the same rate as before, yet the pyrometer needle does not budge!
But, as our wonderment grows and we are still undecided as to the meaning, the needle again begins to advance and continues again regularly and uniformly to higher and higher temperatures as though it had never taken the vacation.
With the piece now at a white heat, we have proceeded far enough with the heating.
Turning off the electric current from the furnace and allowing it to cool we again watch the pyrometer needle as the temperature of the piece in the cooling furnace gradually falls. Lower, lower, lower swings the needle, always at a rate approximately uniform.
But again it suddenly stops and remains immovable, or perhaps even rises slightly, for a period of several seconds, after which it resumes its uniformly-timed downward course as though nothing had happened.
Yes, these pauses of the needle occurred at very nearly the same marking on the pyrometer dial, but not at exactly the same ones. Going up it was at 1350° F., and on the downward way it was at 1250° F. And you are correct in surmising that these two points are closely related. They are parts of the same, if we may so speak, and, in reality they represent one point which is located about halfway between them, the divergence resulting from what is known as “hysteresis” or “lag,” which means, of course, a “being-behind-hand” or tardiness.
For the present we may say that all carbon steels have this “critical” range as shown by such pauses of the pyrometer needle during heating or cooling of the steel.
Now as the piece is most certainly continuing to absorb heat in the furnace as it grows hotter and is losing it uniformly to the air as the furnace cools, we have no alternative but to judge that the pause of the needle on its upward way was caused by some internal affair of the piece of steel itself, for which, at just that stage of its journey, it required and used for its purpose (which was other than making itself hotter), the heat furnished it by the furnace; and, that on the downward journey, at just that same point, it gave out again that same heat. It must have been the setting free of this imprisoned heat, if we may so term it, which kept the piece for those few moments from cooling at the usual rate. Indeed, had we conducted our experiment in a rather dark room and observed the piece closely we would have noticed that during the pause of the pyrometer the piece of steel did brighten or glow somewhat, showing that it had extra heat from some hidden source. Because of this “self-heating” of the steel as shown by the pyrometer and the brightening, the temperature at which the phenomenon occurs has been named the “point of recalescence,” which means the point at which it spontaneously becomes hotter.