Loss of Magnetism
Now another curious thing took place had we but noticed it.
We all know that iron and steel are our most magnetic materials. From childhood we have seen pins, needles, steel pens, and various other steel or iron objects jump to a magnet held near them.
What, now, when we find that our piece of steel in the furnace when at a red or higher heat is entirely unresponsive or dead to the attraction of a strong magnet?
Strange! Do you suppose that our magnet has lost its power?
Let us see.
Suppose that every minute or so, while watching the pyrometer needle go slowly down again after turning off the heat, we put the magnet to the steel.
Continually lower comes the temperature of the piece—1500°, 1400°, 1375°, 1350°, 1325°, 1300°, 1275° F.,—and lo! the piece jumps, and from this all the way down to cold it responds to the attraction of our magnet. Just to make sure that we are not “seeing things,” we again start our furnace, and, as the steel heats, we test it with the magnet.
So far there is no doubt about its being magnetic!
At 900° F., the pieces begins to show dark red, at 1000°, 1100°, 1150°, 1200°, 1250°, stronger and stronger red. At all of these temperatures the steel is attracted. So it is at 1275° and at 1300° F.
But just as we are thinking that we must have been mistaken before, we find that again the steel is suddenly “dead” to the pull of the magnet!
And at what temperature? The pyrometer indicates 1320° F. But was not this the same or very nearly the same reading at which the pyrometer needle paused on the way up, and do you not remember that it was only a little below 1250° F. that it paused on the way down, and the disagreement of the two temperatures we ascribed to “lag”?
No, we made no mistake. Steel loses all of its magnetic properties at the “critical range” and has none above it.