Now it should be noted particularly that the specimen with which we have been experimenting is a tool steel of .90% carbon or thereabouts. This is important, for, while all of the carbon steels show this same critical temperature, at which occurs the point of recalescence, those containing from .45% to about .85% carbon have another point somewhat higher on the temperature scale, and steels which contain from .10% to .45% of carbon have two others, or three points in all. Further, steels having less than .10% of carbon and iron with no carbon at all have the two upper points but no point at 1290° F. This lower one has disappeared.
All of this means that if instead of a piece of .90% carbon steel we had used one having .60% of carbon, say, we would have found two different critical ranges or points at which the pyrometer paused, the one at 1290° F., and another when we got to 1360° F. Had the steel been one containing .30% carbon we would have discovered pauses at three different points, viz., at 1290° F., at 1395° F., and at 1480° F. With very low carbon steel or with wrought iron, the pyrometer would have registered two pauses, one at about 1395° F., and the other at 1650° F.
When records are carefully kept of the time which is required for the temperature to rise or lower over each and every twenty-five degree period, say, on the upward and downward way, and these are “plotted,” what are called “heating” and “cooling” curves can be drawn through the stars and dots so set down and these form a record of the behavior of the pyrometer needle at each temperature along the scale. Two illustrations of such curves are shown.[[9]]
[9]. Special apparatus is now obtainable for determination of critical points, heating and cooling curves.
Now if on properly spaced, dotted, vertical lines, which we will let represent these various alloys, we mark points number three, two and one as shown by our “cooling” curves, calling the topmost point three, it is readily seen that the points are related. The lines and the alloys which they represent are,
(a) The wrought iron, (b) .15% carbon steel, (c) .30% carbon steel, (d) .45% carbon steel, (e) .60% carbon steel, (f) .75% carbon steel, and (g) the steel with .90% carbon.
Critical Point Diagram of Pure Iron and the Steels
Of course many more cooling curves, especially of steels with other percentages of carbon would be desirable, but we have enough that we are safe in sketching the horizontal and oblique lines, Ar1, Ar2, Ar3, Ar3·2 and Ar3·2·1, through the points which we have arranged.
For convenience, metallurgists everywhere mark these points Ar1, Ar2 and Ar3, the first being the lower, and Ar3 the upper one. Arcm represents an upper point found in steels having more than .9% of carbon. The letter “r” is derived from the French word, “refroidissement,” meaning “cooling.” The corresponding points disclosed during heating are marked Ac1, Ac2, Ac3·2·1, etc., from the word, “chauffage” meaning “heating.” The “A” apparently “just happened.” Before the upper critical points were known it had been used by Tschernoff to designate the temperature at which steels harden.