overheated mild steel were inserted. One
peg was withdrawn and quenched at each of the temperatures indicated
by the numbered arrows, and after suitable preparation these pegs
were photographed in order to show the changes in structure taking
place during heating and cooling operations. The illustrations here
reproduced are selected from those photographs with the object
of presenting pictorially the changes involved in the refining of
overheated steel or steel castings. Figures 66 to 79 with their
captions show much that is of value to steel users.

[Illustration: FIG. 66.--The structure of overheated mild steel
from which all the pegs were made (magnified 25 diameters). The
pegs withdrawn at 720 deg.C., or earlier, had this structure and were
quite soft.]

[Illustration: FIG. 67.--Peg withdrawn at 750 deg.C. (magnified 25
diameters). The structure is apparently unaltered, but the peg was
hard and, unlike the earlier ones, would not bend double.]

[Illustration: FIG. 68.--A portion of 66 magnified 200 diameters
to show that the dark (pearlite) areas are laminated.]

[Illustration: FIG. 69.--A portion of 67 magnified 200 diameters,
showing that pearlite areas are no longer laminated and providing
reason for observed hardness.]

[Illustration FIG. 70.--Peg withdrawn at 780 deg.C. (magnified 25
diameters), showing inter-diffusion of transformed pearlite and
ferrite areas.]

[Illustration: FIG. 71.--Peg withdrawn at 800 deg.C. (magnified 25
diameters), showing inter-diffusion so far advanced that the original
outline of the crystals is now only faintly suggested.]

[Illustration: FIG. 72.--Peg withdrawn at 850 deg.C. (magnified 100
diameters) after inter-diffusion was completed. Note the regular
outlines and the small size of the crystals as compared with 67.]

[Illustration: FIG. 73.--To facilitate comparison 67 was enlarged
to the same magnification as 62, and the one superimposed on the
other. The single large crystal occupied as much space as 8,000
of the smaller ones.]

[Illustration: FIG. 74.--The peg withdrawn on cooling at 800 deg.C.
(magnified 100 diameters) shows the first reappearance of free
ferrite. All pegs withdrawn at higher temperatures were like Fig.
72.]

[Illustration: FIG. 75.--Peg withdrawn after cooling to 760 deg.C. The
increased amount of free ferrite arranges itself about the crystals
as envelopes.]

[Illustration: FIG. 76.-Peg withdrawn after cooling to 740 deg.C.]

[Illustration