The E wheel has an exceedingly coarse structure with
traces throughout of inequality of carbon content



35



and disappearance of the grain. This is especially
noticeable in photographs Nos. 19 and 20 and ap-
pears in the others to a greater or less extent, showing
an unevenness of structure that is suggestive of cast
steel. This is discussed elsewhere in connection
with a shelled-out wheel of the same make. The
penetration of work was apparently very slight as is
shown by the large size of the grains in No. 17, taken
at the surface of the tread, and the increasing size of
structure as shown in Nos. 18, 19 and 20 taken at
depths of J in., J in., and i in. respectively.
The F wheel has a coarser grain than the A, P or
C tire and is slightly coarser than that of the D tire.
The carbon content appears to be about the same
as that of the C tire, or somewhat above 0.60, and
this is checked by the chemical analysis. The sur-
face decarbonization which is so marked in the case
of the C tire appears in this one also, as indicated
by the increase of the amount of ferrite accom-
panied by softening of the surface. The large size
of the grain in this wheel, as illustrated by photo-
graphs Nos. 21 to 26, is caused by the heat treat-
ment to which this wheel was subjected. There has
evidently been no work put upon it after the final
heating. This also explains why there is compara-
tively little enlargement of the grain going down
from the surface of the tread. The photograph No.
