Page 251 - My FlipBook

IRON. 239

w rough t-i roll is, as has just been indicated, that steel has
the property of becoming very hard when heated to redness
antl suddenly plunged into cold water. This property is
given the iron by the presence of a definite quantity of
carbon, varying from 0.75 to 2 per cent. The working
quality of steel is modified by the presence of other foreign
substances. A small quantity of sulphur will render it
red-short or practically unworkable at red-heat, while a
small amount of phosphorus will make it cold-short.
When too much sulphur is present, the addition of a small
quantity of manganese will counteract to some extent its
harmful influence. Silicon imparts hardness and brittleness.
Fusing Point.—The fusing point of pure iron is esti-
mated to be about 2900- F., wdiile the amount of carbon
introduced in forming the different modifications of iron
relatively reduces the point of fusion.
Hardening and Tempering Steel.—The hardening of
steel is effected by subjecting it suddenly to extremes of
temperature. Conversely, the hardened steel, reheated to
redness and allowed to cool slowly, is again converted into
soft steel. Any desired variation between these points may
be obtained by taking the hardened steel, carefully reheating
it to the proper point (see table), and stopping the operation
at that moment by suddenly chilling it; this constitutes
teinperiiig.
In writing upon this subject in the " American System of
Dentistry," Dr. Kirk directs that where small articles, such
as drills, excavators, and other dental instruments, are to
be hardened, they should always receive a protective coating
of some material which will retard or prevent loss of carbon
by oxidation during the heating process. Common soap
answers admirably for this purpose. After being heated
carefully to the proper temperature, which has been pre-
viously determined by experiment, the instrument is suddenly