The news and herald. (Winnsboro, S.C.) 1877-1900, 1880-10-30
The Mystery OF Tempered Stool.

In the whole range Of tho mechanical
arts 1t would be nearly impossible to fluid
apother process at once 80 simple and 80
common In practice, and yet so little un-
derstood in theory, as tho hardening and
tempering of steel. 1t was probably this
fact which led the Institute Of Mechanical
Engineers (of EnglandD toplacc this subject
among those to be specially investigated by
q committee OF It's own members, whose
first report has recently appeared. To
illustrate the facts which require scientific
explanation, the process of hardening and
tempering cold chisel, which is usually
done at one operation, may be briefly de-
scrbed. After heating, the point Is dipped
In cold water, and thus the tool is hard
ened. After cooling, the smith lifts the
steel from the water, and watches it closely
as the heat remaining in the body of the
metal diffuses itself throughout the hard
encd portion. AS the heat spreads the
color passes from white luster to a pale
yellow, to straw color, to brownish orange.
Then the point Is dropped into water again,
and In the full confidence that after cooling
the temper will be that desired. If the
smith delayed, the brown would become
dappIed with purple, and would then pass
successiyely into full purple, light blue,
full blue, dark blue, and color would give
Its own temper upon cooling as bright blue
for swords and watch springs, dark blue for
saws, etc. These are the well known facts,
and yet their chow" and "why" has all
ways been equally mystery to the artisan
and the scientist, although upon the correct
solution Of the problem depends 80 import.
ant matter as knowledge how best to
reach that judicious compromise which
should blend the maximum Of hardness and
toughness. Now, either can DO produced at
pleasure as the colder the bath the harder the
steel, and the slower Gas in OILY the tougher
butextreme hardness Is produced at the OK.
pense Of tcnacity, and Uice versa. The
committees conclusion was suggested by
Edison's experiments upon platinum wire
periments showed that the incandescent
wire became covered with minute fissuros,
due TO expiration of the occluded eases
under the action of heat, and that when
the wire was cooled In a vacuum the fissurcs
closed. BY succession of hcatings and
cOollngs the gases were entirely expelled,
and the platinum became much harder
and denser. AS the committed . suggests It may
be that the first and extreme heating drives
out the gases occluded at ordinary TOM
peratures, thus producing tho denscncss OF
hard steel. When tho metal Is slightly
heated, as in tempering, reabsorption be-
gins, and the characteristic colors arc
duo to the changes In the surface, the
gradual opening of minute tissures, which
arc produced by this reabsorption. In con-
nection with this latest theory may be men,
toned one or two earlier ones. Ono is
that when steel Is heated tho carbon be-
comes liquid, and Is absorbed by tho iron
as water is by sponge,> and that upon a
slow cooling the carbon becomes amorphous
and tho steel Is soft, while, II cooled
quickly, the carbon crystalizes, taking
the properties of diamond. and the steel
becomes, as if it were, diamond set III iron.
This theory of Julien's is, perhaps, tho
most striking one, but while accounting for
hardness, it does not explain the temper Of
steel, to say nothing of objections to tho
postulatcd llquetaction of carbon.
Another theory explains the process OF the
contraction and compression of cooling.
Finally may be mentioned the theory that,
at high temperatures. steel assumes on
amorphous wax like form, ON cooling from
which 1t crystalizos into large crystals, If
the process is slow and undisturbed, but
into small crystals If the process is repid or
disturbed by hammering Such are the
leading theories; scarcely one of them Is
not sustained by striking analogles, but
mono of them, Including the latest, entirely
explains this exceedingly SIMPLE and COM
mon but battling mystery.