Juniata sentinel and Republican. [volume] (Mifflintown, Juniata County, Pa.) 1873-1955, 1880-10-13
The Mystery Of Tempered Steel.

In the whole range Of the mechanical
arts it would be nearly impossible to find
another process at once 50 simple and 50
common in practice, and yet 50 little un-
derstoOd in theory, as the hardening and
tempering of steel. 1t was probably this
fact which led the Institute Of Mechanical
Engineers (of England) toplace this subject
among those to be specially investigated by
committee of its 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
ened portion. As the heat spreads the
color passes from white luster to q 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
dappled with purple, and would then pass
successfully 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 ''how" and ''why" has all
ways been equally mystery to the artisan
and the scientist, although upon the correct
solution Of the problem depends so import
ant q matter as knowledge how best to
reach that judicious compromise which
should blend the maximum of hardness and
toughness- Now, either can be produced at
pleasure as the colder the bath the harder the
steel, and the slower las in oID the tougher
but extreme hardness is produced at the ex-
pense Of tenacity, and rice versa. The
committees conclusion was suggested by
Edison's experiments upon platinum wire
which he made public in 1870. These ex-
periments showed that the incandescent
wire became covered with minute fissures, |
due to expiration of the occluded gases
under the action of heat. and that when |
the wire was cooled in a vacuum the fssures |
closed. By succession of heatings and |
coolings the gases were entirely expelled, |
aAh ihO OttaLAC
and denser. Asthe committee suggests 1t may |
be that the first and extreme heating drives /
out the gases occluded at ordinary tenn
peratures, thus producing the denseness of /
hard steel. When the metal is slightly
heated, as II tempering, reabsorption be-
gins, and the characteristic colorS are
due to the changes in the surface. the
gradual opening of minute assures, which
are produced by this reabsorption. In con-
nection with this latest theory may be men,
toned one or two earlier ones One is
that when steel is heated the carbon be-
comes liquid, and is absorbed by the iron
As water is by sponge, and that upon a
aow cooling the carbon becomes amorphous
quickly, the carbon crystsliEes, taking
the properties of diamond. and the steel
becomes, as if it were, diamond set IN iron.
This theory of Julien's Is, perhaps, the
most striking one, but while accounting for
hardness, it does not explain the temper of
steel, to say nothing OF objections to the
oostulated llQuetaction Of carbon.
Another theory explains the process of the
and compression of cooling.
Finally may be mentioned the theory that,
IT high temperatures, steel assumes an
wax like form, on cooling from
which it crys;alizes into large crystals, if
he process is slow and undisturbed, but
nto small crystals if the process is rapid or
by hammering. Such are the
eading theories; scarcely one of them is
lot sustained by striking analogies, but
lone of them, including the latest, entirely
EXPLAINS this exceedingly simple and CON-
non but battling mystery. l.