Mineral Point tribune. (Mineral Point, Wis.) 1869-1938, 1888-11-29
The Science OF Tempering Steel.
Inr..nl....n. Iln.-i-..--

In the whole range of mechanical arts
it would be nearly impossible. to find
another pocess at once so simple
and so common III principle, and
yet so little understood in theory, as the
hardening and temperiDg Of steel. 1t
was probably this fact which some years
ago led the Institute of Mechanical En-
gineers of England to place the subject
among those to be specially investigat ed
by g committee of its own members, who
reported upon it in due time. To illus
trate the facts that require scientific ex-
PLANTATION, the process of hardening and
tenn ering . cold chisel, which is ueusly
done at one operation, may be briefly des-
cribe6. After heating the point it is
dipped in cold water and thus the tool is
hardened. After cooling, the smith lifts
the steel from the water ana watches 10
closely as the heat remaining in the body
of the metal diffuses itself through the
hardened portion. As the heat spreads
the color passes from a white luster to a
pale yellow, to straw color, to a brownish
grange. Thea the point is dropped into
water again, in tho full confidence that,
after cooling, the temper will be that
desired. If the smith delayed, the brown
would become dappled Nth purple, and
would then pass successively into full
purple, light blue, full blue, dark blue;
aud each color would give its own temper
upon cooling, rs bright blu( for watch
springs. dark blue for saws, etc, These
are the weli-known facts, and yet their
show" and "Why' has always been equally
a mystery TO the artist and to the scient.
list, although upon the correct solution of
the profile n depends so importRnt a mat
ter as knowledge of how best to reach
that judicious-comprom which should
blend he maximum Of hardness and
fOughuess. Now, either can be procured
at pleasure, rs the colder the bath the
harder the steel, and the slower las in
oil)the tougher; but extreme hardness ia
produced at the strength OF tenacity, and
vice versa. The commuters conclusion
was suggested by Edison's experiments
upon wire, which he made public in 1879.
These experiments showed that incaude-
scent platinum wire became covered with
inmate assures, due to the expiration OF
the occluded gases under the action OF
heat, and that when the wite was cooled
in vacuum the assures closed. By
succession of hestings and ooolings the
gases we're entirely expelled, and the
PLATINUM became much harder and
denser. As the committee suggests, it
may be that the first and extreme heat
dries ont the gases occluded at ordinary
temperature, thus producing the dense
ness of hard steel. When the metal is
slightly heated, as in tempering, reRb-
sorption begins, and the charistio colors
are due to the changes in the surface, the
gradual opening of minute assures,
which are produced By re absorption In
connection with this latest theory may
be mentioned one or two earlier ones.
One is thst when steel is heated the car
bon becomes smorphous and the steel is
soft, white if cooled quickly the carbon
crystsliEes, taking the properties Of dia-
mJnd and the steel becomes, { : i. were,
diamond set in iron. This theory of
Julisn's i, perhaps, the most striking
one; but while accounting for hsrdresss
it does rot account for the temper.