The Cecil Whig. [volume] (Elkton, Md.) 1841-current, 1868-12-12
From the Boston Commercial Bulletin.
TEMPERING STEEL.

III we take plate OF highly carD0n!'
Zcd cast steel one hundrcdth of an inch
in thickness thc thinner the better, }
heat it between two cast iron plates
which will prevent it from being de-
northumberland let it slip, when hot,
from between them, edgcwise into cold
water, it will be made hard enough to
cut glass.
1t is worthy of notice that piece of
steel so treated, although no heavier
than before, will be slightly larger; if
it We're smaller its hardness might be
attributed to condensation.
If its surface be rubbed bright and it
is slightly and slowly heated it will
soon assume straw-color, which indi-
Gates Il certain degree of reduced hard
ness known to the expert. If the slow
heating is continued it next becomes
purple, after which blue, and last
black.
Professor Henry, of Smithsonian In
stitute, informed the writer a few wears
since that these colors indicated the
thickness of the scale or film of oxide
upon the surface, and that the blue sur
face might be removed by careful pol
ish;ng. 50 as to expose the purple, and
the operation continued would OK
pose the straw-color. If after the black
scale forms-which is black oxide of
iron, same as scales which are found
about the blacksmith's anvil-thc piece
be cooled slowly, it will be annealcd-
The same temperature will produce the
same degree of hardness in tcmpcriny
whether the scale forms or not If air
should be absolutely excluded no scale
would form. If steel be heated too hot,
part of the carbon will he cvapOrizcd,
Which, bursting out, will disintcgratc
the metal some borax applied will pre-
vent further escape of the carbon, when,
by welding and condensation under the
hammer, it may be partially restored,
but it will be lower steel, and possibly
could not be tempered.
High steel, such as is used for tools,
to be tempered has about one and IL
half per cent of carbon Low steel
about a half of one per cent of carban,
and it is Of the quality used for tires
for locomotive drivers, &c. it cannot
be tempered by heating and cooling,
but may be considerably hardened by
condensation under the hammer when
nearly cold.
Steel is, or should be, composed of
iron and carbon.
Case-hardoning is simply carbonizing
and making steel of the surface De
carbonizing is expanding the carbon,
and the product, if from cast steel, is
homogeneous iron.
| suppose hundreds of mechanics
have wondered, as | have, at these
mysterious changes in the qualities of
steel by the process of hardening, con-
dcnsation and annealing, and have re-
sorted to books written by learned men
for information, without finding their
thirst for knowledge gratitied. There
is no chemical change of quality in
steel which is hardened or annealed !;
hardening is simply the Inauguration of
tensions by unequal cooling, when con-
Quoted in the manner described, and
annealing is relieving those tensions,
by heating the steel until the tension is
relaxed, and then cooling slowly and
uniformly If the thin plate above re-
fcrrcd to could be caught flying, as it
fell hot, from between the cast-iron
plates, between two sponges having
fiat surfaccs-wcll saturated with wa
tern such Il manner as to cool the
whole of both surfaces simultaneously,
it would not he hardened at ALL If the
experiment should be conducted with a
steel plate half an inch thick, then it
would be hardened; because the cool
ing would not be uniform, the surfaces
would be cooled in advance of the mid
dk metal of the mass; steel being COM
parativelv non-conductOr of heat, it is
only possible to cool an extremely thin
sheet in this manner, with sufficient
uniformity not to harden it.
A thin plate of steel such as desert
bed can he hardened quite as well with
the hammer, as in water, saw plates
are SO hardened. A planishing anvil
is used, and smooth-faccd hammer,
slightly rounded; the steel is condensed
and strained by stretching uniformly,
by which the tensions arc inauguratcd,
giving the quality We call tempering.
1t is difficult Wrobablv impossible in
cold water) to temper large piece of
steel without cracking it. When plung-
ed into the water the outside is cooled
suddenly and contracted; cracks occur,
which open slightly at first, but after
wards usually close, when the interior
metal cools and contracts, when the
most dangerous rupturcs occur. These