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438 Forging - S tamping - Heat "Beating
The refractor}' material used, is crushed old lire
brick reduced to approximately '4 in. mesh, including
the fines. This material is put in a cement mixer,
along with diluted high temperature cement in proportions
of 3,000 lbs. of crushed fire brick to 1.000
lbs. of high temperature cement. To each batch of
this -ize is added a half bag of Portland Cement when
the mixture is about ready for ramming.
The mixture is then dumped in between the sides
of the form to a depth of about 4 in. This material
is then rammed, using a sand air rammer. Ramming
FIG. 2—Furnace being lined and ready for drying.
is not done directly on the material itself, but against
a piece of board which is shifted about to get an even
distribution of pressure.
Labor Saving.
It has been found that with the monolithic lining,
three men can ram four bungs in a day. With fire
brick lining, as formerly used, it required one bricklayer
and three or four laboreres to line one bung in
a day.
Service of Monolithic Bungs.
The majority of the bungs lined by this method
have been good for thirty heats of service. This is
a fair average. Prior to this, 10 heats was considered
especially good for fire brick lining, though the average
life would probably be from six to eight heats.
The Protection of Thermo-Couples in
Electrically-Heated Salt Baths
By L. E. Crease
(From American Machinist)
Suitable protecting sheaths for pyrometers gave
practically no trouble until electrically-heated salt
baths were brought into use in the heat-treating departments
of our industrial plants.
The ordinary gas furnace, in which the work is
heated directly by the flames, presents few difficulties
as far as the pyrometer sheaths are concerned. Cast
iron was soon found to be the most inexpensive material
giving satisfactory service.
Gas or oil-fired baritim-chlordie baths, in general
use throughout the country a few- years ago, in which
the thermo-couple must be submerged in order to give
an exact indication of the temperature, were the cause
December, 1925
of numerous experiments with materials ranging from
silica to nickel and chromium alloys. Unfortunately,
in spite of its excellent qualities as an impenetrable
sheath, silica proved too fragile for any but laboratory
use. Mild-carbon steel was finally adopted in the
form of a tube, one end of which was made air-tight
bv fusing it in order that the salt, when heated to
900 or 950 deg. C, would not force its way through
to the couple. These tubes gave satisfactory results.
When electrically-heated barium-chloride baths
were brought into existence, another phenomenon was
encountered. The action of the electric current, combined
with the corrosive properties of the fused salts,
wore away the metal tube in from three to five days
when running at 850 deg. C. The position of the
pyrometer is shown in Fig. 1 in relation to the electrodes.
This illustration explains, at least partially,
the origin of the trouble which seems to reside in the
contact established between electrode. A, and the pyrometer
sheath, D, by the slag, C, which falls to the
bottom of the crucible and is drawn into the corners.
The current thus passes between electrode B and the
sheath, instead of passing from one electrode to the
other.
No practical remedy being available, protecting
tubes of various compositions were again tested, and
the nickel-chromium alloy triumphed in all but one
point. The slag in this case seems to have an affinity
for the salts and soon joins them, leaving a series
of small holes in the tube through which the salt
penetrates.
A non-porous sheath is required to insure the thermo-couple
against an}- conductor which would short-
FIG. 1—Slowing the position of the pyrometer in the sa'.t
bath. FIG. 2—(Right)—Cross-section to show successful
construction.
circuit the two elements, and non-corrosive metal is
required to protect this sheath.
The results of the previous tests led to the solving
of the problem, which was accomplished by combining
the well known properties of steel and nickelchromium
alloys.
In Fig. 2 is shown the end of a pyrometer mounted
with a double sheath, X being the mild-steel sheath
and K the allov sheath.