The manufacture of crucible steel : its history and technology

              40 SCIENTIFIC AMERICAN SUPPLEMENT No. 2011 July 18, 1914
The Manufacture of Crucible Steel
Its History and Technology
By George H. Neilson
T H E pioneers in crucible melting are said to have been
the Chinese, who used the process many centuries ago.
But the art in China never progressed beyond the initial
stage. The real father of the crucible steel industry was
Daniel Huntsman, of Sheffield, Eng., a clock-maker, who
found it impossible to get uniform steel from which to
make his springs, and he hit on the idea of fusing blister
steel in a crucible. This was in the latter part of the
eighteenth century, and the melting of crucible steel has
changed but little since that time. The details have
Fig. 1.—Detail of hole in a crucible furnace.
changed somewhat but the actual process is much the
same.
The material to bo melted is loaded in a crucible,
covered with a cap to keep out the gases, and placed in
a hot hole and left there until melted. The crucibles
have changed, the holes also have been changed in shape
and size, and the method of heating is not tho same, but
the process is practically unchanged. Clay crucibles were
the first of which we havo any definite knowledge. They
held aboul 50 or 75 pounds and lasted but one heat, and
very often cracked and went to pieces before the steel
was melted. Clay crucibles of the present day are much
more durable and arc extensively used in Europe, but
little in this country. They have one decided advantage
* Address of the Retiring Chairman of the Mechanical.Section
ot the Engineers' Society of Western Pennsylvania., published in
the February, 1914, Proceedings, page. 00.
over the plumbago, or graphite, crucibles inasmuch as
they do not throw off any carbon during the melting
process. The plumbago crucible, which is the most
generally used in this country, consists of about equal
parts of plumbago and clay. The greater part of the
plumbago is imported from Ceylon. These crucibles are
capable of withstanding a very severe heat, and can be
used a number of times, depending greatly on the nature
of the mix and also whether the crucibles arc replaced
in the furnace before they get cold. The usual practice
is to get as manj' heats as possible from the crucible
without letting it cool. As soon as the melted steel is
poured out, it is re-charged by hand, or by means of a
mechanical shaker, and the crucible returned to the melt­ing
hole.
Furnace. Tho modern crucible furnace is of the re­generative
type and is heated by gas, generally producer
gas, although where natural gas can be obtained it is
often used. Natural gas is probably a more costly way
to run a furnace, but it has many advantages over pro­ducer
gas. It is easier to regulate, as the flow is constant,
which is not the case with producer gas unless a large
holder is used. It is free from tho poisonous fumes of
the producer gas and is much cleaner. I a m not in a
position to say whether or not it is harder on the crucibles
and furnace than producer gas. The capacity of a. furnace
is spoken of in pots. That is a 24 pot, 36 pot or 60 pot
furnace. That is the number of crucibles the furnace
will accommodate at one lime. The furnace holes, in
which the crucibles are placed, hold o crucibles, so a 36-
pot furnace is one of (1 boles. Tho gas enters the holes
at tho bottom on one side, mixing witli the air immedi­ately
before entering the melting hole, and passes out at
the opposite side and then through checker work to tho
stuck. When the valve is reversed, the direction of tho
s flow of the gas is reversed. This is done every 15 or 20
X minutes, and, in this way, the checker work on both
t sides is kept hot. The gas should not be pulled through
1 the melting hole too rapidly. If it is, it will cut the port
3 holes and also cut the crucibles. The gas should fill the
a melting hole and show a small flame around the covers.
3 This is a sure indication that the gas is getting around
a the crucible and not pulling across tho bottom. The
1 detail of the hole is shown in Fig. 1, and a modern 36-pot
3 furnace in Fig. 2.
Charge or Mix. The basis of good crucible steel is iron,
^ and, consequently, the better the iron the better will be
L tbe steel. Therefore, it is vitally necessary that iron
low in phosphorus and sulphur be used. As the crucibles
generally in use hold from 100 to 12.") pounds, the mix or
charge is weighed up in lots of that weight and placed
r in pans, called weigh pans, from which if is transferred
5 to the crucibles. In order to get the exact analysis, (be
weighing must be carefully done, in many cases to the
exact ounce. When tho crucible is filled it is covered
with a cap. This is done to exclude deleterious gases
which otherwise would impregnate the steel. When the
material to be melted is weighed up, the amount of carbon
given off by the crucible must bo taken into considera­tion.
If this is not done, the carbon content of the ingots
will run higher than expected. The new pots, as a rule,
do not throw off as much carbon as they will tho second
time used, and after the. third heat the amount thrown
out will be immaterial.
Melting. The length of time necessary to reduce the
mix to a molten state varies, depending on the makeup
of the mix itself, and will lake anywhere from 2 to 5
hours. When the sfeel becomes Quid, it is usually good
practice lo "kill it," or, in other words, drive out the
gases which would otherwise resulf in blow holes in the
ingot. This process of '•killing" usually takes from 20
minutes to one hour or longer.
Molds. The niohls in general usr an- known as split
angle molds. They are made in two pieces, held together
by rings and wedges - one ring at the top ami one at
the bottom. The three essential qualities are long life,
smooth finish and light joints. If tl„, jnsirle finish is
not smooth, the ingot will have a rough surface which
may result in defects in the finished bar. If the joints
are not tight, the hoi metal will work through and form
a fin on the ingot. This fin will have to be removed,
which means added cost. If it is not removed, it will work
into the steel and cause complications. The smaller
molds have the bottoms cast with the sides. The larger
molds, 7-ineh and over, have no bottoms as a rule, the
molds being set up on removable bottoms. Before the
molds aro used, the general practice is to smoke them
with rosin,_or some other heavy, greasy, smoke-making
material. This prevents the ingots from sticking, and
also makes a smoother surface. The molds should also
be warmed before using.
Iteming or Pouring. Teeming is a very important
feature and is not merely dumping the hot steel from the
crucible into the mold in a haphazard way. In the first
place, the si ream must be steady; if it is stopped and then
started again there will be a weak spot iu the ingot. The
Fig. 3.—A 10-inch bar mill.
Fig. 2.—A 36-pot furnace.