ON THE SUCCESSFUL ROLLING OF STEEL INGOTS WITH THEIR OWN INITIAL HEAT BY
MEANS OF THE SOAKING PIT PROCESS.
By Mr. JOHN GJERS, Middlesbrough.
[Footnote: Paper read before the Iron and Steel Institute at Vienna.]
When Sir Henry Bessemer, in 1856, made public his great invention, and
announced to the world that he was able to produce malleable steel from
cast iron without the expenditure of any fuel except that which already
existed in the fluid metal imparted to it in the blast furnace, his
statement was received with doubt and surprise. If he at that time had
been able to add that it was also possible to roll such steel into a
finished bar with no further expenditure of fuel, then undoubtedly the
surprise would have been much greater.
Even this, however, has come to pass; and the author of this paper
is now pleased to be able to inform this meeting that it is not only
possible, but that it is extremely easy and practical, by the means to
be described, to roll a steel ingot into, say, a bloom, a rail, or other
finished article with its own initial heat, without the aid of the
hitherto universally adopted heating furnace.
It is well understood that in the fluid steel poured into the mould
there is a larger store of heat than is required for the purpose
of rolling or hammering. Not only is there the mere apparent high
temperature of fluid steel, but there is the store of latent heat in
this fluid metal which is given out when solidification takes place.
It has, no doubt, suggested itself to many that this heat of the ingot
ought to be utilized, and as a matter of fact, there have been, at
various times and in different places, attempts made to do so; but
hitherto all such attempts have proved failures, and a kind of settled
conviction has been established in the steel trade that the theory could
not possibly be carried out in practice.
The difficulty arose from the fact that a steel ingot when newly
stripped is far too hot in the interior for the purpose of rolling, and
if it be kept long enough for the interior to become in a fit state,
then the exterior gets far too cold to enable it to be rolled
successfully. It has been attempted to overcome this difficulty
by putting the hot ingots under shields or hoods, lined with
non-heat-conducting material, and to bury them in non-heat-conducting
material in a pulverized state, for the purpose of retaining and
equalizing the heat; but all these attempts have proved futile in
practice, and the fact remains, that the universal practice in steel
works at the present day all over the world is to employ a heating
furnace of some description requiring fuel.
The author introduced his new mode of treating ingots at the Darlington
Steel and Iron Company's Works, in Darlington, early in June this year,
and they are now blooming the whole of their make, about 125 tons a
shift, or about 300 ingots every twelve hours, by such means.
The machinery at Darlington is not adapted for rolling off in one heat;
nevertheless they have rolled off direct from the ingot treated in the
"soaking pits" a considerable number of double-head rails; and the
experience so gained proves conclusively that with proper machinery
there will be no difficulty in doing so regularly. The quality of the
rails so rolled off has been everything that could be desired; and as
many of the defects in rails originate in the heating furnace, the
author ventures to predict that even in this respect the new process
will stand the test.
Many eminently practical men have witnessed the operation at Darlington,
and they one and all have expressed their great surprise at the result,
and at the simple and original means by which it is accomplished.
The process is in course of adoption in several works, both in England
and abroad, and the author hopes that by the time this paper is being
read, there may be some who will from personal experience be able to
testify to the practicability and economy of the process, which is
carried out in the manner now to be described.
A number of upright pits (the number, say, of the ingots in a cast) are
built in a mass of brickwork sunk in the ground below the level of the
floor, such pits in cross-section being made slightly larger than that
of the ingot, just enough to allow for any fins at the bottom, and
somewhat deeper than the longest ingot likely to be used. In practice
the cross section of the pit is made about 3 in. larger than the large
end of the ingot, and the top of the ingot may be anything from 6 in. to
18 in. below the top of the pit. These pits are commanded by an ingot
crane, by preference so placed in relation to the blooming mill that the
crane also commands the live rollers of the mill.
Each pit is covered with a separate lid at the floor level, and after
having been well dried and brought to a red heat by the insertion of hot
ingots, they are ready for operation.
As soon as the ingots are stripped (and they should be stripped as early
as practicable), they are transferred one by one, and placed separately
by means of the crane into these previously heated pits (which the
author calls "soaking pits") and forthwith covered over with the lid,
which practically excludes the air. In these pits, thus covered, the
ingots are allowed to stand and soak; that is, the excessive molten
heat of the interior, and any additional heat rendered sensible during
complete solidification, but which was latent at the time of placing
the ingots into the pit, becomes uniformly distributed, or nearly so,
throughout the metallic mass. No, or comparatively little, heat being
able to escape, as the ingot is surrounded by brick walls as hot as
itself, it follows that the surface heat of the ingot is greatly
increased; and after the space of from twenty to thirty minutes,
according to circumstances, the ingot is lifted out of the pit
apparently much hotter than it went in, and is now swung round to the
rolls, by means of the crane, in a perfect state of heat for rolling,
with this additional advantage to the mill over an ingot heated in an
ordinary furnace from a comparatively cold, that it is always certain to
be at least as hot in the center as it is on the surface.
[Illustration: Fig. 2]
Every ingot, when cast, contains within itself a considerably larger
store of heat than is necessary for the rolling operation. Some of this
heat is, of course, lost by passing into the mould, some is lost by
radiation before the ingot enters into the soaking pit, and some is lost
after it enters, by being conducted away by the brickwork; but in the
ordinary course of working, when there is no undue loss of time in
transferring the ingots, after allowing for this loss, there remains a
surplus, which goes into the brickwork of the soaking pits, so that this
surplus of heat from successive ingots tends continually to keep the
pits at the intense heat of the ingot itself. Thus, occasionally it
happens that inadvertently an ingot is delayed so long on its way to the
pit as to arrive there somewhat short of heat, its temperature will be
raised by heat from the walls of the pit itself; the refractory mass
wherein the pit is formed, in fact, acting as an accumulator of heat,
giving and taking heat as required to carry on the operation in a
continuous and practical manner.
[Illustration: GJERS' SOAKING PITS FOR STEEL INGOTS.]
During the soaking operation a quantity of gas exudes from the ingot and
fills the pit, thus entirely excluding atmospheric air from entering;
this is seen escaping round the lid, and when the lid is removed
combustion takes place.
It will be seen by analyses given hereinafter that this gas is entirely
composed of hydrogen, nitrogen, and carbonic oxide, so that the ingots
soak in a perfectly non-oxidizing medium. Hence loss of steel by
oxidation does not take place, and consequently the great loss of
yield which always occurs in the ordinary heating furnace is entirely
obviated.
The author does not think it necessary to dilate upon the economical
advantages of his process, as they are apparent to every practical man
connected with the manufacture of steel.
The operation of steel making on a large scale will by this process be
very much simplified. It will help to dispense with a large number of
men, some of them highly paid, directly and indirectly connected with
the heating department; it will do away with costly heating furnaces and
gas generators, and their costly maintenance; it will save all the coal
used in heating; and what is perhaps of still more importance, it will
save the loss in yield of steel; and there will be no more steel spoiled
by overheating in the furnaces.
The process has been in operation too short a time to give precise
and reliable figures, but it is hoped that by the next meeting of the
Institute these will be forthcoming from various quarters.
Referring to the illustrations annexed, Fig. 1 shows sectional
elevation, and Fig. 2 plan of a set of eight soaking pits (marked
A). These pits are built in a mass of brickwork, B, on a concrete
foundation, C; the ingots, D, standing upright in the pits. The pits are
lined with firebrick lumps, 6 in. thick, forming an independent lining,
E, which at any time can be readily renewed. F is a cast iron plate,
made to take in four pits, and dropped loosely within the large plate,
G, which surrounds the pits. H is the cover, with a firebrick lining;
and I is a false cover of firebrick, 1 in. smaller than the cross
section of the pit, put in to rest on the top of the ingot. This false
cover need not necessarily be used, but is useful to keep the extreme
top of the ingot extra hot. J is the bottom of the pit, composed of
broken brick and silver sand, forming a good hard bottom at any desired
level.
Figs. 4 and 5 show outline plan of two sets of soaking pits, K K, eight
each, placed under a 25 ft. sweep crane, L. This crane, if a good one,
could handle any ordinary make--up to 2,000 tons per week, and ought to
have hydraulic racking out and swinging round gear. This crane places
the ingots into the pits, and, when they are ready, picks them out and
swings them round to blooming mill, M. With such a crane, four men and a
boy at the handles are able to pass the whole of that make through the
pits. The author recommends two sets of pits as shown, although one set
of eight pits is quite able to deal with any ordinary output from one
Bessemer pit.
In case of an extraordinarily large output, the author recommends a
second crane, F, for the purpose of placing the ingots in the pits
only, the crane, L, being entirely used for picking the ingots out
and swinging them round to the live rollers of the mill. The relative
position of the cranes, soaking pits, and blooming mill may of course be
variously arranged according to circumstances, and the soaking pits may
be arranged in single or more rows, or concentrically with the crane at
pleasure.
Figs. 4 and 5 also show outline plan and elevation of a Bessemer plant,
conveniently arranged for working on the soaking pit system. A A are
the converters, with a transfer crane, B. C is the casting pit with
its crane, D. E E are the two ingot cranes. F is a leading crane which
transfers the ingots from the ingot cranes to the soaking pits, K K,
commanded by the crane, L, which transfers the prepared ingots to the
mill, M. as before described.
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