CAST STEEL
We have seen how primitive man hunted and fought with no implements and
weapons better than clubs, bows and arrows, and stone hatchets, and how
his wife cracked and ground the corn between flat rocks or in mortars of
stone. In the succeeding “Bronze Age” we found ornaments, idols and
tools being made of copper or the copper alloy, bronze. It was only
after the next great advance that we found man utilizing iron for his
purposes of civilization. This metal, which with us is so common, was in
those days very expensive, so much so that it could be used only for
purposes of war and as the gifts of kings.
But the world was traveling fast and it was not long before the
iron-carbon alloy, steel, was produced. Even so, many hundreds of years
elapsed before the present wonderful age was ushered in through the
great inventions of Henry Bessemer and the Siemens brothers. And while
fine steels for swords and tools have had an incalculably great
influence upon the development of the human race, it was the mammoth
production of Bessemer and open-hearth steel which permitted its general
use as a material for construction of ships, bridges, buildings, and for
railroads, that made this the “Age of Steel.”
Speaking in terms of the power house, it is also the “Age of Cast
Steel.” Twenty-five years ago the manufacturer and power house man were
quite content with their “saturated” steam temperatures and pressures.
With cast iron valves and fittings their plants were well equipped.
[Illustration:
STEEL CASTINGS SHOWING THE RISERS ON THE FLANGES
Castings for use on steam, ammonia, water lines, etc., must be of very
close-grained metal and require much larger risers than castings for
less exacting service.
]
But the world did not stand still. It became known that by heating the
steam out of contact with the water in the boiler it lost the moisture
which it carried and became dry and then could be charged with as much
additional heat as it was desired to give to it. This “superheated”
steam, of course, would do more work and it had also certain other
advantages which the old-fashioned “saturated” steam had not.
But while cast iron fittings gave satisfactory service up to
temperatures, say, around 450° F., they faltered when forced to work
under the new conditions which meant decidedly higher temperatures and
pressures. And, too, the repeated heatings and coolings which were often
necessary, disclosed a disadvantage previously unknown—a so-called
“permanent growth” of the cast iron which was attended by loss of
strength, and altogether it was soon found out that when superheated
steam was to be used, higher types of materials were advisable than
those which had been used under old conditions.
[Illustration:
STEEL CASTINGS IN THE ANNEALING OVEN
]
Superheated steam has rapidly come into general use. Some of the new
locomotives and most of the modern power plants are now built for as
much as 200° superheat, i.e., a total temperature of approximately 600°
Fahrenheit.
Valves and fittings of cast steel not only are the articles “de luxe”
for such service but they have come to be considered the necessary
articles and their advantages have only fairly begun to be appreciated.
Though our most august scientific societies are proposing and debating
upon systems of classification which shall include and satisfactorily
define all of our ferrous metals, a satisfactory one has not yet been
evolved, and, considering the intricacy of our ferrous metallurgy and
the discoveries which are being made almost daily, the outlook for a
strictly logical classification is not yet flattering.
With “Cast Steel” our metallurgical nomenclature is again faulty. Before
what we now call the “steel casting” was known, crucible steel was
poured into ingots, “forged” into tools just as it now is and often went
under the name “Cast Steel” to distinguish it from the contemporaneous
material, wrought iron. So to-day we buy many tools and implements which
bear the name cast steel, which we know to have been forged in bringing
them into their final shape.
But it is not these which we mean by the term, cast steel, but rather
those steel products which get their final form by being “cast” from a
fluid condition into a mold. These are what are rapidly coming to be
understood when the term “cast steel” is used.
Satisfactory metal for steel castings may be made in any of three or
four types of furnaces, but, as was suggested before, the making of
molds for castings is a fine art, as is the preparation of the metal
which is to go into them. Further, the making of that special class of
castings which are to withstand water, steam or air pressure is a very
different thing from the making of steel castings for other purposes,
and this is too often forgotten.
For the former are necessary particularly close-grained castings, free
from flaws or spongy spots. Under the great pressures applied such
defects would certainly allow leakage.
[Illustration:
FLANGES AND FITTINGS OF CAST STEEL
]
Whatever the method of production of steel for castings the metal is
poured into molds to receive its final shape. Because of the intensely
high heat of the steel only sands of great refractoriness (resistance to
heat) can be used as material for the mold. White silica sand is such a
material and is generally used, mixed with enough clay and
molasses-water to give it “bond.” While molds for some steel castings
are made in “green” (i.e., undried or unbaked) sand, baked molds are
preferred for fine finish and surest results. After the making of the
molds in the usual way they are sprayed with very finely powdered white
sand or quartz mixed with a little molasses-water. They are then
thoroughly dried in an oven.
Cast steel shrinks during cooling even more than malleable iron and the
pattern and mold must be made to allow for this. Upon the freezing of
the surfaces of the casting with consequent attainment of rigidity, the
interiors, which freeze last, may have cavities unless means for
avoiding them is provided. For this purpose heavier pieces, which later
can be cut off, are cast upon such parts of the casting as tend to have
“shrink holes.” These may be likened to receptacles filled with fluid
metal, which being larger than the parts of the castings which they
“feed,” hold excess metal in fluid condition until the casting itself
has become solid throughout. Such are usually called “risers” or, in
Europe, “lost heads,” and the molten metal in them flows down into the
interior of the casting and fills the shrink holes which are forming.
Not only must the risers be large enough that the metal in them is the
last to solidify but they must be built high enough above the casting
that sufficient pressure is exerted on the steel entering the shrinking
parts to make its entry sure.
[Illustration:
GRAIN OF STEEL CASTINGS AS THEY COME FROM THE MOLD
]
[Illustration:
GRAIN OF STEEL CASTINGS AFTER
ANNEALING
]
(Magnification 60 diameters)
Baked molds, of course, are comparatively rigid. As the risers which
stand on top of the flanges and other high parts of castings aid in
resisting the natural shortening of long castings during and after
“setting” of the metal, there is great liability that the still red-hot
casting will crack somewhere along its length. It is therefore necessary
to loosen with bars the sand of the mold as soon as the metal of the
casting has set, particularly between the risers, and to break out the
sand of the core inside, around which the shrinking metal might crack
were the sand left in its hard packed condition.
After the casting is shaken out from the mold, it is cleaned and the
risers cut off either by sawing or with the more modern oxy-acetylene
torch flame.
[Illustration:
OTHER TYPICAL STEEL CASTINGS
]
Steel castings should be annealed in order to “refine,” i.e., make finer
the grain of metal and to equalize “strains” which are set up in the
castings during cooling. Coarse grain and internal strains tend to make
the castings brittle. No such extended annealing, however, is necessary
as is the case with malleable cast iron, for no divorcing of carbon from
the iron with separation of free carbon is possible. The castings are
carefully heated to a temperature of about 1600° or 1700° Fahrenheit and
allowed to cool slowly.
After annealing, they are cleaned and excrescences removed by chipping,
after which the castings are tapped, drilled or otherwise machined
according to the purposes for which they are intended.
[Illustration:
CAST STEEL VALVES, STEAM SEPARATOR, AND DIRECT-RETURN TRAP FOR USE
WITH SUPERHEATED STEAM
]
While more costly in manufacture and installation than are those of cast
iron, valves, fittings and other cast steel products are, so far as we
now know, practically permanent. Their notable shock resisting quality
is well shown in the following table which is reprinted from page 188.
[Illustration:
POURING STEEL INTO MOLDS FROM A BOTTOM-POUR LADLE
]
It is to be noted that while malleable cast iron far surpasses
“semi-steel” in this property, though their tensile strengths are
ordinarily somewhere near the same, cast steel, in turn, offers more
than six times the resistance of the malleable iron to shock and has
nearly double its tensile strength. It is this great strength and
resistance to shock, heat and pressure, with freedom from “permanent
growth” under alternate heatings and coolings that make cast steel such
a valuable material for the many purposes for which castings are to-day
employed. Millions of steel castings annually find varied application.
═══════════════════╤════════╤══════╤══════
Alloy │Tensile │Number│Total
│Strength│ of │ Foot
│ │Blows │Pounds
───────────────────┼────────┼──────┼──────
Cast Iron │ 23,400│ 7│ 102
Semi-steel │ 35,050│ 11│ 206
Malleable Cast Iron│ 37,140│ 22│ 1,580
Cast Steel │ 72,120│ 92│10,112
───────────────────┴────────┴──────┴──────
In modern power houses and other commercial steam and hydraulic
installations particularly, steel castings have come to be the materials
usually specified and approximately the only ones which satisfactorily
serve under the severe conditions of to-day.
[Illustration:
POURING FROM A LIP-POUR LADLE
]
Undoubtedly the first steel castings were poured from crucible steel,
though we must remember that the crucible is a melting and not a
refining furnace. This was only natural. In the crucible the metal can
be made very hot and fluid, and if of proper composition and properly
“killed” crucible steel makes very fine castings. Crucible steel
castings, however, are not in as fortunate a position as are other
products of this high grade material. Tool steels ordinarily bring high
enough price that there remains a profit to the manufacturer though his
manufacturing cost is necessarily high. In the steel casting line,
however, there is much keener competition and crucible steel has had
considerable difficulty in maintaining its place. It seems to be a
matter of price alone.
Open-hearth steel is very largely used for steel castings, more than
two-thirds of all made in this country being of this material. About one
half of these are poured from basic open-hearth metal, and the other
half from acid metal. It is generally considered that the product of the
acid-lined furnace is a little freer from over-oxidation.
[Illustration:
TAPPING SIDE OF TWO-TON OIL-BURNING OPEN-HEARTH FURNACE FOR STEEL
CASTINGS
]
Open-hearth steel cannot generally be as hot and fluid as are the steels
made in other types of furnaces. For this reason as well as because of
the larger size of the usual open-hearth furnace, small castings are not
generally poured from this steel. It is for steel castings of
considerable size and where there are sufficient orders to warrant a
steady and large output that the open-hearth has its place. True,
smaller open-hearths are now built, some of them of only two or three
tons capacity, but, in general, the standard open-hearth for steel
castings is of fifteen tons or more capacity and of the style of the
open-hearth furnaces which were described in Chapter IX.
In their proper sphere they are highly satisfactory, but they are
“inelastic” in that they must be run continuously day and night and
should not be allowed to cool until extensive repairs are imperative.
It was mentioned that in the open-hearth process the furnace is always
hotter than the metal which it contains and that the heat which can be
put into the steel is limited by the ability of the refractories of
which the roof and side walls are made to withstand melting. In the
Bessemer process the metal is hotter than the furnace because the heat
is generated by combustion of certain of the metalloids contained in the
metal itself. As metal for castings must be very hot and fluid the
Bessemer process is very satisfactory for the making of steel for
castings.
[Illustration:
A 30–TON BASIC OPEN-HEARTH FURNACE TAPPING
The overflow from the ladle into the pit is slag.
]
It has, also, the advantage of “elasticity.” The supply of metal is
practically continuous and one furnace can make from one to eighteen or
even more heats on day turn only and be shut down for the night turn or
longer and then started again without such loss as would result from the
shutting down of an open-hearth furnace with regenerators.
For the making of metal for steel castings, very small-sized Bessemer
converters are used which make from one to three tons of metal per blow.
Some converters of as little as one-half ton capacity are being used.
While some are of the “bottom-blown” type already described, the
majority are what are called “surface-blown” or “side-blown.” In these,
from four to eight round tuyères, about one and one-half inches in
diameter each, pierce the brick or ganister lining just above the
surface of the bath. They slope downward a little toward the bath so
that when the converter is tipped to its upright or blowing position the
air blast will strike the adjacent edge of the metal and blow across its
surface. This three or four pounds per square inch of air blast keeps
the metal in circulation, meanwhile burning out its silicon, manganese,
and carbon, just as it does in the larger bottom-blown converters.
Surface-blown give hotter metal than do bottom-blown converters and very
fine steel castings are made from their metal. For these converters,
which are practically all acid-lined (i.e., with silica or clay brick or
ganister), metal low in phosphorus and sulphur is regularly drawn from a
cupola specially run for the purpose.
[Illustration:
SMALL SIDE-BLOWN CONVERTER MAKING STEEL FOR CASTINGS
]
The remaining recognized type of furnace for steel for castings is the
comparatively new electric furnace.
Commercial melting of metals by the electric current has been sought for
half a century. In 1879 the first furnaces of promise were patented by
Sir William Siemens, one of the Siemens brothers who became so well
known through their great work with the open-hearth furnace, the gas
producer and many other things metallurgical. While Siemens melted as
much as twenty-two pounds of iron per hour in his furnace, the cost of
the electric current at that time was so high as to be practically
prohibitive for the manufacture of steel in competition with the
open-hearth, Bessemer and crucible processes.
[Illustration:
DRAWING OF SIDE-BLOWN CONVERTER IN BLOWING POSITION, SHOWING EDGE OF
METAL EVEN WITH ROW OF TUYÈRES
]
Little of great moment in the electric furnace line developed during the
nineteen years which followed. Then, in 1898, Stassano in Rome, Italy,
constructed a furnace in which three carbons gave an electric arc above
the surface of the bath. About the same time, Heroult, a Frenchman, was
developing the electric furnace which to-day has become so well known in
this country, and which bears his name. Other well known furnaces of the
arc type are the Gronwall-Dixon, the Snyder, the Girod and the
Rennerfelt.
In general, electric furnaces have more or less round steel shells with
shallow brick, magnesite or sand-lined hearths, and sidewalls and
removable roofs of brick. Heat, of course, is furnished electrically. In
most of them long carbon electrodes are lowered through holes in the
roof until the lower ends strike an arc with the metal on the hearth.
The number of carbons may be from one to four or more depending upon the
style and size of the furnace and the manner in which electrical
connections are made. All of the furnaces mentioned have been used for
the production of steel for castings and the Heroult and Girod are in
use in larger sizes for electric steel for rails and miscellaneous
products. The steel is first cast into ingot molds and is later rolled
down into bars, rods, etc.
[Illustration:
A GRONWALL-DIXON 5–TON ELECTRIC FURNACE TAPPING
]
[Illustration:
FIRST EXPERIMENTAL ARC ELECTRIC FURNACES PATENTED BY SIR WILLIAM
SIEMENS IN 1879
]
All of the above use carbon electrodes and are known as “arc” furnaces.
There is a distinctly different type of furnace which, also, is in use
in commercial sizes. This is the “induction” furnace. In this, what is
known to electricians as a secondary current is “induced” in the bath
itself and heats the metal. Of this type the Kjellin and the
Rochling-Rodenhauser are the best known in this country. While they are
in use in the larger sizes for production of steel for ingots, these two
furnaces do not seem to have been used to any extent for metal for steel
castings.
The details of construction of the furnaces which are used for metal for
castings are more or less different, but they are not of particular
interest to us. The working of all is similar and a general description
should suffice.
[Illustration:
DRAWING OF THE SNYDER ELECTRIC FURNACE
]
[Illustration:
SMALL SNYDER ELECTRIC FURNACE TAPPING
]
Whether starting with furnace cold or hot, materials in molten or in the
more usual “cold” form are charged on the shallow hearth of the furnace.
The charging doors are closed, the current is turned on and the carbon
electrodes are lowered until an arc is struck between the upper
electrodes and the metal on the hearth, which in some way is made to
connect with the negative electrodes. In one or two of the types
mentioned the arc plays between the carbons, all of which are above the
bath.
At first there are great fluctuations in the current intensity because
of the uneven surface presented by scrap steel on the hearth. In a short
time, however, the current steadies. The intense heat of the arcs soon
brings cold steel to a molten condition.
Occasional attention from the attendant is necessary to see that the
melting is even and that any outlying pieces of steel are pushed to the
center where they must melt.
[Illustration:
SKETCH OF THE HEROULT THREE-PHASE ELECTRIC FURNACE
There are three electrodes, all of them above the bath. Only two show
here.
]
In the basic-lined furnaces lime is usually charged with the cold steel.
With the iron oxide which is added from time to time this forms a highly
oxidizing slag, which, after it takes the phosphorus from the metal, is
skimmed off.
As you will remember, the other processes stop at this point, little
further refining being possible. In the electric furnace, however, the
sulphur, also, can be reduced to almost any desired amount by use of a
further addition of lime, and greater heat. Not only can the sulphur be
reduced to very small percentages but the over-oxidized bath can be
brought to neutral condition and the green or black slag made white with
return of its manganese and iron to the bath. This is accomplished by
addition of small amounts of powdered coke or coal. The whole process is
under very accurate control.
With a practically white slag, which is the signal that the deoxidation
of the bath is complete, and the sulphur reduced, the steel is ready to
pour provided it is hot enough. Tests of this are usually made either by
pouring a little of the steel from a small ladle and observing its
fluidity or by observing the quickness with which the end of an iron bar
is melted off when plunged into the metal in the furnace.
[Illustration:
THE HEROULT ELECTRIC FURNACE
]
Of all the metallurgical steel furnaces, the electric furnace is the
most susceptible of accurate control. With the heat applied directly to
the metal in the cleanest way possible, i. e., without the admission of
coal ash or gas or air of the blast, the atmosphere in the tightly
closed electric furnace can be made “oxidizing,” “neutral,” or
“reducing” at will. The metal can be held in the furnace and additions
made, samples taken, and the operations conducted with regulation and
certainty.
This newly devised metallurgical apparatus is coming to be largely used
in the production of tool steels. While it has not displaced the
crucible method for the production of steels of the very highest
qualities, it has proceeded far enough in this direction in the very
limited number of years since its introduction, that it is certain that
the crucible, even for tool steels, is to have a keen competitor. Tool
steels in considerable variety are to-day being quite satisfactorily
made in the electric furnace and it is not at all unlikely that steels
of the very highest grade will shortly be produced by this method.