CAST IRON
From the preceding chapters we now know pretty well the place which cast
iron occupies in the iron family. In the chapters which have succeeded
the one in which we discussed the blast furnace and pig iron, every one
of the products except crucible steel has been produced through some
“refining” operation which greatly changed the composition, structure
and properties of the product. Cast iron is not the result of a refining
operation in this sense of the word. It is produced through simple
mixing of pig irons of various compositions, usually with some admixture
of iron castings of similar composition which have outlived their
usefulness in the industrial world and have been returned as scrap to be
remelted.
When we say that cast iron is not produced through a refining operation,
it must not be inferred that no change in composition occurs during the
remelting. There is some change, notably a loss through oxidation from
the air blast of a little of the silicon and manganese. Aside from this
there usually will be absorption of enough, or sometimes more than
enough, carbon from the coke used in melting to make up for the carbon
which is oxidized. Usually some sulphur also is taken up from the fuel.
There is, however, no such actual or intended alteration of composition
through burning out of the metalloids as is necessary for the production
of wrought iron and steel.
But from this we must not assume that the manufacture of cast iron for
chilled rolls, car wheels, machine parts, valves and fittings, etc., is
an easy proposition. As we will soon see, accurate regulation is
required of metal for proper depths of “chill” for rolls, car wheels and
castings which must have high resistance to wear. Too, the metal for
valves and fittings and other more or less complicated castings for high
steam, air, ammonia, water, etc., must be uniform, of close grain,
strong, yet soft enough to machine easily at the extremely high speeds
which modern efficient tools and methods demand. The production of the
best metal for such work requires the use of properly selected
materials, judicious mixing, and clever operation of the cupola furnace,
that the molten metal delivered to the foundry for the pouring of the
molds may be hot and fluid and of the right composition for the
particular work in hand.
[Illustration:
SAMPLING CARS OF HIGH SILICON PIG IRON
]
It is always interesting and instructive to follow the materials through
their course from the “raw” state to finished products, and, therefore,
we are going to take you on a little trip from the receiving yard of a
firm making cast iron goods where we see the cars of pig iron just in
from the blast furnace and where the materials are sampled and held
pending analysis, to the laboratory where the samples are analyzed, then
to the storage bins where the materials are unloaded, and, later, with
the weighed charges, to the cupolas which convert them into molten cast
iron of the proper composition and quality for high grade castings.
[Illustration:
SAMPLING OTHER PIG IRONS
Pig irons of lower silicon content cannot be broken easily with a
sledge but usually are thrown from a height across an iron block.
]
Twenty years ago it looked as if the iron foundry would be one of the
last strongholds of “rule-of-thumb” to give way to scientific methods.
It does not look so to-day, though there are many foundries which yet
buy and use their pig iron on the basis of _fracture_; i.e., the
foundryman guesses by judging of the color and closeness of grain and
other characteristics of fresh fractures of the pig irons how suitable
they are for his purpose and in what proportion to mix them. A skillful
man can get fair results in this way only so long as he uses the small
number of brands of pig iron with which he is perfectly familiar, and
even then there must be but little fluctuation in composition of the
irons used and he must be allowed considerable latitude in the quality
of the iron which he produces.
Success by this method is even more difficult now than it was ten years
ago, for the advent of many new blast furnaces and their greater variety
of products have made this rule-of-thumb mixing a much more uncertain
matter than it formerly was. Machine-made pigs, which are so generally
on the market now, give fractures which tell little regarding their
compositions.
[Illustration:
DRILLING THE SAMPLES
No oil or other lubricant is allowable and the drillings are taken up
with a magnet that no sand or other impurity may get into the sample
for
analysis.
]
While some foundries still attempt to accomplish this difficult and
sometimes impossible feat, the majority are now applying more scientific
methods to their manufacture of cast iron.
Though the eye cannot tell surely from the fracture the composition or
quality of the iron which is used in making up the charges, chemical
analysis does definitely give this information. Therefore, every car of
pig iron purchased by this firm is sampled and analyzed, the composition
of all other materials used in its mixtures is determined, and,
irrespective of fracture, which may or may not tell the truth regarding
their composition, the raw materials are charged with respect only to
their actual content of the metalloids. The resulting molten iron each
day is analyzed to confirm the correctness of the mixture and to furnish
analysis of the “sprues” which next day are to be used as a part of the
day’s charge. Physical test bars, too, are cast each hour or so, and the
tensile, transverse strengths, hardness, shrinkage, etc. are accurately
determined in testing machines and recorded. In this way absolutely
nothing is left to chance or to guess work, and, as you may surmise, any
slight deviation from the composition desired is shown at once and the
mixture immediately changed to the extent necessary to bring the iron
back to normal. It is surprising within what narrow limits of variation
compositions and physical properties can be held, with furnace
operations continually under such surveillance.
[Illustration:
WEIGHING OUT PORTIONS FOR ANALYSIS
The finely divided mixed drillings are shaken from a thin-bladed
spatula on to the balance pan. Drillings are added or taken off
until the long needle attached to the beam of the balance swings
over an equal number of divisions on each side of the center mark of
the white scale in the middle. Accuracy is 1/453,000 of an
avoirdupois pound; this is approximately the weight of the lead of a
“pencil mark” one inch long.
]
As the basis for its cast iron, many thousands of tons of pig iron are
each year used direct from the blast furnaces. The raw materials come in
railroad cars or by boat. The inspector who represents the metallurgical
department enters each car and inspects the materials, taking from each
a representative sample for analysis. In the case of pig iron this will
be from four to eight half pigs, it having been found by experience that
these represent very well the contents of the car. So each car of
material is held without unloading until it has been determined by
inspection and analysis that it is fully up to the specifications upon
which the iron was purchased.
Arriving at the laboratory, the half pigs from each car are drilled,
equal amounts of the drillings being taken and mixed in an envelope
which bears the name of the brand of iron, the number of the car, the
date, etc. The sample pigs from each car are treated in this same way,
each car being treated individually.
The envelopes containing the drillings then go to the chemists.
Frequently samples from fifteen or twenty cars of pig iron, with as many
other samples of various derivation, are being analyzed at the same time
for the four or six different constituents which it is necessary for the
metallurgists to know and control in order that a highly satisfactory
product may result. Though a hundred different determinations may be in
progress at the same time, spelling “chaos” in the mind of one not
entirely familiar with the details of the work, it will be interesting
to single out and explain briefly how the samples are analyzed.
[Illustration:
CLOSER VIEW OF THE WEIGHING
]
In such analytical work everything is based upon weight; i.e.,
constituents are determined and reported in percentages by weight. In
chemical laboratory work everywhere the metric system is used, the
cumbersome English system of weights and measures being practically
impossible. Thus, the metric system is the international scientific
standard. The unit taken is the gram, which is equivalent to ¹⁄₄₅₃ part
of an avoirdupois pound. One gram of pig iron drillings is such an
amount as could be held on an ordinary ten-cent piece.
Working with such small amounts of the sample, exactness and skill are
extremely necessary. The balances used are necessarily very
delicate—just as delicate as were the scales upon which the jeweler
weighed your diamonds—you remember, of course. On these balances we can
weigh an inch-long mark made by an ordinary lead pencil.
[Illustration:
DISSOLVING IN ACIDS
This is done under a hood that the irritating fumes given off may be
kept from the room.
]
As the results of the analysis have to be known inside of three or four
hours that the cars may be quickly unloaded in order to avoid demurrage,
which is the penalty for holding cars longer than the allowable time,
separate portions of each sample are weighed out for determination of
the silicon, manganese, sulphur, phosphorus, graphitic carbon, and
combined carbon. These are necessary in order to determine that the iron
is up to the quality specified in the purchase contract and also to
provide for its most efficient use in the manufacture of iron castings.
The exactly weighed portions are put into clean, numbered beakers, which
are small pieces of high grade glassware that will stand sudden changes
of heat and cold. Some of these portions are dissolved in nitric acid,
some in hydrochloric acid, others in combinations of acids. In each case
the drillings go into solution in the acids, and after various
treatments of boiling, evaporating, filtering, etc., well known to those
of the chemical profession, the desired results are obtained. In some
cases it is by actually weighing a constituent which has been filtered
out and burned to ash of a constant known composition, in others it is
by comparison of color with standards of known composition, and
sometimes it is by other means.
[Illustration:
FILTERING SILICONS
After evaporating the excess acid, baking dry, cooling, and
redissolving in weaker acid, the silicon compound formed may be
filtered out. The iron and other soluble constituents, now in
solution, pass through the filter, which is of pure, porous,
unglazed paper.
]
In all of this analytical work the chemist must take care to lose not
one drop of the solution or one grain of the ash from the burned
“precipitate,” as the “filtered out” constituent is called.
The pig iron is always bought upon guarantee that it will contain a
certain percentage of silicon—the element which in cast iron is known as
a “softener.” But this is not the only thing necessary in the iron that
is purchased. It must also show proper specified quantities of
manganese, phosphorus and carbon, which also are very desirable elements
in iron castings, and as little of that undesirable element, sulphur, as
possible. Therefore they pay in proportion to the content of silicon,
manganese, phosphorus, and carbon—and penalize the seller for sulphur.
[Illustration:
“BURNING OFF” THE SILICONS
The paper and contents, in a little crucible, are placed in a red-hot
muffle furnace. The paper is such pure cellulose that it leaves no
weighable ash. That which remains after burning is silicon oxide,
which is a perfectly white, fine sand. This is very carefully
weighed. (Ordinary sand is silicon oxide usually slightly colored
with iron.)
]
The laboratory holds copies of the contracts upon which these materials
were bought. If, upon comparison, the analysis obtained complies with
the terms of the contract, an O. K. unloading slip is made out and the
receiving department is given directions into what raw-material bin in
the receiver building it shall be unloaded. If not fully up to the
standard called for in the contract, the purchasing department is
notified and the car is either rejected or accepted upon some proper
terms of adjustment if it can be used without detriment to the product
in which it is to be utilized.
Cars of coke, limestone, fluorspar, etc., are inspected, analyzed and
treated in the same way, so that nothing is left to guess work. The
compositions as determined by the laboratory serve not only as the basis
for acceptance or rejection, but the analyses of accepted materials are
forwarded at once to the metallurgists, who from them figure the
mixtures to be used in the cupolas.
Having great stocks of analyzed raw materials in the labeled bins in the
receiver building, the metallurgists who supervise the mixing and
melting of the iron determine by mathematical calculation just what
irons and how much of each must be taken to give molten iron of the best
composition and properties for the castings.
[Illustration:
TITRATING THE SULPHUR
Sulphur is evolved from the drillings as a gas (hydrogen sulphide).
This is absorbed in a solution of chloride of zinc. The amount of
sulphur is measured by slowly running in from a burette a solution
of iodine of very accurately known strength. The iodine unites with
the sulphur compound as long as any of the latter remains, but the
first drop added thereafter turns blue the whole solution because of
the reaction of the excess of iodine with starch paste that has been
added previously as an indicator. Accuracy is about .005 per cent of
sulphur.
]
The total iron materials charged must have a definite amount of silicon,
of manganese, of phosphorus and of carbon. For a 4,000–pound charge for
soft cast iron, for instance, the total silicon in the materials which
make up the charge must be somewhere near 118 pounds, the manganese and
the phosphorus about 30 pounds each. The usual losses of these materials
through oxidation are known, of course, and sufficient excess has been
allowed that the desired final composition will result.
[Illustration:
TITRATING PHOSPHORUS
A yellow precipitate containing the phosphorus is filtered out on
filter paper. It is redissolved in alkali and titrated with a
standard solution of nitric acid, similarly to the sulphur. The
solution in the flask turns pink with the first drop added after the
phosphorus has been measured.
]
On several scales which are regularly inspected and kept carefully
adjusted, the weighers weigh out the prescribed quantities of the raw
materials. “Buggies” holding nearly one ton each are loaded in turn with
coke, with proper amounts of pig iron, cast iron scrap, sprues from the
foundry castings of the preceding day, and a proper weight of limestone
flux. Each charge of two tons of iron requires four buggies for its
transportation from the raw-material bins and scales to the cupolas.
The old-time way was for laborers to dump the charges into the cupola
and spread the materials by hand, but in modern foundries better ways
have been provided. Here a charging machine operated by compressed air
lifts into the furnace, one by one, the buggies of coke, and of the
other materials, whence, after the dumping of their contents, the
buggies are returned to the receiver building to be filled again.
Thus many charges per hour pass through the yawning charging doors of
the cupolas, being dumped in fast enough to maintain the level
approximately even with the bottom of the charging door.
[Illustration:
READING THE CARBONS
The higher the combined carbon the darker the nitric acid solution of
the iron or steel. The solution is diluted with water or weak acid
until the color matches that of a “known” sample or standard.
Accurately graduated comparison tubes are used.
]
In starting, a wood fire has been made on the sand bottom of the cupola.
This is covered with coke in such a way and in such amount that, when
ready for charging of the metal, a column of glowing coke extends to a
distance of one foot or two above the tuyères. Upon this “bed” in
alternating layers are piled the weighed amounts of pig iron, sprue,
scrap and limestone as described above. Following each charge is a layer
of coke sufficient in amount to replace the “bed” coke which is burned
away in melting the iron charge, thus maintaining the top of the bed of
coke throughout the day at approximately the same height.
[Illustration:
WEIGHING THE GRAPHITIC CARBON
The graphite is filtered out on an asbestos pad in a perforated
platinum crucible. After drying until all moisture is gone it is
weighed, ignited, and weighed again. The loss of weight equals the
weight of the graphite of the sample.
]
Ever since 7 o’clock A.M., when the twelve to sixteen ounces of blast
pressure was put on, the charges have been descending gradually from the
charging door. Encountering the intense heat in the “melting zone” at
the top of the bed of coke a little above the tuyères, the iron melts
and trickles down through the three to five-foot bed of glowing coke on
to the sand cupola bottom or hearth where it accumulates. The tapper,
with his iron bar and “bod stick” with its little ball of moist fire
clay, alternately opens and plugs the tap hole at the bottom of the
furnace as occasion requires, but throughout the day of ten or more
hours there is almost constantly a full stream of iron flowing from the
spout. The big “bull ladle” which receives it, in turn gives it up to
smaller or “shank” ladles, in which it is conveyed along trolleys to
still smaller ladles from which it is poured into the sand molds to form
the castings.
[Illustration:
ANOTHER CLOSE-UP VIEW
]
As in the blast furnace, limestone is added as flux to make liquid and
dispose of sand, dirt, scale, etc., which are detrimental. The liquid
slag formed from union of limestone with these impurities floats upon
the molten iron in the cupola hearth, as it is less than half as heavy
as the iron itself. It flows almost continuously from a higher hole
called the “slag hole,” in the rear of the furnace and just beneath the
tuyères. The slag has little value except as material for filling
purposes, etc. So-called “slag wool” can be made by blowing air through
it. Sometimes the blast from the cupola blows it in such a way that this
pure white “wool” is formed and blows out of the slag hole of the
cupola. About Christmas time some of the workmen take quantities of it
home for decoration and for fireproof whiskers for “Santa Claus.”
These operations go on continuously throughout the day, each cupola
making the particular grade of cast iron or “semi-steel” which is best
adapted to the particular castings to be poured, size, shape and purpose
of
[Illustration:
DETERMINING CARBON BY DIRECT COMBUSTION
In an electric furnace with pure oxygen passing over them, the
drillings burn as would splinters of wood. From the gas given off
the total carbon is determined with great accuracy.
]
[Illustration:
WEIGHING PIG IRON FOR THE CUPOLA
From three to six varieties of pig iron are used in each charge.
]
[Illustration:
THE RECEIVER BUILDING
Here pig irons of different compositions are separately kept in
numbered and labeled bins. The magnet which is used for unloading
and handling the iron may be seen. The grab bucket for sand is at
the right.
]
[Illustration:
WEIGHING THE CHARGE OF COKE
Close weighing is necessary, for a variation of ten or fifteen pounds
may affect the running of the cupola.
]
the castings being the three main determining factors.
[Illustration:
SECTIONAL VIEW OF A CUPOLA
]
When nearing the end of the day, charging ceases, the charging door is
closed and the last charge, gradually descending, melts and flows into
the bull ladle about an hour later. As soon as the bull ladle is emptied
it is run out of the way, the cupola is drained of all iron and
considerable slag through the tap hole, and the bottom doors of the
cupola are dropped by pulling from under them the “props” which have
held them in position. The great mass of bed coke follows with a great
burst of heat, light and flame. This is quickly subdued with a stream of
water and removed. When cool, the slag and accumulations are chipped
from the cupola lining, burned areas are patched with bricks and stiff,
fire-resisting mud, the bottom doors are raised and fastened in
position, and the eight-inch sand bottom is packed in ready for the next
day’s run. After building a fire and getting a good “bed” of glowing
coke, the cupola is ready again for the charging of iron.
[Illustration:
A CUPOLA IN OPERATION
The stream of iron from the cupola spout flows into a “bull ladle” and
from that into “shank-ladles.” The bull ladle serves as a reservoir
and mixer.
]
[Illustration:
CASTINGS OF CAST IRON AS THEY COME FROM THE MOLDS
The “sprues” have not been removed.
]