CAST IRON (_Continued_)
Unlike the modern Bessemer, open-hearth, and other steel products which
are reworked, i.e., rolled, forged, etc., cast iron is a comparatively
old alloy dating back over several centuries. It cannot be rolled,
forged, or otherwise reshaped, so its final form must be given to it at
once by pouring or “casting” the molten metal into a mold. Its castings
serve exceedingly well in the hundreds of places for which they are
adapted. They are comparatively cheap, can be readily duplicated in
small or large quantities, and those from the softer grades of cast iron
may be machined easily.
These cast iron alloys have only from one-third to one-half the strength
of steel or wrought iron and are, comparatively speaking, very brittle.
Where resistance to severe shock must be withstood they should not be
used. Also, some varieties have a “habit” of growing larger upon
repeated heating and cooling. This “permanent growth” is particularly
noticeable when the alternate heating and cooling is at red heat or
over. Pieces of cast iron have been made to gain 15 per cent in linear
dimensions, and it is quite common knowledge among machinists that a
piece of cast iron which is slightly too small can be permanently
expanded by heat.
Nevertheless, the cast irons have large and legitimate fields in which
they are very serviceable. From most of their important present uses
they are not likely soon to be displaced.
[Illustration:
BECAUSE OF THEIR LARGE SIZE MOLDS FOR VERY LARGE CASTINGS HAVE TO BE
MADE ON THE FLOOR OF THE FOUNDRY, OR PARTLY IN PITS
]
Cast irons in considerable variety of compositions and physical
properties are available, as was indicated by alloys Nos. 14 to 19 which
were given in the table on page 83, part of which is here reproduced. In
alloys 3 to 13 the carbon exerts the great influence on the physical
properties, and this is true also of the cast irons. But all of the
latter have a total carbon content of more than 2½ per cent, and, under
certain conditions, some of the carbon assumes a different form from
that which we encountered in the steels. This modified form is
“graphite,” well known to us as a flaky, black, greasy-feeling material,
which is soft and very fragile. Graphite in the iron alloy naturally
weakens it and, as it is itself such a good lubricant, it makes cast
iron machine easily if sufficient amount is present.
A FEW OF THE CAST IRONS
════════════════════════╤════════╤═════════╤════════╤═══════╤══════════
│Silicon,│Graphite,│Combined│ Total │
│Per Cent│Per Cent │Carbon, │Carbon,│
│ │ │Per Cent│ Per │
│ │ │ │ Cent │
────────────────────────┼────────┼─────────┼────────┼───────┼──────────
No. 14. White Cast Iron│ .70│ .10│ 2.65│ 2.75│Very Hard
No. 15. Annealed │ .70│ 2.70│ .05│ 2.75│Machinable
Malleable Iron │ │ │ │ │
No. 16. Cast Iron for │ 1.00│ 1.00│ 2.00│ 3.00│Very Hard
Chilled Castings │ │ │ │ │
No. 17. Semi-steel │ 1.75│ 2.80│ .40│ 3.20│Machinable
No. 18. Gray Cast Iron │ 2.00│ 3.10│ .30│ 3.40│Machinable
No. 19. Soft Gray Cast │ 2.50│ 3.30│ .15│ 3.45│Machinable
Iron │ │ │ │ │
────────────────────────┴────────┴─────────┴────────┴───────┴──────────
Now, the above must be understood as being _typical_ compositions only.
There are, of course, irons of all intermediate compositions, also, and
while the total, graphitic and combined carbons, typically, are about as
indicated, there may be wide variation.
To illustrate what a variety of chemical and physical properties may be
produced, let us assume that the total carbon in a certain cast iron is
3.25 per cent. If this carbon is all in the _chemically combined_ form
(i. e., combined with the iron to form the very hard compound which is
known to the metallographist as “cementite”) the fracture will be white
and the alloy extremely hard. If none of this carbon is combined, but
all is in the form of graphite flakes throughout the alloy, the fracture
will be “gray” and the alloy soft and machinable. It is possible to
produce either of these two conditions or practically any intermediate
stage; i.e., we can almost at will split up the 3.25 per cent of carbon
into varying percentages of graphitic and combined carbon—the total
always equaling 3.25 per cent.
[Illustration:
No. 30. VERY SOFT CAST IRON. NOTE LARGE GRAPHITE FLAKES
]
[Illustration:
No. 31. MEDIUM HARD CAST IRON
The “combined carbon” is in the roundish, dark parts. It is the
“combined carbon” that increases the strength of cast iron and
steel.
]
The “precipitation” of graphite which is necessary for softness is
brought about mainly through the influence of silicon, which we before
termed the “softener.” Other conditions being equal, the higher the
silicon (if not above 4 per cent), the higher will be the graphite and
the lower the combined carbon; and vice versa, the lower the silicon the
lower will be the graphite and the higher the combined carbon. It is
mainly due to the “combined carbon” which is left after precipitation of
the graphite that the alloy has greater strength, hardness, and closer
grain. So, just as the steels are stronger and harder as the carbon
increases (in steel all the carbon is combined), so, other conditions
being equal, the strengths and hardnesses of the cast irons, within
usual limits, increase as the combined carbon increases.
[Illustration:
No. 92d. SEMI-STEEL. A CLOSER GRAINED AND YET STRONGER CAST IRON
]
[Illustration:
No. 33e. MOTTLED CAST IRON
So-called because it is a mixture of white and gray iron.
]
Just here it is interesting to remember that from the standpoint of
metallography _cast irons are simply steels in which there is what we
might call an impurity or an adulterant, graphite crystals_. It will be
seen at once that could these graphite crystals be removed from the cast
irons shown in photomicrographs No. 74, No. 92d, No. 30 and No. 31, we
would have alloys quite similar in appearance to the steels shown in
photomicrographs No. 3b and No. 22c which appeared on pages 77 and
78.[7]
Footnote 7:
Magnification 70 diameters.
[Illustration:
No. 7. WHITE CAST IRON
]
So the softer cast irons which are used for valves and fittings, machine
parts, radiators, hollowware, etc., have high silicon. Parts that do not
have to be machined can be of “harder” iron; i. e., made of iron having
lower silicon content.
Manipulation of the silicon content is not the only method by which the
hardness of cast iron can be influenced. Graphite can “precipitate”
(i.e., separate throughout the casting) only if sufficient time is given
it to do so. That is, the cooling of the casting after pouring must be
sufficiently slow. In a sand mold the iron remains molten for a time,
and after solidification it cools slowly enough that the greater portion
of the carbon separates as graphite. Therefore, castings of proper
composition made in sand molds are soft and machinable.
If the iron is poured into a mold the surfaces of which are made of
iron, the molten metal upon entering becomes solid almost as soon as it
takes the form of the mold, and it cools with great rapidity. Under such
conditions the carbon of the alloy is denied the time necessary to
change into the graphitic form and the casting has a white fracture and
is so hard that it cannot be machined.
[Illustration:
SECTION OF CHILLED CAR WHEEL
Showing white iron rim.
]
There are many purposes for which the alloy should have extreme hardness
and the great resistance to wear which accompanies such hardness. The
wearing faces of gears, brake shoes, rolls, and car wheels, for
instance, must be hard. For such products, white cast iron, the
extremely hard condition of the alloy, just referred to, is utilized.
Such castings are usually produced with a white cast iron face, but with
a gray iron interior, gray iron being less brittle and less likely to
break under shock or strain. A car wheel, for instance, has
approximately an inch in depth of white iron on the surface which lies
next to the rail on which it runs.
[Illustration:
CHILLED CAST IRON
The white edge resulted from the more rapid cooling against an iron
chill, as did the white rim of the wheel shown above.
]
Such are known as “chilled castings.” Molds for them are usually made of
sand, with pieces of iron (called “chills”) imbedded where white iron is
to be produced. The molten iron next to the sand surfaces cools in the
usual way and is gray and soft, while that which lies next to the
“chill” is white and extremely hard. The “depth” of the chilled layer
can be increased or diminished according to the thickness of the iron
“chill” used, its temperature, and by the composition and temperature of
the molten cast iron with which the mold is poured.
The sulphur and total carbon of the molten cast iron also have
considerable influence on the depth of “chill.”
[Illustration:
TWO-PART MOLDING FLASKS
]
There is a cast iron alloy which is familiarly known as “semi-steel.” It
is simply a high grade and stronger “gray” iron and must be classed as a
cast iron, as our table on page 180 shows. While it could undoubtedly be
made from materials which are commonly used for cast iron, it is
practically always produced by charging with these a certain amount of
steel scrap to bring about the lower silicon, phosphorus and total
carbon desired.
[Illustration:
A HOLLOW CYLINDER
The casting which we are about to make.
]
Because steel has a higher tensile strength than has cast iron, many
have inferred that it was the steel addition which made semi-steel
stronger than the ordinary soft cast iron alloy. The rather unfortunate
name, “semi-steel,” apparently was given because of the steel used and
the intermediate strength which the resulting product possessed.
However, during the melting down of the charge the steel scrap becomes
molten and its constituents merge with those of the other iron materials
charged. We get out of the cupola, then, a mixture which, disregarding
the losses and gains due to the air of the blast, the fuel, etc., is an
average of the materials charged. We, therefore, no longer have any
steel, but a cast iron which has a somewhat lower silicon, phosphorus
and carbon than the softer cast irons. The greater strength of the alloy
is due to its composition and only indirectly to the fact that steel was
used in its production. The physical properties of the steel charged
have been entirely obliterated in the melting process.
[Illustration:
SPLIT PATTERN OF WOOD, SURFACE-COATED WITH SHELLAC VARNISH
]
This view that semi-steel only indirectly gets its increase in strength
from the steel charged is confirmed by its structural appearance under
the microscope, as was shown in numbers 74 and 92d which were given on
page 79, and the photomicrographs given here, and by its extreme
brittleness under hammer blows. Under such shock it is but little more
resistant than cast iron.
[Illustration:
CORE THAT MAKES HOLE IN CASTING
]
This weakness under “shock” was shown by tests from which the table
which follows was compiled. Bars one inch square and thirteen inches
long laid on supports exactly twelve inches apart, were struck at the
center by a twenty-five pound weight. It took seven blows to break the
cast iron bar, the semi-steel bar required eleven, while cast steel
withstood ninety-two blows. Even this does not adequately express the
great resistance of the cast steel (another alloy not yet discussed),
for the height of the “drop” was being increased one inch with every
blow, and the cast steel bar, on account of its bending, had to be
regularly turned. The total foot-pounds exerted by the blows are given
in the table which follows:
[Illustration:
DRAG, OR BOTTOM HALF OF MOLD, AFTER PATTERN IS WITHDRAWN
]
[Illustration:
DRAG WITH CORE IN PLACE AND COPE, OR TOP HALF OF MOLD READY TO CLOSE
]
[Illustration:
TRANSPARENT MOLD, SHOWING RELATIVE POSITIONS OF CORE, CASTING, SPRUE,
ETC.
]
══════════════════════╤════════╤══════╤══════
Alloy │Tensile │Number│Total
│Strength│ of │ Foot
│ │Blows │Pounds
──────────────────────┼────────┼──────┼──────
Cast Iron │ 23,400│ 7│ 102
Semi-Steel │ 35,050│ 11│ 206
Malleable Cast Iron[8]│ 37,140│ 22│ 1,580
Cast Steel[8] │ 72,120│ 92│10,112
──────────────────────┴────────┴──────┴──────
Footnote 8:
On account of bending, the malleable iron and the steel bars had to be
turned several times.
Semi-steel is a very close-grained alloy of ten or twelve thousand
pounds per square inch greater strength than cast iron. It is a most
satisfactory material for medium and larger sized castings for which
cast iron formerly was used.
[Illustration:
CAST IRON TEE, COCK PLUGS, AND RETURN BENDS, WITH SPRUES AND GATES
ATTACHED
]
Did we say that cast iron was very brittle? So it is, comparatively
speaking. But just as the chemist will tell you that there are no
substances which are absolutely insoluble, just so does cast iron appear
to be extremely brittle only when compared with the iron alloys of
considerably less brittleness.
The three pictures of the hooped and twisted casting illustrate how
unwise it might be to speak with absoluteness. A few years ago the
casting illustrated on pages 190 and 192 was brought to this country by
a visitor to Europe, with an expression of regret that cast iron
produced in this country did not have such qualities of elasticity as
had cast iron made abroad. Whereupon, without any change whatever in his
iron mixture or cupola practice, the superintendent of a well-known
foundry made castings which were exact duplicates of that submitted. The
three photographs shown further on (Figs. A, B, C) were of one of these
castings. The ability to bend without breaking is, of course, largely
due to the shape.
[Illustration:
PLUGS AND WHEELS AS THEY COME FROM THE MOLD
Occasionally as many as 200 castings can be made in one mold.
]
[Illustration:
TYPE OF MOLDING MACHINE
]
As a matter of fact, such castings were not at all new in this country,
having been furnished by American foundries for electrical work for many
years. Cast iron springs, piston rings, and many other articles of cast
iron are regularly made, which show such elastic quality.
We have said considerable about “castings.” In general we know what
castings are, but in the minds of some there may be a little uncertainty
as to the manner in which they are produced.
[Illustration:
ANOTHER OF THE MANY KINDS OF MOLDING MACHINES
]
There are few lines of human endeavor which require greater judgment and
skill than does the making of molds for castings. Sound judgment based
on long experience, knowledge of conditions under which the work
immediately in hand must be done, observation, and accurate, deductive
reasoning as to the causes of failure are absolutely necessary for
success.
[Illustration:
CASTING, WHICH BECAUSE OF ITS LENGTH AND SMALL CROSS SECTIONS,
REQUIRES VERY FLUID CAST IRON. (FIG. A)
]
In general, molding may be said to be done in “pit” or on the “floor”
for large work, on the “bench” for smaller work, or by “machine.” Pit,
floor, and bench molding are applicable for production of castings of
all sizes and descriptions and this general type which we might term
“hand molding,” is the form that has been practiced longest. Molding
machines are more or less recent inventions which have enabled certain
standard shapes and sizes of castings which are in sufficient demand to
be produced in great numbers by unskilled workmen and therefore at less
cost than is possible by the older hand method.
Each design for casting may be said to demand individual treatment, and
the molder must select that method out of the many which alone, perhaps,
can be successful. The subject is such a broad one that little will be
here attempted further than to give by description and illustration the
predominant points of the making of molds and castings. A simple,
typical case of bench molding will be taken, that the relation of
pattern, mold, core and casting may be clear.
The molding sands used are usually natural sands which contain greater
or lesser amounts of clay, which, when moist, acts as a “binder” of the
grains of sand. When used without drying, the mold is said to be a
“green sand” mold; if dried, a “dry” or “baked” mold, as the case may
be. The majority are “green sand” molds.
For the usual casting of which only a few or several duplicates are
wanted, the “split” pattern is generally the most convenient.
The two halves of the mold, the “cope” (top) and the “drag” (bottom),
are separately made in the two parts of the “flask” or molding box by
“ramming” properly selected and “tempered” (moistened, mixed, and
sieved) sand over the halves of the pattern. Of these, the drag is made
first over the lower half of the separable pattern placed flat side down
upon a bottom board. After “ramming,” i.e., packing the sand, just hard
enough but not too hard, this half mold is reversed and the top half of
the pattern placed upon the lower half, now at the upper face of the
drag and flat side up. A little “parting powder” or fine, dry sand is
sprinkled over the fresh surface of the half mold so that the upper
half, next to be made, will not stick to the lower half, but can be
lifted off at the proper time.
[Illustration:
IT MAY BE BENT DOUBLE READILY WITHOUT BREAKING. (FIG. B)
]
The cope half of the two-part “flask” is now put on, filled and rammed
with sand as was the drag. Any extra sand is scraped off with a straight
edge and at the proper place a hole is cut with the “sprue cutter”
straight down through the cope to the “parting.” More commonly, perhaps,
this “sprue” hole is made by withdrawing a “sprue” stick (of wood) about
which sand had been packed during the making of the cope. It is through
this hole that the molten metal will be poured into the mold.
[Illustration:
BECAUSE OF ITS ABILITY TO WITHSTAND BENDING AND 180–DEGREE TWISTS IT
IS OFTEN JOCULARLY REFERRED TO AS THE “RUBBER CASTING.” (FIG. C)
]
Lifting the cope or top half, it is turned upside down, and, after
cutting in the drag the “runner” or “gate” connecting the “sprue” hole
with the casting, the halves of the pattern are carefully drawn that the
sand may not be disturbed. Now in the cavity left in the drag, to make
the hole in the casting, is hung the baked “core” of sand, held together
by flour or rosin or a “drying” oil. The cope is carefully replaced upon
the drag, thus “closing” the mold.
As will be noted from the drawings, there is left between the core and
the mold a space all around, which will be filled by the metal of the
casting when poured. Therefore the surface of the core shapes the
inside, and the mold itself the outside and ends of the casting.
The molten metal, entering through the vertical “sprue” hole, flows
along the “runner” and into the mold through the more or less
constricted entrance called the “gate.” The gases formed during pouring
and the air with which the mold was filled are driven out through the
porous bodies of sand of the mold and core. Had the mold been rammed too
hard the gases could not escape through the sand and an imperfect
casting would result.
The poured mold is allowed to stand until the metal has solidified and
cooled sufficiently, when the casting is “shaken out.” The sand is
returned to the molder to be used again. The sprue and runner are broken
from the casting, which, after cleaning by “tumbling” with others in a
revolving mill, or otherwise, goes to the machining and assembling
shops.
Some form of the above general method is everywhere used for the
production of all kinds of castings, except for those which can be made
by machine at a lower cost for molds.
This kind of molding, which we have termed “hand work,” requires expert
molders and is too slow and expensive for the hundreds of standard
shapes and sizes of castings which are in great and constant demand. The
latter are made on cleverly devised molding machines working with
compressed air or by hand power applied through a lever. The pattern is
attached to the machine, set and very accurately adjusted by a skilled
mechanic. Thereafter the sand is rammed, the runner formed and the
pattern drawn by the machine itself, all of these very critical
movements being therefore rapidly and unerringly duplicatable any
desired number of times by unskilled labor, which has but to put on the
parts of the “flasks,” feed in the sand, set the cores, close and remove
the mold, and begin the next.
Sometimes there is but one, but for the smaller sizes there are often
ten or twenty, and, occasionally, as many as two hundred pieces or
castings in a single mold.