A GENERAL GLIMPSE AHEAD
We have arrived at the parting of the ways. From the vast beds of iron
ore, the coal fields and coke ovens, and from the quarries of limestone,
all roads have led to the blast furnace. This we have visited and we now
know how pig iron is made.
From this point the several paths diverge. One by one we are to follow
them to get acquainted with the interesting country which they traverse
and the regions to which they lead. However, before choosing any one of
these paths for our first trip, it will be to our advantage to pause, to
study for a moment our position and get a general view of the country
ahead of us. We should know the relative locations and importance of the
places we are going to visit, for only by getting a comprehensive idea
of the general plan of this ferrous (meaning iron) world, can we
understand to the best advantage the position of each of the main
products, wrought iron, steel, cast iron, malleable iron, etc., and
acquire a satisfactory knowledge of them. In the last chapter pig iron
was called the intermediate stage between the ore and the finished iron
product, and such the sketch given shows it to be. It is only as an
intermediate product that pig iron has value, for nowhere in the
commercial world has it a purpose except as a material to be chemically
and structurally transformed into other materials which may themselves
be used without further transformation. It is desirable that we fully
realize the position of this very important semi-raw material, pig iron,
before passing on to the study, one at a time, of the refined ferrous
products which _are_ susceptible of _direct use_ in our commercial life.
[Illustration:
ELECTRIC ARC, MICROSCOPE, AND CAMERA, PART OF METALLOGRAPHIC APPARATUS
]
From Table A (page 71) it will be seen that from blast furnace metal
several well-known products are formed by the various processes of
refinement. Each of these methods of purification may be said to result
in a certain general composition and structure which give to the
material formed its character and properties.
The Metallographic Method of Classification
By aid of the microscope it is possible actually to look into the
structure of these materials.
TABLE A
IRON AND STEEL PRODUCTION OF THE UNITED STATES FOR 1912
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 3000 Furnaces. │ │ Bar Iron, Sheets. │
│ │ │ │ │ WROUGHT IRON. │ │ Pipe, Boiler Tubes, │
│ │ │ │↗│ 1,637,582 tons │→│Stay Bolts. Stock for│
│ │ │ │ │ (750,000 tons Pig │ │ Cementation and │
│ │ │ │ │ Iron used). │ │ Crucible Steel. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 102 Plants. │ │ Rails, Axles, Rods, │
│ │ │ │ │ BESSEMER STEEL. │ │ Wire, Nails. │
│ │ │ │↗│ 1,327,900 tons │→│ Structural Shapes. │
│ │ │ │ │(11,000,000 tons Pig│ │ Pipe, Bar Steel, │
│ │ │ │ │ used). │ │ Steel Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 78 Plants. │ │ │
│ │ │ │ │ ACID OPEN-HEARTH │ │Rails, Plate, Axles. │
│ │ │ │↗│ STEEL. │→│ Steel Castings. │
│ │ │ │ │ 1,139,221 tons │ │ Miscellaneous. │
│ │ │ │ │ (500,000 tons Pig │ │ │
│ │ │ │ │ used). │ │ │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 100 Plants. BASIC │ │Rails, Plate, Axles, │
│ IRON ORE │ │ │ │ OPEN-HEARTH STEEL. │ │ Rods. Structural │
│59,975,000│↖│ │↗│ 19,641,502 tons │→│Shapes. Pipe, Boiler │
│ TONS │ │ │ │(11,500,000 tons Pig│ │ Tubes, Bar Steel, │
│ │ │ │ │ used). │ │ Steel Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ │ │ Stove, Radiator, │
│ │ │466 BLAST │ │ │ │ Machinery Parts, │
│COKE USED │ │ FURNACES │ │ CAST IRON. │ │ Valves, Pipe │
│35,491,292│←│ PIG IRON │→│ 6,750,000 tons │→│Fittings, etc., Bath │
│ TONS │ │ MADE │ │(5,000,000 tons Pig │ │ Tubs, Sinks, │
│ │ │29,726,937│ │ used). │ │ Carwheels, │
│ │ │ TONS │ │ │ │ Brakeshoes, Rolls, │
│ │ │ │ │ │ │ Misc. Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ │ │ Railway Car and │
│ CHARCOAL │ │ │ │ MALLEABLE IRON. │ │ Agricultural │
│35,436,017│↙│ │↘│ 1,000,000 tons │→│ Machinery Castings, │
│ BUSHELS │ │ │ │ (750,000 tons Pig │ │ Pipe Fittings, │
│ │ │ │ │ used). │ │ Miscellaneous │
│ │ │ │ │ │ │ Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│LIMESTONE │ │ │ │ 108 Plants. │ │Tool Steels, Ordnance│
│15,092,166│↙│ │↘│ CRUCIBLE STEEL. │→│Materials, Bar Steel,│
│ TONS │ │ │ │ 121,517 tons. │ │ Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 14 Plants. │ │ Rails, Bar Steel, │
│ │ │ │↘│ ELECTRIC STEEL. │→│ Castings. │
│ │ │ │ │ 18,309 tons. │ │ │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 4 Plants. │ │ │
│ │ │ │↘│ OTHER STEEL │→│ Tool Steels, Bar │
│ │ │ │ │ PROCESSES. │ │Steel, Miscellaneous.│
│ │ │ │ │ 2853 tons. │ │ │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
* Figures for Pig Iron Used are Estimates, no Conclusive Statistics
being available.
[Illustration:
PART OF A CHEMICAL LABORATORY WHERE 6,000 SAMPLES A YEAR ARE ANALYZED
]
By cutting in any direction through a piece of metal or alloy, polishing
the surface exposed very clean and smooth and then etching (corroding)
slightly with acid, the exposed grains of the metal may be seen when
sufficiently magnified. Not only may the grains of metal be seen, but
certain of the other constituents which are present are visible.
Photographs, also, can be taken by attaching a camera to the microscope.
This method of analysis, which is known as “metallography,” has proved
as valuable an aid to the metallurgist as it proved to the geologist
when applied to the study of rocks and geological specimens.
[Illustration:
MACHINE FOR DETERMINING STRENGTH OF IRON AND STEEL BARS
]
The metallography of wrought iron, cast iron, malleable iron and steel
differentiates them to us with considerable accuracy, as is shown by a
glance at the accompanying photomicrographs, as photographs taken
through the microscope are called.
For our immediate purpose of gaining a general knowledge of the relative
positions of these products, this method of analysis probably cannot be
excelled.
After the processes of manufacture of these materials have been taken up
one by one in later chapters, the photomicrographs will be even better
understood than now, as will the differences of chemical composition and
physical properties of the alloys, such as strength, hardness,
brittleness, forging quality, etc. The photomicrographs are given at
this time to show that the materials are structurally very different and
to aid in the general classification.
To make them comparative, all have been taken at the same magnification
of seventy diameters; i.e., the microscope has made everything shown
just seventy times as large as it actually was in the alloy.
[Illustration:
No. 198. PHOTOMICROGRAPH OF SAND CAST PIG IRON. THE THICK BLACK LINES
ARE GRAPHITE FLAKES
]
As stated before, the alloy _pig iron_ normally contains from 3 to 5½
per cent of carbon. This was absorbed during the journey through the
blast furnace. As long as the iron was molten all of this carbon was in
the “combined” form; i.e., in chemical combination with the iron itself.
Cold iron, however, cannot retain in the chemically combined form as
much carbon as does molten iron, so, during the solidification and
cooling of the alloy, more or less of its carbon was precipitated, i.e.,
thrown out of solution and from chemical combination with the iron, the
amount depending mainly upon the speed of the cooling. It appeared then
as the “free” carbon (crystalline graphite) which remained distributed
throughout the alloy and may be seen as the jet black flakes in
photomicrograph No. 198.
Every pure metal is supposed to be composed of crystals or grains which
would have been of the true cubic form if the severe internal pressure
during solidification and cooling had not distorted them.
Photomicrograph No. 99b represents quite well a pure metal. It is that
of an extremely mild steel made by special methods in the open-hearth
steel furnace. It is so highly refined that it can hardly be called
steel at all but is often called “_open-hearth iron_” or “_ingot iron_.”
It is probably the purest iron on the market in commercial quantities
to-day. While in the chemical laboratory iron of considerably greater
purity can be made, for a commercial product this is remarkably pure,
seldom containing more than ¼ per cent of elements other than the metal,
iron.
[Illustration:
No. 99b. OPEN-HEARTH IRON. PROBABLY THE PUREST COMMERCIAL IRON PRODUCT
]
The boundary lines of the crystals or grains may be plainly seen. Each
grain should show practically white. The dark parallel lines, the dots,
and the grayish portions result from inequalities in the polishing and
etching.
[Illustration:
No. 1d. SECTION OF WROUGHT IRON CUT LENGTHWISE OF THE BAR. BLACK
PATCHES AND FILAMENTS ARE “SLAG” OR “CINDER”
]
After noting the appearance of photomicrograph No. 99b, which is of a
nearly pure iron, one need have no difficulty in realizing that pig iron
and the steels are alloys and not simple metals. The truth is that of
all alloys some of the well-known iron products which we are studying
are by far the most complicated, much more so than are the nonferrous
alloys, which include the brasses, bronzes, babbitts, German silver,
etc.
[Illustration:
PHYSICAL TEST ROOM WHERE 15,000 SPECIMENS ARE TESTED EACH YEAR
]
This should not worry us, however, for we shall not attempt to follow
them into their complications.
[Illustration:
No. 3b. STEEL CONTAINING .10 PER CENT OF CARBON. THE CARBON IS IN THE
BLACK PATCHES
]
The important point just now is to observe the crooked black flakes of
crystalline graphite in this photomicrograph No. 198. It is largely
because of these flakes of brittle, soft graphite that pig iron and the
cast irons are so fragile. One has no difficulty in realizing that these
flakes, which cut in every direction through the metal, make it
structurally weak, especially toward a sudden blow.
No. 1d shows a typical section of _wrought iron_ cut lengthwise of the
rolled bar. It will be noted that, as in photomicrograph No. 99b, there
are no graphite flakes.
In the process of wrought iron manufacture practically all of the carbon
of the original pig iron is burned out, leaving little besides the iron
itself and some viscous cinder or slag. In the rolling or hammering out
of the resulting white-hot “bloom,” the slag enclosures which remain
after the squeezing process are extended lengthwise through the bar.
Upon observing with the microscope any prepared section of wrought iron
which has been cut lengthwise of the bar the filaments of slag may be
plainly seen, all parallel or practically so. When such filaments of
slag can be discerned in a longitudinal section it is practically an
absolute indication that the material in question is wrought iron.
[Illustration:
No. 22c. STEEL WITH .50 PER CENT OF CARBON
]
[Illustration:
No. 36a. STEEL WITH 1.98 PER CENT OF CARBON
]
Photomicrograph No. 3b is that of _mild steel_ containing .10 per cent
(⅒ per cent) of carbon. Here we have neither the graphite flakes of No.
198 nor the slag filaments of No. 1d. We can plainly see the boundary
lines of the grains. The irregular dark patches which are evenly
distributed throughout are the defining features of steel. In what might
be called a chemical-mechanical combination, these dark patches contain
all of the small percentage of carbon which gives to carbon steel its
definite properties.
During the manufacture of this alloy all but a small amount of carbon is
eliminated by burning it out, as happens with wrought iron. But the
steel is _molten_ or _fluid_ when finished and the slag which has been
formed floats on top and is also eliminated, which does not occur with
wrought iron, which is thick and pasty at its finishing temperature.
[Illustration:
No. 74. GRAY CAST IRON. THE CROOKED BLACK LINES ARE GRAPHITE FLAKES
]
Therefore, steel contains no graphite and no slag but has only the small
percentage of carbon which was purposely put back to give to it its
valuable properties.
[Illustration:
No. 92d. SEMI-STEEL, A STRONGER GRAY CAST IRON
]
Photomicrograph No. 22c is typical for steel which contains .50 per cent
(½ per cent) of carbon. The irregular patches containing the carbon are
much larger and more frequent. It will be seen, therefore, that through
metallography the various iron alloys, to a considerable extent, may be
analyzed as well as classified.
Photomicrographs Nos. 74 and 92d represent soft and stronger grades of
ordinary _gray cast iron_ respectively. It will be noted at once that
both much resemble pig iron in structure, as of course they should, for
simple remelting in the cupola does not effect much modification in
composition or structure.
Occasionally castings are made from molten iron direct from the blast
furnace, but such practice is not very satisfactory and is little done.
It forms probably the only exception to the statement made above that
pig iron has no useful purpose in the commercial world except as
something to be transformed by some refining process into another
material.
[Illustration:
No. 132. GRAY CAST IRON 200 YEARS OLD
]
[Illustration:
No. 109. MALLEABLE CAST IRON BEFORE ANNEALING
]
The remelting for the well-known cast iron is usually of such selected
brands of pig iron and cast scrap as will produce cast iron best adapted
to the purposes intended. The resulting alloy still contains 3 per cent
or 3½ per cent of carbon.
No. 132 is interesting. It is a photomicrograph of a piece of cast iron
which is approximately 200 years old. From the standpoint of the
metallurgist it is the same as other photomicrographs of cast iron, the
difference in appearance resulting probably from casting conditions,
etc.
_Malleable cast iron_ is made much as is gray cast iron except that it
is brought to such a composition that sections of castings made from the
melt show a white fracture when broken. Castings of gray iron of course
show a gray fracture. The former are extremely hard and are as brittle
as glass until they have gone through a careful long anneal or heat
softening process. By maintaining them at cherry-red heat away from air
for sixty hours or more and cooling slowly, they become “malleable”;
i.e., not brittle at all but capable of considerable distortion without
cracking.
[Illustration:
No. 35. MALLEABLE CAST IRON ANNEALED
]
From the viewpoint of malleability, malleable iron may be considered to
occupy a position between gray cast iron and steel.
Photomicrographs Nos. 109 and 35 are those of malleable iron before and
after the annealing treatment. The former shows the typical structure of
_white cast iron_, while the latter plainly shows the minute lumps of
pure carbon and the surrounding grains of pure iron metal, the two
having become divorced through the annealing process. Note that the
large amount of carbon in this, the “temper carbon” form, does not make
the alloy brittle through cutting of the grains as does the crystalline
graphite form of carbon.
The above gives very briefly the most essential points in the
classification of the irons and steels from the structural standpoint.
True it has not entered into the vast complications of the higher carbon
or tool steels which are those which will take a “temper”; i.e., the
tool steel’s quality of hardening by quenching in water from a cherry
red heat. The simpler points of these will be taken up later. But enough
has been given that we now understand something of the relative
positions of the great main products, some reasons therefor, and their
general structures.
[Illustration:
NO. 8. BAR ROLLED FROM SCRAP; CONTAINS BOTH WROUGHT IRON AND STEEL
]
It has been seen that their micrographs quite definitely differentiate
them and probably no one will have difficulty in recognizing and naming
such specimens. As testing this it may be interesting to note
photomicrograph No. 8. Two different iron alloys made up the material
from which this photomicrograph was taken. Apparently the bar was one
made by heating and rolling together scrap metals. Such material is on
the market. The photomicrograph shows that some of the scrap used was
wrought iron and some of it mild steel of about .08% carbon.
Classification by Chemical Analysis and by Physical Tests
So far not much has been said about composition except that upon it to a
great extent depends the structure and physical properties of the alloy.
Composition and physical characteristics as well as structures are
necessary to a fair
TABLE B—CHEMICAL COMPOSITION AND PHYSICAL PROPERTIES
═══════════════╤═══════╤═════════╤════╤══════╤═══════════╤═══════════
│ Per │Per Cent │Per │ Per │ Beginning │ Tensile
│ Cent │Graphitic│Cent│ Cent │ of │Strength[3]
│Silicon│ Carbon │ │Total │Freezing[2]│
│ │ │ │Carbon│ │
───────────────┼───────┼─────────┼────┼──────┼───────────┼───────────
1. Sand Cast │ 2.50│ 3.75│ .25│ 4.00│ 2100│
Pig Iron │ │ │ │ │ │
2. Machine │ 2.50│ 3.00│1.00│ 4.00│ 2100│
Cast Pig │ │ │ │ │ │
Iron │ │ │ │ │ │
───────────────┼───────┼─────────┼────┼──────┼───────────┼───────────
3. Open-Hearth│ .03│ .00│ .02│ .02│ 2740│ 50,000
Iron │ │ │ │ │ │
4. Wrought │ Slag│ .00│ .05│ .05│ 2740│ 52,000
Iron │ │ │ │ │ │
│ │ │ │ │ │
5. Mild Steel │ .05│ .00│ .10│ .10│ 2740│ 55,000
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
6. Medium │ .05│ .00│ .25│ .25│ 2720│ 65,000
Steel │ │ │ │ │ │
7. Rail Steel │ .05│ .00│ .60│ .60│ 2680│ 90,000
8. Low Carbon │ .15│ .00│ .75│ .75│ 2660│ 100,000
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
9. Medium │ .15│ .00│1.00│ 1.00│ 2630│ 120,000
Carbon │ │ │ │ │ │
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
10. High Carbon│ .15│ .00│1.25│ 1.25│ 2600│ 135,000
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
11. Very High │ .15│ .00│1.50│ 1.50│ 2560│ 124,000
Carbon │ │ │ │ │ │
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
12. High Steel │ │ .00│1.75│ 1.75│ 2530│
│ │ │ │ │ │
13. White Iron │ │ .00│2.50│ 2.50│ 2460│
│ │ │ │ │ │
14. White Cast │ .70│ .10│2.65│ 2.75│ 2400│ 41,000
Iron │ │ │ │ │ │
│ │ │ │ │ │
15. Annealed │ .70│ 2.70│ .05│ 2.75│ │ 40,000
Malleable│ │ │ │ │ │
Cast Iron│ │ │ │ │ │
│ │ │ │ │ │
16. Cast Iron │ 1.00│ 1.00│2.00│ 3.00│ 2330│ 35,000
for │ │ │ │ │ │
Chilled │ │ │ │ │ │
Castings │ │ │ │ │ │
17. Semi-Steel │ 1.75│ 2.80│ .40│ 3.20│ 2300│ 35,000
│ │ │ │ │ │
│ │ │ │ │ │
18. Gray Cast │ 2.00│ 3.10│ .30│ 3.40│ 2200│ 25,000
Iron │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
19. Soft Gray │ 2.50│ 3.30│ .15│ 3.45│ 2200│ 23,000
Cast Iron│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
───────────────┴───────┴─────────┴────┴──────┴───────────┴───────────
═══════════════╤═════════════╤══════════╤═════════════════
│Elongation[4]│ Welding │ Principal Uses
│ │Properties│
│ │ │
│ │ │
───────────────┼─────────────┼──────────┼─────────────────
1. Sand Cast │ │ │Further Refining.
Pig Iron │ │ │
2. Machine │ │ │Further Refining.
Cast Pig │ │ │
Iron │ │ │
───────────────┼─────────────┼──────────┼─────────────────
3. Open-Hearth│2½″ │ Good │Bars, pipe,
Iron │ │ │ sheet.
4. Wrought │2¼″ │ Fine │Bars, pipe,
Iron │ │ │ boiler tubes,
│ │ │ stay bolts.
5. Mild Steel │2½″ │ Good │Bars, pipe, wire,
│ │ │ sheet,
│ │ │ shafting,
│ │ │ tubes.
6. Medium │2¼″ │ Good │Bars, plate,
Steel │ │ │ structural.
7. Rail Steel │1¾″ │ Fair │Rails, gears.
8. Low Carbon │1¼″ │ Fair │Hammers, cold
Tool │ │ │ chisels, saws,
Steel │ │ │ springs.
9. Medium │1″ │ Poor │Lathe tools,
Carbon │ │ │ chisels,
Tool │ │ │ drills, dies,
Steel │ │ │ springs.
10. High Carbon│1⅛″ │ Slight │Lathe tools,
Tool │ │ │ chisels, files,
Steel │ │ │ saws.
11. Very High │ ¼″ │ Slight │Razors, lancets,
Carbon │ │ │ graving tools,
Tool │ │ │ saws for steel.
Steel │ │ │
12. High Steel │0 │ None │Dies for wire
│ │ │ drawing.
13. White Iron │0 │ None │Dies for wire
│ │ │ drawing.
14. White Cast │0 │ None │For
Iron │ │ │ malleableizing,
│ │ │ carwheels.
15. Annealed │ ½″ │ None │Railway and
Malleable│ │ │ agricultural
Cast Iron│ │ │ castings,
│ │ │ fittings.
16. Cast Iron │0 │ None │Rolls, gears,
for │ │ │ brakeshoes.
Chilled │ │ │
Castings │ │ │
17. Semi-Steel │0 │ None │Gears, steam
│ │ │ cylinders,
│ │ │ valves.
18. Gray Cast │0 │ None │Machine parts,
Iron │ │ │ grates,
│ │ │ radiators,
│ │ │ valves, soil
│ │ │ pipe.
19. Soft Gray │0 │ None │Stoves,
Cast Iron│ │ │ hollowware,
│ │ │ pipe fittings,
│ │ │ misc.
───────────────┴─────────────┴──────────┴─────────────────
Footnote 2:
In degrees Fahrenheit. For our purposes, the same as the _melting
points_ of the alloys.
Footnote 3:
Pounds required to pull apart lengthwise a bar one inch square.
Footnote 4:
Number of inches that a bar eight inches long will stretch before it
breaks.
[Illustration:
IRON AND STEEL TEST PIECES AND INSTRUMENTS USED IN MEASURING THEIR
SIZE, ELASTIC LIMIT AND ELONGATION
]
understanding of the subject. There is given therefore, a table showing
approximate comparative values of chemical compositions and physical
properties of the alloys under discussion.
It should be distinctly understood that the figures given in Table B are
_approximate_ only and are intended to be average, or rather, perhaps,
typical.
There are all sorts of conditions which in practice modify the figures
given in the table, and criticism may be maintained justly against some
of the too specific statements which here it was necessary to make. The
classification is given with considerable hesitation and only because,
arranged in this way, it brings out existing relationships which
otherwise would escape notice and which we cannot afford to overlook.
But please do not gain the impression that these alloys are divided into
distinct classes. There are no dividing lines at all. One group merges
into the next so gradually that it is impossible to tell where the one
ends and the other begins.
It is to be hoped that no one will make himself miserable by trying to
digest these rather formidable figures of Table B all at one sitting.
They are given mainly for comparison and for reference. It is suggested
that after noting carefully the similarities and differences to which
attention is called, they be reserved until the processes of manufacture
of the various alloys are taken up one by one. Reference to these
figures on those occasions should be profitable.
The main points to be noted at this time are:
1. Open-hearth iron is practically pure iron, having no constituents or
slag inclusions which materially affect its properties.
2. Wrought iron, for all practical purposes, is pure iron except for its
content of slag. It is the only one of the iron family which does
normally contain slag.
3. Neither open-hearth iron nor wrought iron contains carbon in
appreciable quantities.
4. The distinguishing and active element of the steel family is carbon.
With increase of carbon the hardness of the alloy increases as does its
tensile strength, but the ductility (elongation or stretch) decreases.
5. Other conditions being equal, the more carbon the alloy contains the
more easily it melts; i.e., at lower temperature. So the purer irons
such as open-hearth iron, wrought iron, and mild steel (i.e., steel with
low carbon, usually under 0.15 per cent) have relatively high and the
cast irons lower melting points.
TABLE C—PERCENTAGES OF THE SO-CALLED “METALLOIDS” BY WEIGHT AND BY
VOLUME
═══════════════════╤═══════════╤══════════╤═════════╤══════════
│ Silicon │Manganese │ Sulphur │Phosphorus
│ │ │ │
───────────────────┼────┬──────┼────┬─────┼───┬─────┼────┬─────
│Wt. │ Vol. │Wt. │Vol. │Wt.│Vol. │Wt. │Vol.
───────────────────┼────┼──────┼────┼─────┼───┼─────┼────┼─────
Gray Cast Iron │2.50│=10.0=│ .70│ =.7=│.09│=.36=│ .75│=3.2=
Semi-Steel │1.75│ =7.0=│1.00│=1.0=│.10│=.40=│ .35│=1.5=
Malleable Cast Iron│ .70│ =2.8=│ .50│ =.5=│.15│=.60=│ .17│ =.7=
Cast Steel │ .30│ =1.2=│ .50│ =.5=│.05│=.20=│ .04│=.17=
Mild Steel │ .05│ =.2=│ .40│ =.4=│.04│=.16=│ .03│=.13=
Open-hearth Iron │ .03│ =.1=│ .02│=.02=│.01│=.06=│ .01│=.06=
Wrought Iron │1.20│ =5.0=│ .08│=.08=│.01│=.04=│ .15│=.65=
───────────────────┴────┴──────┴────┴─────┴───┴─────┴────┴─────
═══════════════════╤═══════════╤═══════╤══════════
│ Total │Iron[5]│ Total
│ Carbon │ │Metalloids
───────────────────┼────┬──────┼───────┼───┬──────
│Wt. │ Vol. │ Wt. │Wt.│ Vol.
───────────────────┼────┼──────┼───────┼───┼──────
Gray Cast Iron │3.45│=12.1=│ 92.5 │7.5│=26.4=
Semi-Steel │3.20│=11.2=│ 93.6 │6.4│=21.1=
Malleable Cast Iron│2.75│ =9.6=│ 95.7 │4.3│=14.3=
Cast Steel │ .35│ =1.2=│ 98.8 │1.2│ =3.3=
Mild Steel │ .10│ =.35=│ 99.4 │ .6│ =1.2=
Open-hearth Iron │ .02│ =.07=│ 99.9 │ .1│ =.3=
Wrought Iron │ .05│ =.17=│ 97.7 │2.3│ =5.9=
───────────────────┴────┴──────┴───────┴───┴──────
Footnote 5:
These alloys contain small amounts of other elements so these
percentages of iron are a little high, though approximately correct.
Note the purity (high percentage of iron) of the open-hearth iron.
6. In the cast irons, the carbon occurs not in one only but in two
different forms; i.e., as graphitic carbon, commonly called graphite
(Gr. C.) and the combined form (C. C.). The sum of these is usually
between 3.00 per cent and 3.50 per cent. It is not so much the total
amount of the carbon that causes the differences in structure and
physical properties which have been noted in 1 and 2 and in 16 and 17
above, as it is the _relative proportions_ in which these two varieties
occur.
[Illustration:
EXHIBITION CASE SHOWING FOUR WELL-KNOWN IRON ALLOYS WITH THEIR
METALLOIDS
]
7. No one knows just when, with increase of carbon, steel ceases to be
steel and becomes white cast iron. There is no definite dividing line
either in chemical or physical properties. The changes are extremely
gradual throughout the scale. Aided by the microscope, modern physical
chemistry has disclosed the fact that alloys of iron with carbon
“freeze” from molten to solid condition according to two different laws.
The change from one to the other occurs somewhere between 1.7 per cent
and 2.2 per cent of carbon as is described in Chapter XXII. This is our
only basis for calling alloys with less than 2 per cent of carbon,
steels, and those with greater amounts, cast irons.
8. For our immediate purposes the other “metalloids” or constituents are
of secondary importance and will not be taken up now. From this it must
not be understood that they can be slighted by the metallurgist and
furnace man in his work. They cannot. Every one of them is of importance
and must be accounted for in the final product or trouble results.
Volumetric Analysis of the Iron Alloys
There is a way in which we may visually get a very intimate idea of the
relative composition of these alloys.
The cabinet of which a photograph is given is partitioned into four
sections. Each one of these contains a bar and six specimen jars. As you
may or may not be able to read from the labels, the bars, all of exactly
the same size, are soft cast iron, semi-steel (a stronger cast iron),
annealed malleable iron and cast steel. The six jars above each bar
contain the exact amounts of the various constituents other than iron
which are in the bar beneath.
As none of the constituents except manganese are as heavy as iron, their
volumes per unit of weight are correspondingly greater. Putting it into
approximate figures we have the percentages by weight and by volume
shown in Table C.
This means, of course, that _of the cast iron plates of your cook stove
or steam or water radiators fully one-quarter (26 per cent by volume) is
not iron at all_ but brittle substances of little or no strength. These
elements, silicon, sulphur, phosphorus, and carbon, are commonly called
“metalloids.” While the first three named are not in “free” form in the
alloy and therefore allow of some doubt as to just the space they
require, we have good reason to suppose that the figures given are not
far from correct.
With such a volume of weakening constituents and particularly with the
graphite flakes cutting through and separating the iron grains as the
photomicrographs show, can one wonder that cast iron is fragile—more so
than steel or wrought iron?
To sum up, naming only the most familiar alloys and the two or three
qualifying features of each which stand forth with particular boldness,
we have:
_Pig Iron_—Very High Carbon. Brittle.
_Gray Cast Iron_—High Carbon. Brittle.
_Malleable Cast Iron_—High Carbon. Made Malleable by Annealing.
_Wrought Iron_—Slag. Little or no Carbon. Very Malleable without
Annealing.
_Mild Steel_—Very Low Carbon. No Slag. Very Malleable without Annealing.
_Carbon Tool Steel_—Medium Carbon. No Slag. Of Medium Malleability.
So steel which is not called “iron” at all is a very pure metal in
comparison with gray and malleable cast iron and usually has a larger
percentage of the chemical element, iron from which all are derived,
than has the well-known wrought iron itself.