Chapter XII: Part 12

30. _Chilling cast iron_, i.e. hastening its cooling by casting it in
a cool mould, favours the formation of cementite rather than of
graphite in the freezing of the eutectic at aBc, and also, in case of
hyper-eutectic iron, in the passage through region 3. Like the
hardening of steel, it hinders the transformation of the austenite,
whether primary or eutectic, into pearlite + cementite, and thus
catches part of the iron in transit in the hard [beta] state. The
annealing of such iron may occur in either of two degrees--a small
one, as in making common chilled cast iron objects, such as railway
car wheels, or a great one, as in making malleable cast iron. In the
former case, the objects are heated only to the neighbourhood of Ac1,
say to 730 deg. C., so that the [beta]-iron may slip into the a state,
and the transformation of the austenite into pearlite and cementite
may complete itself. The joint effect of such chilling and such
annealing is to make the metal much harder than if slowly cooled,
because for each 1% of graphite which the chilling suppresses, 15% of
the glass-hard cementite is substituted. Thus a cast iron which, if
cooled slowly, would have been "grey," i.e. would have consisted
chiefly of graphite with pearlite and ferrite (which are all
relatively soft bodies), if thus chilled and annealed consists of
cementite and pearlite. But in most such cases, in spite of the
annealing, this hardness is accompanied by a degree of brittleness too
great for most purposes. The process therefore is so managed that only
the outer shell of the casting is chilled, and that the interior
remains graphitic, i.e. grey cast iron, soft and relatively malleable.

31. In making _malleable castings_ the annealing, i.e. the change
towards the stable state of ferrite + graphite, is carried much
farther by means of a much longer and usually a higher heating than in
the manufacture of chilled castings. The castings, initially of white
cast iron, are heated for about a week, to a temperature usually above
730 deg. C. and often reaching 900 deg. C. (1346 deg. and 1652 deg.
F.). For about 60 hours the heat is held at its highest point, from
which it descends extremely slowly. The molecular freedom which this
high temperature gives enables the cementite to change gradually into
a mixture of graphite and austenite with the result that, after the
castings have been cooled and their austenite has in cooling past Ac1
changed into pearlite and ferrite, the mixture of cementite and
pearlite of which they originally consisted has now given place to one
of fine or "temper" graphite and ferrite, with more or less pearlite
according to the completeness of the transfer of the carbon to the
state of graphite.

Why, then, is this material malleable, though the common grey cast
iron, which is made up of about the same constituents and often in
about the same proportion, is brittle? The reason is that the
particles of temper graphite which are thus formed within the solid
casting in its long annealing are so finely divided that they do not
break up the continuity of the mass in a very harmful way; whereas in
grey cast iron both the eutectic graphite formed in solidifying, and
also the primary graphite which, in case the metal is hyper-eutectic,
forms in cooling through region 3 of fig. 1, surrounded as it is by
the still molten mother-metal out of which it is growing, form a
nearly continuous skeleton of very large flakes, which do break up in
a most harmful way the continuity of the mass of cast iron in which
they are embedded.

In carrying out this process the castings are packed in a mass of iron
oxide, which at this temperature gradually removes the fine or
"temper" graphite by oxidizing that in the outer crust to carbonic
oxide, whereon the carbon farther in begins diffusing outwards by
"molecular migration," to be itself oxidized on reaching the crust.
This removal of graphite doubtless further stimulates the formation of
graphite, by relieving the mechanical and perhaps the osmotic
pressure. Thus, first, for the brittle glass-hard cementite there is
gradually substituted the relatively harmless temper graphite; and,
second, even this is in part removed by surface oxidation.

32. _Fineness of Structure._--Each of these ancient processes thus
consists essentially in so manipulating the temperature that, out of
the several possible constituents, the metal shall actually consist of
a special set in special proportions. But in addition there is another
very important principle underlying many of our thermal processes,
viz. that the state of aggregation of certain of these constituents,
and through it the properties of the metal as a whole, are profoundly
affected by temperature manipulations. Thus, prior exposure to a
temperature materially above Ac3 coarsens the structure of most steel,
in the sense of giving it when cold a coarse fracture, and enlarging
the grains of pearlite, &c., later found in the slowly cooled metal.
This coarsening and the brittleness which accompanies it increase with
the temperature to which the metal has been exposed. Steel which after
a slow cooling from about 722 deg. C. will bend 166 deg. before
breaking, will, after slow cooling from about 1050 deg. C., bend only
18 deg. before breaking. This injury fortunately can be cured either
by _reheating_ the steel to Ac3 when it "refines," i.e. returns
spontaneously to its fine-grained ductile state (_cooling_ past Ar3
does not have this effect); or by breaking up the coarse grains by
_mechanical distortion_, e.g. by forging or rolling. For instance, if
steel has been coarsened by heating to 1400 deg. C., and if, when it
has cooled to a lower temperature, say 850 deg. C. we forge it, its
grain-size and ductility when cold will be approximately those which
it would have had if heated only to 850 deg. Hence steel which has
been heated very highly, whether for welding, or for greatly softening
it so that it can be rolled to the desired shape with but little
expenditure of power, ought later to be refined, either by reheating
it from below Ar3 to slightly above Ac3 or by rolling it after it has
cooled to a relatively low temperature, i.e. by having a low
"finishing temperature." Steel castings have initially the extremely
coarse structure due to cooling without mechanical distortion from
their very high temperature of solidification; they are "annealed,"
i.e. this coarseness and the consequent brittleness are removed, by
reheating them much above Ac3, which also relieves the internal
stresses due to the different rates at which different layers cool,
and hence contract, during and after solidification. For steel
containing less than about 0.13% of carbon, the embrittling
temperature is in a different range, near 700 deg. C., and such steel
refines at temperatures above 900 deg. C.

33. _The Possibilities of Thermal Treatment._--When we consider the great number of different regions in fig. 1, each with its own set of constitutents, and remember that by different rates of cooling from different temperatures we can retain in the cold metal these different sets of constituents in widely varying proportions; and when we further reflect that not only the proportion of each constituent present but also its state of aggregation can be controlled by thermal treatment, we see how vast a field is here opened, how great a variety of different properties can be induced in any individual piece of steel, how enormous the variety of properties thus attainable in the different varieties collectively, especially since for each percentage of carbon an incalculable number of varieties of steel may be made by alloying it with different proportions of such elements as nickel, chromium, &c. As yet there has been only the roughest survey of certain limited areas in this great field, the further exploration of which will enormously increase the usefulness of this wonderful metal.

34. _Alloy steels_ have come into extensive use for important special purposes, and a very great increase of their use is to be expected. The chief ones are nickel steel, manganese steel, chrome steel and chrome-tungsten steel. The general order of merit of a given variety or specimen of iron or steel may be measured by the degree to which it combines strength and hardness with ductility. These two classes of properties tend to exclude each other, for, as a general rule, whatever tends to make iron and steel hard and strong tends to make it correspondingly brittle, and hence liable to break treacherously, especially under shock. Manganese steel and nickel steel form an important exception to this rule, in being at once very strong and hard and extremely ductile. _Nickel steel_, which usually contains from 3 to 3.50% of nickel and about 0.25% of carbon, combines very great tensile strength and hardness, and a very high limit of elasticity, with great ductility. Its combination of ductility with strength and hardening power has given it very extended use for the armour of war-vessels. For instance, following Krupp's formula, the side and barbette armour of war-vessels is now generally if not universally made of nickel steel containing about 3.25% of nickel, 0.40% of carbon, and 1.50% of chromium, deeply carburized on its impact face. Here the merit of nickel steel is not so much that it resists perforation, as that it does not crack even when deeply penetrated by a projectile. The combination of ductility, which lessens the tendency to break when overstrained or distorted, with a very high limit of elasticity, gives it great value for shafting, the merit of which is measured by its endurance of the repeated stresses to which its rotation exposes it whenever its alignment is not mathematically straight. The alignment of marine shafting, changing with every passing wave, is an extreme example. Such an intermittently applied stress is far more destructive to iron than a continuous one, and even if it is only half that of the limit of elasticity, its indefinite repetition eventually causes rupture. In a direct competitive test the presence of 3.25% of nickel increased nearly sixfold the number of rotations which a steel shaft would endure before breaking.

35. As actually made, _manganese steel_ contains about 12% of manganese and 1.50% of carbon. Although the presence of 1.50% of manganese makes steel relatively brittle, and although a further addition at first increases this brittleness, so that steel containing between 4 and 5.5% can be pulverized under the hammer, yet a still further increase gives very great ductility, accompanied by great hardness--a combination of properties which was not possessed by any other known substance when this remarkable alloy, known as Hadfield's manganese steel, was discovered. Its ductility, to which it owes its value, is profoundly affected by the rate of cooling. Sudden cooling makes the metal extremely ductile, and slow cooling makes it brittle. Its behaviour in this respect is thus the opposite of that of carbon steel. But its great hardness is not materially affected by the rate of cooling. It is used extensively for objects which require both hardness and ductility, such as rock-crushing machinery, railway crossings, mine-car wheels and safes. The burglar's blow-pipe locally "draws the temper," i.e. softens a spot on a hardened carbon steel or chrome steel safe by simply heating it, so that as soon as it has again cooled he can drill through it and introduce his charge of dynamite. But neither this nor any other procedure softens manganese steel rapidly. Yet this very fact that it is unalterably hard has limited its use, because of the great difficulty of cutting it to shape, which has in general to be done with emery wheels instead of the usual iron-cutting tools. Another defect is its relatively low elastic limit.

36. _Chrome steel_, which usually contains about 2% of chromium and 0.80 to 2% of carbon, owes its value to combining, when in the "hardened" or suddenly cooled state, intense hardness with a high elastic limit, so that it is neither deformed permanently nor cracked by extremely violent shocks. For this reason it is the material generally if not always used for armour-piercing projectiles. It is much used also for certain rock-crushing machinery (the shoes and dies of stamp-mills) and for safes. These are made of alternate layers of soft wrought iron and chrome steel hardened by sudden cooling. The hardness of the hardened chrome steel resists the burglar's drill, and the ductility of the wrought iron the blows of his sledge.

Vanadium in small quantities, 0.15 or 0.20%, is said to improve steel greatly, especially in increasing its resistance to shock and to often-repeated stress. But the improvement may be due wholly to the considerable chromium content of these so-called vanadium steels.

37. _Tungsten steel_, which usually contains from 5 to 10% of tungsten and from 1 to 2% of carbon, is used for magnets, because of its great retentivity.

38. _Chrome-tungsten or High-speed Steel._--Steel with a large content of both chromium and tungsten has the very valuable property of "red-hardness," i.e. of retaining its hardness and hence its power of cutting iron and other hard substances, even when it is heated to dull redness, say 600 deg. C. (1112 deg. F.) by the friction of the work which it is doing. Hence a machinist can cut steel or iron nearly six times as fast with a lathe tool of this steel as with one of carbon steel, because with the latter the cutting speed must be so slow that the cutting tool is not heated by the friction above say 250 deg. C. (482 deg. F.), lest it be unduly softened or "tempered" (S 29). This effect of chromium, tungsten and carbon jointly consists essentially in raising the "tempering temperature," i.e. that to which the metal, in which by suitable thermal treatment the iron molecules have been brought to the allotropic [gamma] or [beta] state or a mixture of both, can be heated without losing its hardness through the escape of that iron into the [alpha] state. In short, these elements seem to impede the allotropic change of the iron itself. The composition of this steel is as follows:--

The usual limits. Apparently the best.
Carbon 0.32 to 1.28 0.68 to 0.67
Manganese 0.03 " 0.30 0.07 " 0.11
Chromium 2.23 " 7.02 5.95 " 5.47
Tungsten 9.25 " 25.45 17.81 " 18.19

39. _Impurities._--The properties of iron and steel, like those of most of the metals, are profoundly influenced by the presence of small and sometimes extremely small quantities of certain impurities, of which the most important are phosphorus and sulphur, the former derived chiefly from apatite (phosphate of lime) and other minerals which accompany the iron ore itself, the latter from the pyrite found not only in most iron ores but in nearly all coal and coke. All commercial iron and steel contain more or less of both these impurities, the influence of which is so strong that a variation of 0.01%, i.e. of one part in 10,000, of either of them has a noticeable effect. The best tool steel should not contain more than 0.02% of either, and in careful practice it is often specified that the phosphorus and sulphur respectively shall not exceed 0.04 and 0.05% in the steel for important bridges, or 0.06 and 0.07% in rail steel, though some very prudent engineers allow as much as .085% or even 0.10% of phosphorus in rails.

40. The specific effect of _phosphorus_ is to make the metal cold-short, i.e. brittle in the cold, apparently because it increases the size and the sharpness of demarcation of the crystalline grains of which the mass is made up. The specific effect of _sulphur_ is to make the metal red-short, i.e. brittle, when at a red heat, by forming a network of iron sulphide which encases these crystalline grains and thus plays the part of a weak link in a strong chain.

41. _Oxygen_, probably dissolved in the iron as ferrous oxide FeO, also makes the metal red-short.

42. _Manganese_ by itself rather lessens than increases the malleableness and, indeed, the general merit of the metal, but it is added intentionally, in quantities even as large as 1.5% to palliate the effects of sulphur and oxygen. With sulphur it forms a sulphide which draws together into almost harmless drops, instead of encasing the grains of iron. With oxygen it probably forms manganous oxide, which is less harmful than ferrous oxide. (See S 35.)

43. _Ores of Iron._--Even though the earth seems to be a huge iron meteor with but a thin covering of rocks, the exasperating proneness of iron to oxidize explains readily why this metal is only rarely found native, except in the form of meteorites. They are four important iron ores, magnetite, haematite, limonite and siderite, and one of less but still considerable importance, pyrite or pyrites.

44. _Magnetite_, Fe3O4, contains 72.41% of iron. It crystallizes in
the cubical system, often in beautiful octahedra and rhombic
dodecahedra. It is black with a black streak. Its specific gravity is
5.2, and its hardness 5.5 to 6.5. It is very magnetic, and sometimes
polar.

45. _Haematite_, or red haematite, Fe2O3, contains 70% of iron. It
crystallizes in the rhombohedral system. Its colour varies from
brilliant bluish-grey to deep red. Its streak is always red. Its
specific gravity is 5.3 and its hardness 5.5 to 6.5.

46. _Limonite_, 2Fe2O3, 3H2O, contains 59.9% of iron. Its colour
varies from light brown to black. Its streak is yellowish-black, its
specific gravity 3.6 to 4.0, and its hardness 5 to 5.5. Limonite and
the related minerals, turgite, 2Fe2O3 + H2O, and gothite, Fe2O3 + H2O,
are grouped together under the term "brown haematite."

47. _Siderite_, or spathic iron ore, FeCO3, crystallizes in the
rhombohedral system and contains 48.28% of iron. Its colour varies
from yellowish-brown to grey. Its specific gravity is 3.7 to 3.9, and
its hardness 3.5 to 4.5. The clayey siderite of the British coal
measures is called "clay band," and that containing bituminous matter
is called "black band."

48. _Pyrite_, FeS2, contains 46.7% of iron. It crystallizes in the
cubic system, usually in cubes, pentagonal dodecahedra or octahedra,
often of great beauty and perfection. It is golden-yellow, with a
greenish or brownish-black streak. Its specific gravity is 4.83 to
5.2, its hardness 6 to 6.5. Though it contains far too much sulphur to
be used in iron manufacture without first being desulphurized, yet
great quantities of slightly cupriferous pyrite, after yielding nearly
all their sulphur in the manufacture of sulphuric acid, and most of
the remainder in the wet extraction of their copper, are then used
under the name of "blue billy" or "purple ore," as an ore of iron, a
use which is likely to increase greatly in importance with the gradual
exhaustion of the richest deposits of the oxidized ores.

49. _The Ores actually Impure._--As these five minerals actually exist in the earth's crust they are usually more or less impure chemically, and they are almost always mechanically mixed with barren mineral matter, such as quartz, limestone and clay, collectively called "the gangue." In some cases the iron-bearing mineral, such as magnetite or haematite, can be separated from the gangue after crashing, either mechanically or magnetically, so that the part thus enriched or "concentrated" alone need be smelted.

50. _Geological Age._--The Archaean crystalline rocks abound in deposits of magnetite and red haematite, many of them very large and rich. These of course are the oldest of our ores, and from deposits of like age, especially those of the more readily decomposed silicates, has come the iron which now exists in the siderites and red and brown haematites of the later geological formations.

51. _The World's Supply of Iron Ore._--The iron ores of the earth's crust will probably suffice to supply our needs for a very long period, perhaps indeed for many thousand years. It is true that an official statement, which is here reproduced, given in 1905 by Professor Tornebohm to the Swedish parliament, credited the world with only 10,000,000,000 tons of ore, and that, if the consumption of iron should continue to increase hereafter as it did between 1893 and 1906, this quantity would last only until 1946. How then can it be that there is a supply for thousands of years? The two assertions are not to be reconciled by pointing out that Professor Tornebohm underestimated, for instance crediting the United States with only 1.1 billion tons, whereas the United States Geological Survey's expert credits that country with from ten to twenty times this quantity; nor by pointing out that only certain parts of Europe and a relatively small part of North America have thus far been carefully explored for iron ore, and that the rest of these two continents and South America, Asia and Africa may reasonably be expected to yield very great stores of iron, and that pyrite, one of the richest and most abundant of ores, has not been included. Important as these considerations are, they are much less important than the fact that a very large proportion of the rocks of the earth's crust contain more or less iron, and therefore are potential iron ores.

TABLE II.--_Professor Tornebohm's Estimate of the World's Ore Supply._

+--------------------+---------------+------------+------------+
| Country. | Workable | Annual | Annual |
| | Deposits. | Output. |Consumption.|
+--------------------+---------------+------------+------------+
| | tons. | tons. | tons. |
| United States | 1,100,000,000 | 35,000,000 | 35,000,000 |
| Great Britain | 1,000,000,000 | 14,000,000 | 20,000,000 |
| Germany | 2,200,000,000 | 21,000,000 | 24,000,900 |
| Spain | 500,000,000 | 8,000,000 | 1,000,000 |
| Russia and Finland | 1,500,000,000 | 4,000,000 | 6,000,000 |
| France | 1,500,000,000 | 6,000,000 | 8,000,000 |
| Sweden | 1,000,000,000 | 4,000,000 | 1,000,000 |
| Austria-Hungary | 1,200,000,000 | 3,000,000 | 4,000,000 |
| Other countries | | 5,000,000 | 1,000,000 |
+--------------------+---------------+------------+------------+
| Total |10,000,000,000 |100,000,000 |100,000,000 |
+--------------------+---------------+------------+------------+

_Note to Table._--Though this estimate seems to be near the truth as
regards the British ores, it does not credit the United States with
one-tenth, if indeed with one-twentieth, of their true quantity as
estimated by that country's Geological Survey in 1907.

52. _What Constitutes an Iron Ore._--Whether a ferruginous rock is or is not ore is purely a question of current demand and supply. That is ore from which there is reasonable hope that metal can be extracted with profit, if not to-day, then within a reasonable length of time. Rock containing 2(1/2)% of gold is ah extraordinarily rich gold ore; that with 2(1/2)% of copper is a profitable one to-day; that containing 2(1/2)% of iron is not so to-day, for the sole reason that its iron cannot be extracted with profit in competition with the existing richer ores. But it will become a profitable ore as soon as the richer ore shall have been exhausted. Very few of the ores which, are mined to-day contain less than 25% of iron, and some of them contain over 60%. As these richest ores are exhausted, poorer and poorer ones will be used, and the cost of iron will increase progressively if measured either in units of the actual energy used in mining and smelting it, or in its power of purchasing animal and vegetable products, cotton, wool, corn, &c., the supply of which is renewable and indeed capable of very great increase, but probably not if measured in its power of purchasing the various mineral products, e.g. the other metals, coal, petroleum and the precious stones, of which the supply is limited. This is simply one instance of the inevitable progressive increase in cost of the irrecreatable mineral relatively to the recreatable animal and vegetable. When, in the course of centuries, the exhaustion of richer ores shall have forced us to mine, crush and concentrate mechanically or by magnetism the ores which contain only 2 or 3% of iron, then the cost of iron in the ore, measured in terms of the energy needed to mine and concentrate it, will be comparable with the actual cost of the copper in the ore of the copper-mines of to-day. But, intermediate in richness between these two extremes, the iron ores mined to-day and these 2 and 3% ores, there is an incalculably great quantity of ore capable of mechanical concentration, and another perhaps vaster store of ore which we do not yet know how to concentrate mechanically, so that the day when a pound of iron in the ore will cost as much as a pound of copper in the ore costs to-day is immeasurably distant.

53. _Future Cost of Ore._--The cost of iron ore is likely to rise much less rapidly than that of coal, because the additions to our known supply are likely to be very much greater in the case of ore than in that of coal, for the reason that, while rich and great iron ore beds may exist anywhere, those of coal are confined chiefly to the Carboniferous formation, a fact which has led to the systematic survey and measurement of this formation in most countries. In short, a very large part of the earth's coal supply is known and measured, but its iron ore supply is hardly to be guessed. On the other hand, the cost of iron ore is likely to rise much faster than that of the potential aluminium ores, clay and its derivatives, because of the vast extent and richness of the deposits of this latter class. It is possible that, at some remote day, aluminium, or one of its alloys, may become the great structural material, and iron be used chiefly for those objects for which it is especially fitted, such as magnets, springs and cutting tools.

In passing, it may be noted that the cost of the ore itself forms a
relatively small part of the cost even of the cruder forms of steel,
hardly a quarter of the cost of such simple products as rails, and an
insignificant part of the cost of many most important finished
objects, such as magnets, cutting tools, springs and wire, for which
iron is almost indispensable. Thus, if the use of ores very much
poorer than those we now treat, and the need of concentrating them
mechanically, were to double the cost of a pound of iron in the
concentrated ore ready for smelting, that would increase the cost of
rails by only one quarter. Hence the addition to the cost of finished
steel objects which is due to our being forced to use progressively
poorer and poorer ores is likely to be much less than the addition due
to the progressive rise in the cost of coal and in the cost of labour,
because of the ever-rising scale of living. The effect of each of
these additions will be lessened by the future improvements in
processes of manufacture, and more particularly by the progressive
replacement of that ephemeral source of energy, coal, by the secular
sources, the winds, waves, tides, sunshine, the earth's heat and,
greatest of all, its momentum.

54. _Ore Supply of the Chief Iron-making Countries._--The United States mine nearly all of their iron ores, Austria-Hungary, Russia and France mine the greater part of theirs, but none of these countries exports much ore. Great Britain and Germany, besides mining a great deal of ore, still have to import much from Spain, Sweden and in the case of Germany from Luxemburg, although, because of the customs arrangement between these last two countries, this importation is not usually reported. Belgium imports nearly all of its ore, while Sweden and Spain export most of the ore which they mine.

55. _Great Britain_ has many valuable ore beds, some rich in iron,
many of them near to beds of coal and to the sea-coast, to canals or
to navigable rivers. They extend from Northamptonshire to near
Glasgow. About two-thirds of the ore mined is clayey siderite. In 1905
the Cleveland district in North Yorkshire supplied 41% of the total
British product of iron ores; Lincolnshire, 14.8%; Northamptonshire,
13.9%; Leicestershire, 4.7%; Cumberland, 8.6%; North Lancashire, 2.7%;
Staffordshire, 6.1%; and Scotland, 5.7%. The annual production of
British iron ore reached 18,031,957 tons in 1882, but in 1905 it had
fallen to 14,590,703 tons, valued at L3,482,184. In addition
7,344,786 tons, or about half as much as was mined in Great Britain,
were imported, 78.5% of it from Spain. The most important British ore
deposit is the Lower Cleveland bed of oolitic siderite in the Middle
Lias, near Middlesborough. It is from 10 to 17 ft. thick, and its ore
contains about 30% of iron.

56. _Geographical Distribution of the British Works._--Most of the
British iron works lie in and near the important coal-fields in
Scotland between the mouth of the Clyde and the Forth, in Cleveland
and Durham, in Cumberland and Lancashire, in south Yorkshire,
Derbyshire, and Lincolnshire, in Staffordshire and Northamptonshire,
and in south Wales in spite of its lack of ore.

The most important group is that of Cleveland and Durham, which makes
about one-third of all the British pig iron. It has the great
Cleveland ore bed and the excellent Durham coal near tidewater at
Middlesbrough. The most important seat of the manufacture of cutlery
and the finer kinds of steel is at Sheffield.

57. The _United States_ have great deposits of ore in many different
places. The rich beds near Lake Superior, chiefly red haematite,
yielding at present about 55% of iron, are thought to contain between
1(1/2) and 2 billion tons, and the red and brown haematites of the
southern states about 10 billion tons. The middle states, New York,
New Jersey and Pennsylvania, are known to have many great deposits of
rich magnetite, which supplied a very large proportion of the American
ores till the discovery of the very cheaply mined ores of Lake
Superior. In 1906 these latter formed 80% of the American production,
and the southern states supplied about 13% of it, while the rich
deposits of the middle states are husbanded in accordance with the law
that ore bodies are drawn on in the order of their apparent
profitableness.

The most important American iron-making district is in and about
Pittsburg, to whose cheap coal the rich Lake Superior ores are brought
nearly 1000 m., about four-fifths of the distance in the large ore
steamers of the Great Lakes. Chicago, nearer to the Lake ores, though
rather far from the Pittsburg coal-field, is a very important centre
for rail-making for the railroads of the western states. Ohio, the
Lake Erie end of New York State, eastern Pennsylvania and Maryland
have very important works, the ore for which comes in part from Lake
Superior and in part from Pennsylvania, New York and Cuba, and the
fuel from Pennsylvania and its neighbourhood. Tennessee and Alabama in
the south rely on southern ore and fuel.

58. _Germany_ gets about two-thirds of her total ore supply from the
great Jurassic "Minette" ore deposit of Luxemburg and Lorraine, which
reaches also into France and Belgium. In spite of its containing only
about 36% of iron, this deposit is of very great value because of its
great size, and of the consequent small cost of mining. It stretches
through an area of about 8 m. wide and 40 m. long, and in some places
it is nearly 60 ft. thick. There are valuable deposits also in
Siegerland and in many other parts of the country.

59. _Sweden_ has abundant, rich and very pure iron ores, but her lack
of coal has restricted her iron manufacture chiefly to the very purest
and best classes of iron and steel, in making which her thrifty and
intelligent people have developed very rare skill. The magnetite ore
bodies which supply this industry lie in a band about 180 m. long,
reaching from a little north of Stockholm westerly toward the
Norwegian frontier, between the latitudes 59 deg. and 61 deg. N. In
Swedish Lapland, near the Arctic circle, are the great Gellivara,
Kirunavara and Luossavara magnetite beds, among the largest in Europe.
From these beds, which in some parts are about 300 ft. thick, much ore
is sent to Germany and Great Britain.

60. _Other Countries._--Spain has large, rich and pure iron ore beds,
near both her northern and her southern sea coast. She exports about
90% of all the iron ore which she mines, most of it to England. France
draws most of her iron ore from her own part of the great Minette ore
deposit, and from those parts of it which were taken from her when she
lost Alsace and Lorraine. Russia's most valuable ore deposit is the
very large and easily mined one of Krivoi Rog in the south, from which
comes about half of the Russian iron ore. It is near the Donetz
coal-field, the largest in Europe. There are also important ore beds
in the Urals, near the border of Finland, and at the south of Moscow.
In Austria-Hungary, besides the famous Styrian Erzberg, with its
siderite ore bed about 450 ft. thick, there are cheaply mined but poor
and impure ores near Prague, and important ore beds in both northern
and southern Hungary. Algeria, Canada, Cuba and India have valuable
ore bodies.

61. _Richness of Iron Ores._--The American ores now mined are
decidedly richer than those of most European countries. To make a ton
of pig iron needs only about 1.9 tons of ore in the United States, 2
tons in Sweden and Russia, 2.4 tons in Great Britain and Germany, and
about 2.7 tons in France and Belgium, while about 3 tons of the native
British ores are needed per ton of pig iron.

62. _The general scheme of iron manufacture_ is shown diagrammatically in fig. 6. To put the iron contained in iron ore into a state in which it can be used as a metal requires essentially, first its deoxidation, and second its separation from the other mineral matter, such as clay, quartz, &c. with which it is found associated. These two things are done simultaneously by heating and melting the ore in contact with coke, charcoal or anthracite, in the iron blast furnace, from which issue intermittently two molten streams, the iron now deoxidized and incidentally carburized by the fuel with which it has been in contact, and the mineral matter, now called "slag." This crude cast iron, called "pig iron," may be run from the blast furnace directly into moulds, which give the metal the final shape in which it is to be used in the arts; but it is almost always either remelted, following path 1 of fig. 6, and then cast into castings of cast iron, or converted into wrought iron or steel by purifying it, following path 2.

If it is to follow path 1, the castings into which it is made may be
either (a) grey or (b) chilled or (c) malleable. Grey iron castings
are made by remelting the pig iron either in a small shaft or "cupola"
furnace, or in a reverberatory or "air" furnace, with very little
change of chemical composition, and then casting it directly into
suitable moulds, usually of either "baked," i.e. oven-dried, or
"green," i.e. moist undried, sand, but sometimes of iron covered with
a refractory coating to protect it from being melted or overheated by
the molten cast iron. The general procedure in the manufacture of
chilled and of malleable castings has been described in SS 30 and 31.

If the pig iron is to follow path 2, the purification which converts
it into wrought iron or steel consists chiefly in oxidizing and
thereby removing its carbon, phosphorus and other impurities, while it
is molten, either by means of the oxygen of atmospheric air blown
through it as in the Bessemer process, or by the oxygen of iron ore
stirred into it as in the puddling and Bell-Krupp processes, or by
both together as in the open hearth process.

On its way from the blast furnace to the converter or open hearth
furnace the pig iron is often passed through a great reservoir called
a "mixer," which acts also as an equalizer, to lessen the variation in
composition of the cast iron, and as a purifier, removing part of the
sulphur and silicon.

63. _Shaping and Adjusting Processes._--Besides these extraction and purification processes there are those of adjustment and shaping. The _adjusting processes_ adjust either the ultimate composition, e.g. carburizing wrought iron by long heating in contact with charcoal (cementation), or the proximate composition or constitution, as in the hardening, tempering and annealing of steel already described (SS 28, 29), or both, as in the process of making malleable cast iron (S 31). The _shaping processes_ include the _mechanical_ ones, such as rolling, forging and wire-drawing, and the _remelting_ ones such as the crucible process of melting wrought iron or steel in crucibles and casting it in ingots for the manufacture of the best kinds of tool steel. Indeed, the remelting of cast iron to make grey iron castings belongs here. This classification, though it helps to give a general idea of the subject, yet like most of its kind cannot be applied rigidly. Thus the crucible process in its American form both carburizes and remelts, and the open hearth process is often used rather for remelting than for purifying.

64. The _iron blast furnace_, a crude but very efficient piece of apparatus, is an enormous shaft usually about 80 ft. high and 20 ft. wide at its widest part. It is at all times full from top to bottom, somewhat as sketched in figs. 7 and 8, of a solid column of lumps of fuel, ore and limestone, which are charged through a hopper at the top, and descend slowly as the lower end of the column is eaten off through the burning away of its coke by means of very hot air or "blast" blown through holes or "tuyeres" near the bottom or "hearth," and through the melting away, by the heat thus generated, both of the iron itself which has been deoxidized in its descent, and of the other minerals of the ore, called the "gangue," which unite with the lime of the limestone and the ash of the fuel to form a complex molten silicate called the "cinder" or "slag."

GG, Flanges on the ore bucket;
HH, Fixed flanges on the top of the furnace;
J, Counterweighted false bell;
K, Main bell;
O, Tuyere;
P, Cinder notch;
RR', Water cooled boxes;
S, Blast pipe;
T, Cable for allowing conical bottom of bucket to drop.]

Lumps of Coke [symbol]
Lumps* of Iron Ore [symbol]
Lumps* of Lime [symbol]
Drops of Slag [symbol]
Drops of Iron [symbol]
Layer of Molten Slag [symbol]
Layer of Molten Iron [symbol]

* The ore and lime actually exist here in powder. They are shown in
lump form because of the difficulty of presenting to the eye their
powdered state.]

Interpenetrating this descending column of solid ore, limestone and coke, there is an upward rushing column of hot gases, the atmospheric nitrogen of the blast from the tuyeres, and the carbonic oxide from the combustion of the coke by that blast. The upward ascent of the column of gases is as swift as the descent of the solid charge is slow. The former occupies but a very few seconds, the latter from 12 to 15 hours.

In the upper part of the furnace the carbonic oxide deoxidizes the iron oxide of the ore by such reactions as xCO + FeO_x = Fe + xCO2. Part of the resultant carbonic acid is again deoxidized to carbonic oxide by the surrounding fuel, CO2 + C = 2CO, and the carbonic oxide thus formed deoxidizes more iron oxide, &c. As indicated in fig. 7, before the iron ore has descended very far it has given up nearly the whole of its oxygen, and thus lost its power of oxidizing the rising carbonic oxide, so that from here down the atmosphere of the furnace consists essentially of carbonic oxide and nitrogen.

But the transfer of heat from the rising gases to the sinking solids, which has been going on in the upper part of the furnace, continues as the solid column gradually sinks downward to the hearth, till at the "fusion level" (A in fig. 7) the solid matter has become so hot that the now deoxidized iron melts, as does the slag as fast as it is formed by the union of its three constituents, the gangue, the lime resulting from the decomposition of the limestone and the ash of the fuel. Hence from this level down the only solid matter is the coke, in lumps which are burning rapidly and hence shrinking, while between them the molten iron and slag trickle, somewhat as sketched in fig. 8, to collect in the hearth in two layers as distinct as water and oil, the iron below, the slag above.

As they collect, the molten iron is drawn off at intervals through a hole A (fig. 8), temporarily stopped with clay, at the very bottom, and the slag through another hole a little higher up, called the "cinder notch." Thus the furnace may be said to have four zones, those of (1) deoxidation, (2) heating, (3) melting, and (4) collecting, though of course the heating is really going on in all four of them.

In its slow descent the deoxidized iron nearly saturates itself with carbon, of which it usually contains between 3.5 and 4%, taking it in part from the fuel with which it is in such intimate contact, and in part from the finely divided carbon deposited within the very lumps of ore, by the reaction 2CO = C + CO2. This carburizing is an indispensable part of the process, because through it alone can the iron be made fusible enough to melt at the temperature which can be generated in the furnace, and only when liquid can it be separated readily and completely from the slag. In fact, the molten iron is heated so far above its melting point that, instead of being run at once into pigs as is usual, it may, without solidifying, be carried even several miles in large clay-lined ladles to the mill where it is to be converted into steel.

65. The _fuel_ has, in addition to its duties of deoxidizing and carburizing the iron and yielding the heat needed for melting both the iron and slag, the further task of desulphurizing the iron, probably by the reaction FeS + CaO + C = Fe + CaS + CO.

The desulphurizing effect of this transfer of the sulphur from union
with iron to union with calcium is due to the fact that, whereas iron
sulphide dissolves readily in the molten metallic iron, calcium
sulphide, in the presence of a slag rich in lime, does not, but by
preference enters the slag, which may thus absorb even as much as 3%
of sulphur. This action is of great importance whether the metal is to
be used as cast iron or is to be converted into wrought iron or steel.
In the former case there is no later chance to remove sulphur, a
minute quantity of which does great harm by leading to the formation
of cementite instead of graphite and ferrite, and thus making the
cast-iron castings too hard to be cut to exact shape with steel tools;
in the latter case the converting or purifying processes, which are
essentially oxidizing ones, though they remove the other impurities,
carbon, silicon, phosphorus and manganese, are not well adapted to
desulphurizing, which needs rather deoxidizing conditions, so as to
cause the formation of calcium sulphide, than oxidizing ones.

66. The _duty of the limestone_ (CaCO3) is to furnish enough lime to form with the gangue of the ore and the ash of the fuel a lime silicate or slag of such a composition (1) that it will melt at the temperature which it reaches at about level A, of fig. 7, (2) that it will be fluid enough to run out through the cinder notch, and (3) that it will be rich enough in lime to supply that needed for the desulphurizing reaction FeS + CaO + C = Fe + CaS + CO. In short, its duty is to "flux" the gangue and ash, and wash out the sulphur.

67. In order that the _slag_ shall have these properties its composition usually lies between the following limits: silica, 26 to 35%; lime, _plus_ 1.4 times the magnesia, 45 to 55%; alumina, 5 to 20%. Of these the silica and alumina are chiefly those which the gangue of the ore and the ash of the fuel introduce, whereas the lime is that added intentionally to form with these others a slag of the needed physical properties.

Thus the more gangue the ore contains, i.e. the poorer it is in iron,
the more limestone must in general be added, and hence the more slag
results, though of course an ore the gangue of which initially
contains much lime and little silica needs a much smaller addition of
limestone than one of which the gangue is chiefly silica. Further, the
more sulphur there is to remove, the greater must be the quantity of
slag needed to dissolve it as calcium sulphide. In smelting the rich
Lake Superior ores the quantity of slag made was formerly as small as
28% of that of the pig iron, whereas in smelting the Cleveland ores of
Great Britain it is usually necessary to make as much as 1(1/2) tons of
slag for each ton of iron.

68. _Shape and Size of the Blast-Furnace._--Large size has here, as in most metallurgical operations, not only its usual advantage of economy of installation, labour and administration per unit of product, but the further very important one that it lessens the proportion which the outer heat-radiating and hence heat-wasting surface bears to the whole. The limits set to the furnace builder's natural desire to make his furnace as large as possible, and its present shape (an obtuse inverted cone set below an acute upright one, both of them truncated), have been reached in part empirically, and in part by reasoning which is open to question, as indeed are the reasons which will now be offered reservedly for both size and shape.

First the width at the tuyeres (fig. 7) has generally been limited to about 12(1/2) ft. by the fear that, if it were greater, the blast would penetrate so feebly to the centre that the difference in conditions between centre and circumference would be so great as to cause serious unevenness of working. Of late furnaces have been built even as wide as 17 ft. in the hearth, and it may prove that a width materially greater than 12(1/2) ft. can profitably be used. With the width at the bottom thus limited, the furnace builder naturally tries to gain volume as rapidly as possible by flaring or "battering" his walls outwards, i.e. by making the "bosh" or lower part of his furnace an inverted cone as obtuse as is consistent with the free descent of the solid charge. In practice a furnace may be made to work regularly if its boshes make an angle of between 73 deg. and 76 deg. with the horizontal, and we may assume that one element of this regularity is the regular easy sliding of the charge over this steep slope. A still steeper one not only gives less available room, but actually leads to irregular working, perhaps because it unduly favours the passage of the rising gas along the walls instead of up and through the charge, and thus causes the deoxidation of the central core to lag behind that of the periphery of the column, with the consequence that this central core arrives at the bottom incompletely deoxidized.

In the very swift-running furnaces of the Pittsburg type this outward flare of the boshes ceases at about 12 ft. above the tuyeres, and is there reversed, as in fig. 7, so that the furnace above this is a very acute upright cone, the walls of which make an angle of about 4 deg. with the vertical, instead of an obtuse inverted cone.

In explanation or justification of this it has been said that a much
easier descent must be provided above this level than is needed below
it. Below this level the solid charge descends easily, because it
consists of coke alone or nearly alone, and this in turn because the
temperature here is so high as to melt not only the iron now
deoxidized and brought to the metallic state, but also the gangue of
the ore and the limestone, which here unite to form the molten slag,
and run freely down between the lumps of coke. This coke descends
freely even through this fast-narrowing space, because it is perfectly
solid and dry without a trace of pastiness. But immediately above this
level the charge is relatively viscous, because here the temperature
has fallen so far that it is now at the melting or formation point of
the slag, which therefore is pasty, liable to weld the whole mass
together as so much tar would, and thus to obstruct the descent of the
charge, or in short to "scaffold."

The reason why at this level the walls must form an upright instead of
an inverted cone, why the furnace must widen downward instead of
narrowing, is, according to some metallurgists, that this shape is
needed in order that, in spite of the pastiness of the slag in this
formative period of incipient fusion, this layer may descend freely as
the lower part of the column is gradually eaten away. To this very
plausible theory it may be objected that in many slow-running
furnaces, which work very regularly and show no sign of scaffolding,
the outward flare of the boshes continues (though steepened) far above
this region of pastiness, indeed nearly half-way to the top of the
furnace. This proves that the regular descent of the material in its
pasty state can take place even in a space which is narrowing
downwards. To this objection it may in turn be answered that, though
this degree of freedom of descent may suffice for a slow-running
furnace, particularly if the slag is given such a composition that it
passes quickly from the solid state to one of decided fluidity, yet it
is not enough for swift-running ones, especially if the composition of
the slag is such that, in melting, it remains long in a very sticky
condition. In limiting the diameter at the tuyeres to 12(1/2) ft., the
height of the boshes to one which will keep their upper end below the
region of pastiness, and their slope to one over which the burning
coke will descend freely, we limit the width of the furnace at the top
of the boshes and thus complete the outline of the lower part of the
furnace.

The height of the furnace is rarely as great as 100 ft., and in the belief of many metallurgists it should not be much more than 80 ft. There are some very evident disadvantages of excessive height; for instance, that the weight of an excessively high column of solid coke, ore and limestone tends to crush the coke and jam the charge in the lower and narrowing part of the furnace, and that the frictional resistance of a long column calls for a greater consumption of power for driving the blast up through it. Moreover, this resistance increases much more rapidly than the height of the furnace, even if the rapidity with which the blast is forced through is constant; and it still further increases if the additional space gained by lengthening the furnace is made useful by increasing proportionally the rate of production, as indeed would naturally be done, because the chief motive for gaining this additional space is to increase production.

The reason why the frictional resistance would be further increased is
the very simple one that the increase in the rate of production
implies directly a corresponding increase in the quantity of blast
forced through, and hence in the velocity of the rising gases, because
the chemical work of the blast furnace needs a certain quantity of
blast for each ton of iron made. In short, to increase the rate of
production by lengthening the furnace increases the frictional
resistance of the rising gases, both by increasing their quantity and
hence their velocity and by lengthening their path.

Indeed, one important reason for the difficulties in working very high
furnaces, e.g. those 100 ft. high, may be that this frictional
resistance becomes so great as actually to interrupt the even descent
of the charge, parts of which are at times suspended like a ball in
the rising jet of a fountain, to fall perhaps with destructive
violence when some shifting condition momentarily lessens the
friction. We see how powerful must be the lifting effect of the rising
gases when we reflect that their velocity in a 100 ft. furnace rapidly
driven is probably at least as great as 2000 ft. per minute, or that
of a "high wind." Conceive these gases passing at this great velocity
through the narrow openings between the adjoining lumps of coke and
ore. Indeed, the velocity must be far greater than this where the edge
or corner of one lump touches the side of another, and the only room
for the passage of this enormous quantity of gas is that left by the
roughness and irregularity of the individual lumps.