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.
CHAPTER VI
WROUGHT IRON
We of America, and especially of the West, never have been particularly devoted to the study of our genealogies. However, it is likely that most of us at one time or other in our mind’s eye have seen that little country town back in Massachusetts or New York State, whence came our forebears. Since those days of long ago, when, with you upon his knee, grandfather waxed reminiscent and related tales of his boyhood, haven’t you many times wished that Father Time could carry you back with him a hundred and fifty years and allow you for a few hours to walk among those good people with their quaint dress and customs? How interested you would be in the relatives and friends whose queerly transcribed verses, bearing date of a century and a half ago, adorn the yellow pages of your great aunt’s autograph album, which is one of your treasures!
To the present rather unsentimental world we admit of much the same sort of reverent feeling as we traverse the paths, centuries old, of the wrought iron region. Here is something which links us with the antiquities and we “take off our hats and tread lightly” over the lands and the centuries in and through which this primal material, wrought iron, has been produced.
But we come back to a plain statement of the case. Let not the fact be overlooked that it is wrought iron which in some form or other has been made during the forty or more centuries that iron has been known. It undoubtedly was the pioneer of the iron family and through all of the centuries it has maintained its importance. From it as a base during the earlier centuries were made the steels for which the ancients were so famous, and similar practice has prevailed from those early times up to the present day.
Wrought Iron Bar Steel has no “fiber” and can not be split in this way.
We say that the iron made by the ancients was in reality a variety of what we now term wrought iron, for it fulfilled three of the principal requirements for wrought iron, viz., it was not necessarily melted during production, it was a malleable metal, and it contained cinder or slag, but practically no carbon.
Early iron and steel, of course, were comparatively rare, so much so as to be available only for implements of war and for particular purposes. Moreover, all of the steel made was of the high carbon, hard variety, none of it being of the low carbon sort which we know as the soft or mild steels. The latter have all come since the invention of the Bessemer process in 1855 and the Siemens-Martin (the open-hearth) process of a few years later.
As you will remember the next development in the iron industry was the use of the larger blast furnace and much more fuel, with higher temperatures and the production of cast iron which, by reason of the absorption of considerable carbon from the fuel, was fluid enough to run out of the furnace. And, as you also know, this blast furnace metal has come to be the great intermediate product in what has come to be known as the Indirect Process of making iron and steel.
Pickling in Weak Acid Exposes the “Fibers” of Wrought Iron
As pig iron contains 3½% or more of carbon, while wrought iron has none or comparatively little of this embrittling element, any process for converting the pig iron into wrought iron must eliminate the carbon.
Following the advent of pig iron as a commercial product several furnaces and processes were developed to turn it into wrought iron. All of these were based upon the elimination of the carbon by oxidation, which means the burning of the carbon in presence of oxygen of the air.
The best known ones were the Walloon or Swedish Lancashire process, the South Wales process, the Finery Fire, the Running-Out Fire and the Charcoal Finery or Knobbling process. The Walloon furnace, largely used in Sweden, is shown in the illustration on page [94].
While by the process generally used to-day wrought iron is more easily and cheaply produced, it must be said that by the processes above named has been made as fine iron as has ever been produced. The Sheffield manufacturers, whose high grade steel made by the crucible process, is so widely and favorably known for cutlery, tools, etc., in many cases yet demand the Walloon process iron made from high grade Swedish ores as the starting point for their product.
Bar Iron Will Stand Severe Bending Even After It Has Been Nicked
Swedish Walloon Furnace
One of several types of furnaces that preceded the Reverberatory. Some of them are still used.
In these furnaces which consisted mainly of a shallow hearth filled with glowing charcoal, with tuyères for blast supply, the pigs of iron were heated until molten. At high temperatures the metalloids of pig iron will readily burn in a blast of air. After the air blown over and upon the molten metal has burned out most of the carbon, the main metalloid, the melting point of the purified metal becomes so much higher that the heat of the furnace is not sufficient to keep it molten. It becomes more and more pasty and stiff therefore. This is what the iron maker calls “coming to nature.” It is the signal that the carbon is about gone and that he must be careful or the iron will suffer in quality. At this stage he sees to it that the mass of pasty iron is well protected by glowing charcoal until it is removed to be hammered or worked into a bar.
This in general is the process which was used from about the fourteenth century up to 1783 when Cort invented the reverberatory furnace which has since been the type generally used.
In the processes just referred to, all very similar, it should be noted that the iron was in contact with the fuel, which, therefore, had to be charcoal, the fuel with little or no sulphur.
Cort’s Reverberatory Furnace
This is the type of furnace generally used to-day.
Charcoal was an expensive fuel, and, moreover, there was an insufficient supply because of the great destruction of forests necessary for its production. Naturally the thing to do was to substitute coal, which was plentiful. But the sulphur of the coal spoiled the iron.
This proved to be a great barrier until 1783 when Henry Cort of England succeeded in making wrought iron in a new type of furnace wherein the iron was refined in one compartment while the fuel was made to furnish its heat from another; i. e., the fuel and the metal were not in contact and the refined metal did not suffer from the sulphur content of the coal.
The figure illustrating Cort’s furnace plainly shows how this was accomplished and it represents almost as well the furnace used to-day. In the one-hundred and thirty-three years that have elapsed since it was designed, his furnace has been changed only in certain details, and but two important changes have been made in his process—the use of an iron bottom instead of the sand bed by S. B. Rogers in 1804 and the introduction of “pig boiling” by Joseph Hall in 1830.
Cort’s process is known as “dry puddling” and his trouble was the excessive loss of iron, due to his use of the sand bottom and the absence of a proper cinder. The loss was said to have been from 50% to 70% of the iron charged; i. e., it took about 2 tons of pig iron to make one ton of wrought iron. Because of the great demand for iron and the fact that he was using such a cheap fuel, coal, his process at that time was a success financially.
Early Iron Rolls
While Cort did not originate the idea, it was he who first made the rolling process successful, and, therefore, he is given credit for the invention.
But Cort did not stop here. He saw the desirability of a quicker and more economical method of reducing the “blooms” or balls of iron into bars or other finished shapes than the hammering process up to that time used. He accomplished this by the use of power driven rolls, such as those shown in the cut.
These two inventions of Cort’s were epoch making, though of the two the more important one was the invention of the rolls from which has developed the modern rolling mill.
There remains to be described in a little more detail the making of wrought iron as practiced to-day. The puddling furnace used has a grate upon which is built the coal fire. The long flame passes over the fire wall and is deflected by the roof down upon the charge of pig iron piled upon the “fettling” or lining of iron oxide (iron ore or mill scale) over the air or water cooled iron plates which form the bottom and the sides of the hearth.
With a long iron bar the “puddler” (the expert attendant) turns the pigs until they have melted down into the bath of slag or cinder charged and continually being formed by the chemical union of iron from the ore or pig and some of the sand and other impurities present. This cinder, which is largely a silicate of iron, is a protection for the bath of molten iron and its use prevents excessive oxidation and loss of metal.
During the melting of the charge the heat is kept as high as possible and the molten iron is “puddled,” i. e., stirred, by the “puddler” or his helper with a “rabbler” or iron bar. In order to take out the phosphorus and sulphur, which for best results should be removed before the carbon is eliminated, the heat of the furnace is lowered somewhat as soon as all of the pig has melted, and some iron oxide is mixed into the “bath” of molten iron and cinder. Most of the phosphorus and sulphur are chemically acted upon and pass into the cinder which covers the iron. Soon the mass begins to boil or seethe and small blue flames break through the cinder covering. This indicates that the carbon is being oxidized by the oxygen of the iron ore which was added, the oxygen, as in the blast furnace, having a greater “affinity” for carbon than it has for the iron.
This “pig boiling” goes on for twenty or thirty minutes, the “puddler” meanwhile “rabbling” the charge in order to hurry the reactions and to make sure that all parts of the bath of molten metal are uniformly exposed to the oxidizing conditions.
The Making of Wrought Iron To-day. Charging the Pig Iron
Soon the metal begins to “come to nature” and little lumps of pure iron here and there through the bath stiffen up into little pasty balls. Some may be seen sticking out through the cinder covering of the bath and others must be torn loose from the bottom with the rabbler. This “balling” period lasts from fifteen to twenty minutes, during which time all of the iron has “come to nature.”
The Coal Pile and Fire Box of a Reverberatory Furnace
Turning the Pigs to Insure Even Melting
The puddler quickly separates into two or three white-hot balls the 400 pounds or so of spongy iron. These balls, full of and dripping with cinder, are seized, one at a time, with long tongs, removed from the furnace and rushed to the rotary squeezer through which they are twisted and turned, ever becoming longer and smaller in diameter until they emerge from the other and narrower side much compacted and with but little of the cinder which they originally carried. Without being given time to cool, the blooms are seized and shoved into the “muck rolls,” whence after a few passes they emerge as long flat bars, very rough and imperfect in appearance.
After shearing into short lengths, many pieces of this “muck bar,” as it is called, are made into “box piles” and tied together with wires. These are charged into the furnace of the finishing mill. After coming to a white heat the box piles go to the finishing rolls where they are rolled into bars, rods, plates, or other shapes desired. By repeated cutting, piling, heating, and rolling, double refined and other high grades of wrought iron are made, each repiling and rerolling, of course, producing a more compact and better product.
Puddling the Melted Charge
Considerable Cinder Overflows During the “Boiling” Stage
Wrought iron has a “fiber,” as may be seen from two of the illustrations presented. In “coming to nature” small particles of iron crystallize out as they become purer. These measure perhaps ¹⁄₃₂″ or ¹⁄₁₆″ in diameter. As it goes to the squeezer the ball of “sponge” is made up of such grains of iron loosely welded together with the interstices filled with cinder. The squeezer, and later the rolls, elongate the particles of iron into threads, which, welded together, make the bar.
It is thought by some that films of cinder, too thin to be seen under the microscope, surround each fiber of iron and that these afford protection from rusting and give to wrought iron the excellent welding quality that it possesses.
Drawing One of the Balls from the Furnace
On the Way to the “Squeezer”
Steel has no fiber and for this reason it cannot be split as was the wrought iron bar shown in the illustration of page [92].
It will have been noticed that only small amounts of iron can be refined at one time. This, indeed, has been the unfortunate part of wrought iron manufacture, for it may readily be seen that production of any such material in lots as small as a quarter ton results in labor costs which are almost prohibitive in these days of machine-made goods. Not only is the output of a furnace small but much skill and judgment are necessary for the production of a high grade product. Very sturdy and strong men, too, are required as puddlers, for the work is heavy and the extremes of heat and cold to which they are exposed necessitate men of rugged health.
Cross-Section of a “Squeezer”
This material in bar iron, engine stay bolts, butt and lap-welded pipe and certain other products has long held a high place. Though not as strong as steel, its very excellent welding properties and comparative freedom from “crystallization” and treacherous breakage under long continued vibration or sudden jar have fostered its application in such products as stay bolts, chain-links, cable hooks and others where failure might have serious consequences. Too, it is thought by many that pipes, sheets and other articles of wrought iron resist the corrosive influences of moist air, soil, etc., particularly well. The cinder films in which the fibers are supposed to be encased are given credit for such protective influence.
“Box Piles” Ready for the Re-Heating Furnace
Though the product has always been a favorite with iron users, the industry has suffered during the last sixty years by reason of the high cost of manufacture, which has very largely restricted the application of wrought iron to certain uses for which first cost is not a main factor.
The production of wrought iron rails has rapidly dwindled since the year 1880, when they began generally to be replaced by rails made from Bessemer steel.
The Ball Going Into the “Squeezer”
Shearing the Muck Bar into Short Lengths for “Piling”
The Bloom from the Squeezer Goes to the “Muck” Rolls Which Roll It Down into “Muck Bar”
The Re-Heated “Box Piles” Being Rolled into “Skelp” for Pipe, in the Finishing Mills
Cross Piling Gives Cross Fibers in the Finished Bar—a Desirable Quality
Of recent years, mild steel has been the great competitor of wrought iron. With marvelous energy, skill, and much capital the manufacturers of Bessemer and open-hearth steels have adapted their products very well to the needs of the iron user, while the enormous tonnages turned out in short time by use of ingenious furnaces and other devices has resulted in a low cost of production.
Had any of the several mechanical puddling furnaces devised for the manufacture of wrought iron proved really successful, things might be more rosy commercially for this very excellent material.
According to the 1916 Statistical Report of the American Iron and Steel Institute, the recent yearly production in this country of wrought iron and steel merchant bars, plates and sheets, and skelp for pipe in gross tons has been:
| Merchant Bars | Plate and Sheets | Skelp for Pipe | ||||
|---|---|---|---|---|---|---|
| Iron | Steel | Iron | Steel | Iron | Steel | |
| 1905 | 1,322,439 | 2,271,162 | 72,156 | 3,460,074 | 452,797 | 983,198 |
| 1906 | 1,481,348 | 2,510,852 | 74,373 | 4,107,783 | 391,517 | 1,137,068 |
| 1907 | 1,440,356 | 2,530,632 | 74,038 | 4,174,794 | 444,536 | 1,358,091 |
| 1908 | 685,233 | 1,301,405 | 54,033 | 2,595,660 | 297,049 | 853,534 |
| 1909 | 952,230 | 2,311,301 | 76,202 | 4,158,144 | 370,151 | 1,663,230 |
| 1910 | 1,074,163 | 2,711,568 | 91,118 | 4,864,366 | 350,578 | 1,477,616 |
| 1911 | 835,625 | 2,211,737 | 89,427 | 4,398,622 | 322,397 | 1,658,276 |
| 1912 | 944,790 | 2,752,324 | 75,044 | 5,800,036 | 327,012 | 2,119,804 |
| 1913 | 1,026,632 | 2,930,977 | 64,729 | 5,686,308 | 312,746 | 2,189,218 |
| 1914 | 563,171 | 1,960,460 | 56,590 | 4,662,656 | 264,340 | 1,718,091 |
| 1915 | 657,107 | 3,474,135 | 20,253 | 6,057,441 | 262,198 | 2,037,266 |
| 1916 | 993,948 | 5,625,598 | 13,303 | 7,440,677 | 355,445 | 2,572,229 |
There is considerable so-called wrought iron on the market to-day which is not truly wrought iron, by which we mean iron “puddled” from pig iron.