About 1740 Benjamin Huntsman introduced the " crucible process " of melting steel in small crucibles, and thus freeing it from the slag, or rich iron silicate, with which it, like wrought iron, was mechanically mixed, whether it was made in the old forge or in the puddling furnace.
In the puddling process molten cast iron is converted into wrought iron, i.e.
They are almost entirely confined to puddling furnaces.
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.
The ultimate source of the oxygen may be the air, as in the Bessemer process, or rich iron oxide as in the puddling process, or both as in the open-hearth process; but in any case iron oxide is the chief immediate source, as is to be expected, because the oxygen of the air would naturally unite in much greater proportion with some of the great quantity of iron offered to it than with the small quantity of these impurities.
Second, in puddling iron which is to be used as a raw material for making very fine steel by the crucible process, quality is the thing of first importance.
Now in the series of operations, the blastfurnace, puddling and crucible processes, through which the iron passes from the state of ore to that of crucible tool steel, it is so difficult to detect just which are the conditions essential to excellence in the final product that, once a given procedure has been found to yield excellent steel, every one of its details is adhered to by the more cautious ironmasters, often with surprising conservatism.
In 1901 the number of persons engaged in working of the raw material was 23,263, of whom 8258 were employed in steel smelting and founding, 7781 at blast furnaces in the manufacture of pig-iron, and 7224 at puddling furnaces and rolling mills.
The first puddling works were opened in 1839, and Troy was long the centre of the New York iron and steel industry; in 1865 the second Bessemer steel works in the United States were opened here.
Sometimes the stuff is disintegrated with water in a " puddling machine," which was used, especially in Australia, when the earthy matters are tenacious and water scarce.
There are large smelting-works, foundries, puddling-works, rolling-mills and manufactures of iron and plated wares.
As the iron oxide is stirred into the molten metal laboriously by the workman or "puddler " with his hook or "rabble," it oxidizes the silicon to silica and the phosphorus to phosphoric acid, and unites with both these products, forming with them a basic iron silicate rich in phosphorus, called " puddling " or " tap cinder."
- Puddling Furnace.
- The few men who have, and are willing to exercise, the great strength and endurance which the puddler needs when he is stirring the pasty iron and balling it up, command such high wages, and with their little Soo-lb charges turn out their iron so slowly, that many ways of puddling by machinery have been tried.
- In common practice the cast iron as it runs from the blast-furnace is allowed to solidify and cool completely in the form of pigs, which are then graded by their fracture, and remelted in the puddling furnace itself.
At Hourpes, in order to save the expense of this remelting, the molten cast iron as it comes from the blast-furnace is poured directly into the puddling furnace, in large charges of about 2200 lb, which are thus about four times as large as those of common puddling furnaces.
Attractive as are these advances in puddling, they have not been widely adopted, for two chief reasons: First, owners of puddling works have been reluctant to spend money freely in plant for a process of which the future is so uncertain, and this unwillingness has been the more natural because these very men are in large part the more conservative fraction, which has resisted the temptation to abandon puddling and adopt the steel-making processes.
In case of direct puddling and the use of larger charges this conservatism has some foundation, because the established custom of allowing the cast iron to solidify gives a better opportunity of examining its fracture, and thus of rejecting unsuitable iron, than is afforded in direct puddling.
So, too, when several puddlers are jointly responsible for the thoroughness of their work, as happens in puddling large charges, they will not exercise such care (nor indeed will a given degree of care be so effective) as when responsibility for each charge rests on one man.
The removal of phosphorus, a very important duty of the puddling process, requires that the cinder shall be " basic," i.e.
It should also be nearly free from sulphur, because of the great difficulty of removing this element in the puddling process.
The " refinery process " of fitting pig iron for the puddling process by removing the silicon without the carbon, is sometimes used because of this difficulty in making a pig iron initially low in both sulphur and silicon.
The coke thus at once supplies by its combustion the heat needed for melting the iron and keeping it hot, and by itself dissolving in the molten metal returns carbon to it as fast as this element is burnt out by the blast, so that the " refined " cast iron which results, though still rich in carbon and therefore easy to melt in the puddling process, has relatively little silicon.
- When Bessemer discovered that by simply blowing air through molten cast iron rapidly he could make low-carbon steel, which is essentially wrought iron greatly improved by being freed from its essential defect, its necessarily weakening and embrittling slag, the very expensive and exhausting puddling process seemed doomed, unable to survive the time when men should have familiarized themselves with the use of Bessemer steel, and should have developed the evident possibilities of cheapness of the Bessemer process.
Many of the furnaces used for this heating are in a general way like the puddling furnace shown in fig.
14, except that they are heated by gas, that the hearth or bottom of the chamber in which they are heated is nearly flat, and that it is usually very much larger than that of a puddling furnace.
We may note with interest that the three great iron producers so closely related by blood-Great Britain, the United States and Germany and Luxemburg-made in 1907 81% of the world's pig iron and 83% of its steel; and that the four great processes by which nearly all steel and wrought iron are made-the puddling, crucible and both the acid and basic varieties of the Bessemer and open-hearth processes, as well as the steam-hammer and grooved rolls for rolling iron and steel-were invented by Britons, though in the case of the openhearth process Great Britain must share with France the credit of the invention.
The placer-miner's cradle and rocking-trough were replaced by puddling troughs stirred by a revolving comb worked by horse power; reservoirs were constructed for the scanty water-supply, bucket elevators were introduced to carry away the tailings; and the natives were confined in compounds.
The town is flourishing and rapidly increasing, and possesses very extensive wire factories (in connexion with which there are puddling and rolling works), machine works, and manufactories of gloves, baskets, leather, starch, chemicals, varnish, oil and beer.
Its industries include iron foundries, rolling mills, puddling furnaces, and manufactures of iron, steel and brass wares and of machines.
The largest iron-works in Silesia is situated at KOnigshiitte, and includes puddling works, rollingmills, and zinc-works.
The former have a very extended application in many branches of industry, being used by both founders and smelters in the fusion of metals; in the concentration of poor metallic compounds by fusion into regulus; in the reduction of lead and tin ores; for refining copper and silver; and for making malleable iron by the puddling processes and welding.
It has been found to stand well for the linings of rotatory puddling furnaces, where, under long-continued heating, it changes into a substance as hard and infusible as natural emery.
Puddling furnaces are usually entirely cased with iron plates, and blast furnaces with hoops round each course of the stack, or in those of thinner constructions the firebrick work is entirely enclosed in a wrought iron casing or jacket.
With reverberatory calciners, however, where the work is done upon a horizontal bed, a considerable amount of hand labour is expended in raking out the charge when finished, and in drawing slags from fusion furnaces; and more particularly in the puddling process of refining iron the amount of manual exertion required is very much greater.
In reverberatory and air furnaces used in the different operations of iron manufacture, where an extremely high temperature has to be maintained in spaces of comparatively small extent, such as the beds of puddling, welding and steel-melting furnaces, the temperature of the exhaust gases is exceedingly high, and if allowed to pass directly into the chimney they appear as a great body of flame at the top. It is now general to save a portion of this heat by passing the flame through flues of steam boilers, air-heating apparatus, or both - so that the steam required for the necessary operations of the forge and heated blast for the furnace itself may be obtained without further expenditure of fuel.