For rails of basic open-hearth steel, which is rapidly ousting Bessemer steel, the Civil Engineers' specifications allowed from o 65 to 0-75% of carbon with 0-05% of phosphorus, while the specifications of the American Railway Engineering and Maintenance of Way Association provided for a range of 0.75 to 0-85% of carbon, with a maximum of 0.03% of phosphorus.
In many respects the glassmelting tank resembles the open-hearth steel furnace, but there are certain interesting differences.
In the specification for bridge material, drawn up by the British Engineering Standards Committee, it is provided that the steel shall be acid or basic open-hearth steel, containing not more than o.
Crucible steel was first successfully produced in 1832, Bessemer and open-hearth in 1864.
Between 1860 and 1870 the invention of the Bessemer and open-hearth processes introduced a new class of iron to-day called " mild " or " carbon wcarbon steel," which lacked the essential property of steel, the hardening power, yet differed from the existing forms of wrought iron in freedom from slag, and from cast iron in being very malleable.
The third period has for its great distinction the invention of the Bessemer and open-hearth processes, which are like Huntsman's crucible process in that their essence is their freeing wrought iron and low carbon steel from mechanically entangled cinder, by developing the hitherto unattainable temperature, rising to above 1500° C., needed for melting these relatively infusible products.
Knowing this, and having in the Siemens regenerative gas furnace an independent means of generating this temperature, the Martin brothers of Sireuil in France in 1864 developed the open-hearth process of making steel of any desired carbon-content by melting together in this furnace cast and wrought iron.
After the remarkable development of the blast furnace, the Bessemer, and the open-hearth processes, the most important work of this, the third period of the history of iron, is the birth and growth of the science and art of iron metallography.
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.
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.
Indeed, few if any of the direct processes have attempted to make this separation, or to make it complete, leaving it for some subsequent operation, such as the open hearth process.
Until relatively lately the cast iron for the Bessemer and open-hearth processes was nearly always allowed to solidify in pigs, which were next broken up by hand and remelted at great cost.
This is kept molten by a flame playing above it, and successive lots of the cast iron thus mixed are drawn off, as they are needed, for conversion into steel by the Bessemer or open-hearth process.
This is very useful if the iron is intended for either the basic Bessemer or the basic open-hearth process, for both of which silicon is harmful.
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.
But in the crucible and the open-hearth processes the temperature attainable is limited by the danger of melting the furnace itself, both because some essential parts of it, which, unfortunately, are of a destructible shape, are placed most unfavourably in that they are surrounded by the heat on all sides, and because the furnace is necessarily hotter than the steel made within it.
In it the steel heats the converter, whereas in the open-hearth and crucible processes the furnace heats the steel.
The open-hearth process consists in making molten steel out of pig or cast iron and " scrap," i.e.
- Open-Hearth Process.
version of cast iron into steel, of course, consists in lessening its content of the several foreign elements, carbon, silicon, phosphorus, &c. The open-hearth process does this by two distinct steps: (I) by oxidizing and removing these elements by means of the flame of the furnace, usually aided by the oxygen of light charges of iron ore, and (2) by diluting them with scrap steel or its equivalent.
The charge may be melted down on the " open hearth " itself, or, as in the more advanced practice, the pig iron may be brought in the molten state from the blast furnace in which it is made.
It is in large part because of this shallowness, which contrasts so strongly with the height and roominess of the Bessemer converter, that the process lasts hours where the Bessemer process lasts minutes, though there is the further difference that in the open-hearth process the transfer of heat from flame to charge through the intervening layer of slag is necessarily slow, whereas in the Bessemer process the heat, generated as it is in and by the metallic bath itself, raises the temperature very rapidly.
The general plan of the open-hearth process was certainly conceived by Josiah Marshall Heath in 1845, if not indeed by Reaumur in 1722, but for lack of a furnace in which a high enough temperature could be generated it could not be carried out until the development of the Siemens regenerative gas furnace about 1860.
Siemens Open-Hearth Furnace.
The working chamber indeed is the furnace proper, in which the whole of the open-hearth process is carried out, and the function of all the rest of the apparatus, apart from the tilting mechanism, is simply to pre-heat the air and gas, and to lead them to the furnace proper and thence to the chimney.
A locomotive carries a train of these cars to the track running beside a long line of open-hearth furnaces.
Three special varieties of the open-hearth process, the Bertrand-Thiel, the Talbot and the Monell, deserve notice.
charged with oxygen, and superheated, in an open-hearth furnace.
The oxygenated metal is prepared by melting cast iron diluted with as much scrap steel as is available, and oxidizing it with the flame and with iron ore as it lies in a thin molten layer, on the hearth of a large open-hearth furnace; the thinness of the layer hastens the oxidation, and the large size of the furnace permits considerable frothing.
To enlarge the scale of operations makes strongly for economy in the open-hearth process as in other high temperature ones.
Yet the use of an open-hearth furnace of very great capacity, say of 200 tons per charge, has the disadvantage that such very large lots of steel, delivered at relatively long intervals, are less readily managed in the subsequent operations of soaking and rolling down to the final shape, than smaller lots delivered at shorter intervals.
Besides minor advantages, this plan has the merit of avoiding an ineffective period which occurs in common open-hearth procedure just after the charge of cast iron has been melted down.
In common open-hearth procedure, although the temperature is low early in the process, viz.
At the Carnegie works Mr Monell gets the two dephosphorizing conditions, low temperature and basicity of slag, early in the process, by pouring his molten but relatively cool cast iron upon a layer of pre-heated lime and iron oxide on the bottom of the open-hearth furnace.
In the most promising form of this process an acid converter and a basic open-hearth furnace are used.
But if the conversion is only begun in the converter and finished on the open-hearth, then there is no need of regulating the temperature in the converter closely, and variations in the silicon-content of the pig iron thus become almost harmless in this respect.
In the basic open-hearth process, on the other hand, silicon is harmful because the silica which results from its oxidation not only corrodes the lining of the furnace but interferes with the removal of the phosphorus, an essential part of the process.
But if the silicon of the pig iron is removed by a preliminary treatment in the Bessemer converter, then its presence in the pig iron is harmless as regards the open-hearth process.
Compared with the Bessemer process, which converts a charge of even as much as 20 tons of pig iron into steel in a few minutes, and the open-hearth process which easily treats charges of 75 tons, the crucible process is, of course, a most expensive one, with its little 80-lb charges, melted with great consumption of fuel because the heat is kept away from the metal by the walls of the crucible, themselves excellent heat insulators.
In steel-making, electric furnaces are used for two distinct purposes, first for making steel sufficiently better than Bessemer and open-hearth steels to replace these for certain important purposes, and second for replacing the very expensive crucible process for making the very best steel.
When the Heroult furnace is used for completing the purification of molten steel begun in the Bessemer or open-hearth process, and this is its most appropriate use, the process carried out in it may be divided into two stages, first dephosphorization, and second deoxidation and desulphurization.
It is by forming calcium sulphide that sulphur is removed in the manufacture of pig iron in the iron blast furnace, in the crucible of which, as in the electric furnaces, the conditions are strongly deoxidizing But in the Bessemer and open-hearth processes this means of removing sulphur cannot be used, because in each of them there is always enough oxygen in the atmosphere to re-oxidize any calcium as fast as it is deoxidized.
The melting can be done much more cheaply in a cupola or open-hearth furnace, and the first part of the purification much more cheaply in a Bessemer converter or open-hearth furnace.
Electric furnaces are at an advantage over others as regards the removal of sulphur and of iron oxide from the molten steel, because their atmosphere is free from the sulphur always present in the flame of coal-fired furnaces, and almost free from oxygen, because this element is quickly absorbed by the carbon and silicon of the steel, and in the case of arc furnaces by the carbon of the electrodes themselves, and is replaced only very slowly by leakage, whereas through the Bessemer converter and the open-hearth furnace a torrent of air is always rushing.
why the molten metal can be freed from mechanically suspended slag more perfectly in them than in the Bessemer converter or the open-hearth furnace.
It is practically unattainable in the open-hearth furnace, because here the oxygen of the furnace atmosphere indirectly oxidizes the carbon of the metal which is kept boiling by the escape of the resultant carbonic oxide.
In short the electric furnaces can be used to improve the molten product of the Bessemer converter and open-hearth furnace, essentially because their atmosphere is free from sulphur and oxygen, and because they can therefore remove sulphur, iron oxide and mechanically suspended slag, more thoroughly than is possible in these older furnaces.
They are not likely to displace either the open-hearth furnace or the Bessemer converter, because their normal work is only to improve the product of these older furnaces.
But the great mass of the steel of commerce is made by the Bessemer and the open-hearth processes..
Open-hearth steel is generally thought to be better than Bessemer, and the acid variety of each of these two processes is thought to yield a better product than the basic variety.
cannot be made to yield cast iron either free enough from phosphorus for the acid Bessemer or the acid open-hearth process, neither of which removes that most injurious element, or rich enough in phosphorus for the basic Bessemer process, which must rely on that element as its source of heat.
But cast iron for the basic open-hearth process can be made from almost any ore, because its requirements, comparative freedom from silicon and sulphur, depend on the management of the blast-furnace rather than on the composition of the ore, whereas the phosphorus-content of the cast iron depends solely on that of the ore, because nearly all the phosphorus of the ore necessarily passes into the cast iron.
Thus the basic open-hearth process, is the only one which can make steel from cast iron containing, more than o io% but less than i 80%of phosphorus.
basic form, made all of the world's rail steel; but even for this work it has now begun to be displaced by the basic open-hearth process, partly because of the fast-increasing scarcity of ores which yield pig iron low enough in phosphorus for the acid Bessemer process, and partly because the increase in the speed of trains and in the loads on the individual engineand car-wheels has made a demand for rails of a material better than Bessemer steel.
The increased importance of Germany and Luxemburg may be referred in large part to the invention of the basic Bessemer and open-hearth processes by Thomas, who by them gave an inestimable value to the phosphoric ores of these countries.
2 Bessemer and open hearth only.
This rich matte is then mixed with coke and salt-cake and melted down in an open hearth furnace.
At the Boston and Great Falls (Montana) works tilting reverberatories, modelled after open hearth steel furnaces, were first erected; but they were found to possess objectionable features.
Several neighbouring cities and towns are also extensively engaged in the same industry, and in 1902 Allegheny county produced about 24% of the pig-iron, nearly 34% of the Bessemer steel, more than 44% of the open-hearth steel, more than 53% of the crucible steel, more then 24% of the steel rails, and more than 59% of the structural shapes that were made in that year in the United States.
fireplace with open hearth (flue not tested ).
Early dwellings had their fires in an open hearth on the floor, in the middle of the main room.
We baptized the infant and had all the usual trappings of Xmas, a tree, and plum pudding and even an open hearth.
open-hearth furnace at Brymbo 1886 General Election.
open-hearth fireplaces would be allowed.
open-hearth fire and laundry demonstrations.
The third period has for its great distinction the invention of the Bessemer and open-hearth processes, which are like Huntsman's crucible process in that their essence is their freeing wrought iron and low carbon steel from mechanically entangled cinder, by developing the hitherto unattainable temperature, rising to above 1500Ã‚° C., needed for melting these relatively infusible products.
The restriction of the basic Bessemer process to pig iron containing at least i 80% of phosphorus has prevented it from getting a foothold in the United States; the restriction of the acid Bessemer process to pig iron very low in phosphorus, usually to that containing less than o ro% of that element, has almost driven it out of Germany, has of late retarded, indeed almost stopped, the growth of its use in the United States, and has even caused it to be displaced at the great Duquesne works of the Carnegie Steel Company by the omnivorous basic open-hearth.
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