This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Steel rolled colder than Iron.
Two Principal Methods.
Open Hearth Process.
It consists of melting a combination of irons in a basin-shaped hearth under a reverberatory roof by playing on them what is really a blowpipe with a flame oxidizing (removing carbon) from the mass: or reducing it (adding carbon) at will (but usually oxidizing). The combination consists of pig-iron and wrought scrap-iron or old steel rails, or else of pig-iron and iron ore, or sometimes a combination of all the above ; when melted down, the one with or in the other, the mass is stirred about to render the mixture homogeneous, and after preliminary testing of samples, is duly poured into ingot moulds. Thus the cast-iron gives up its excess of carbon to the rest of the charge having less or none, and the resultant product, which can be controlled with a nicety that is surprising, is a steel of high grade, with carbon between limits that are well governed from the start to the close of the conversion. The ingots are allowed to cool and are then sold to mills not owning steel furnaces, who, of course, have to re-heat the ingots before rolling them. Where a mill manufactures its own ingots, however, the hot ingot is run directly to the blooming mill and rolled into "blooms" while still hot.
In the pneumatic or Bessemer process the carbon is entirely removed from the pig-iron by oxidation and then the necessary amount of carbon to produce the desired quality of steel is afterwards re-introduced. This seemingly roundabout method is adopted, because it would be impossible otherwise to determine promptly or exactly enough when the decarbonization should cease, the entire removal of all the carbon taking only some twenty minutes. It is easy to determine, however, when the carbon, silicon, etc., are completely oxidized, and then to introduce carbon in a known proportion which is done by adding "spiegeleisen" as hereafter mentioned. It would not be so easy, however, otherwise to know at just what point sufficient carbon was removed.
The above operations are conducted in an egg-shaped vessel open on top, known as the "converter," hung like a cannon on "trunnions," into which melted pig-iron is poured from a cupola furnace, blast or wind furnace. This converter has a perforated lining in the bottom through which a forced vertical blast, at a pressure of 20 to 25 pounds per square inch, enters and bubbles up through the molten metal. The action of this blast is distinctly oxidizing, the oxygen in the air seeming to first combine with the silicon in the pig, forming silica or quartz, which rises as slag to the top. The manganese and carbon in the pig are next consumed, the carbon passing off in gaseous combination with the oxygen and being thus eliminated. By the changing appearance of the "flame," which at first is of a bright yellow appearance but very turbulent and full of flying sparks, then settles to a more steady and clearer flame of pink or amethyst color, and finally settles down and disappears almost completely from the mouth of the converter, the various stages and final total elimination of the carbon are known, and at the end the blast is shut off. The metal in the converter is at this moment approximately pure iron, slightly oxidized or "burnt," and is in a spongy friable mass and non-malleable. Molten "spiegeleisen" is then poured in and its addition is accompanied by a violent boiling reaction, accompanied by a reappearance of the flame at the mouth of the converter. This Spiegeleisen is simply a German pig-iron very high in carbon and manganese of known proportions, and is so-called from its mirrorlike, shiny, crystallized formation.
Condition known by tests.
The object of the carbon in this addition is to turn the pure iron to steel; the object of the manganese is to take up any excess of oxygen in the melted iron by forming oxides of manganese, thus preventing the steel from being what is known as "red short" or burned.
This mixture of decarbonized iron and spiegel is allowed to rest in the converter for a short time or is sometimes mechanically stirred up and then, after pouring off the slag, the contents of the converter are tipped into a ladle swinging radially on a crane-arm over a semi-circular row of ingot moulds into which the melted steel is poured from an opening in the bottom of the ladle controlled by a plug. These ingots weigh 4 to 5 tons each : they are allowed to cool as they are in the mould, or else in a "soaking pit," which is simply a device for allowing them to cool slowly on the annealing principle ; this slower cooling improves the quality of the product by more thoroughly eliminating the included gases and avoiding the internal strains due to otherwise too-rapid cooling. But, as already remarked, where the blooming mill is close at hand, the ingots are not allowed to cool, but are run hot to the rollers.
Condition known by flame.
The lining of the converter is usually "ganister" (silicious sand) which is of an acid (silicle) reaction and gives the name of "Acid process" to that usually adopted in this country. In England and on the Continent and presently in Alabama, where ores run too high in phosphorous and sulphur to permit of their successful use in the Bessemer process, the acid lining is sometimes replaced with a basic or alkaline lining made from dolomite, etc. The lime and magnesia in this lining unite with the sulphur and phosphorous in the pig forming a fusible slag, thus rendering their elimination possible. This expense is not required, however, with most pig made from American ores as may be inferred from the fact that it does not represent the ordinary practice in this country.
It is not necessary within these limits to explain the "Clapp-Griffiths" process, about the merits of which so much controversy has been had lately, further than to state that it is an intended modification of the Bessemer process, by which it is hoped to modify and cheapen it to the point of rendering it adaptable to plants of small capacity, and also to secure a better control over the quality and homogeneity of the product desired.