The fact that the phosphorus of the iron remained in the steel notwithstanding the active combustion and high temperature led to the dictum that at high temperatures phosphorus could not be eliminated from iron. This conclusion was credited because in some of the so-called direct processes of making iron where the temperature was never high enough to melt steel all, or nearly all, of the phosphorus was removed from the iron.
For many years steel-makers the world over worked upon this basis, and devoted themselves to procuring for their work iron containing not more than ten (.10) phosphorus, now universally known and quoted as Bessemer iron.
Two young English chemists, Sidney Gilchrist Thomas and Percy C. Gilchrist, being careful thinkers, concluded that the question was one of chemistry and not one of temperature; accordingly they set to work to obtain a basic lining for the vessel and to produce a basic slag from the blow which should retain in it the phosphorus of the iron. After the usual routine of experiment, and against the doubtings of the experienced, they succeeded, and produced a steel practically free from phosphorus. For the practical working of their process it was found better, or necessary, to use iron low in silicon and high in phosphorus, using the phosphorus as a fuel to produce the high temperature that is necessary instead of the silicon of the acid process. In the acid process it is found necessary to have high silicon—two percent or more—to produce the temperature necessary to keep the steel liquid; in the Thomas-Gilchrist process phosphorus takes the place of silicon for this purpose.
In this way the basic Bessemer process was worked out and became prominent.
The basic Bessemer process is of great value to England and to the continent of Europe by enabling manufacturers to use their native ores, which are usually too high in phosphorus for the acid process, so that before this invention nearly all of the ores for making Bessemer steel were imported from Sweden, Spain, and Africa.
The basic process has found little development in the United States, because the great abundance of pure ore keeps the acid process the cheaper, except in one or two special localities. Where the basic process is profitable in the United States, it is worked successfully.
At about the time that Bessemer made his invention William Siemens, afterward Sir William, invented the well-known regenerative gas-furnace. A Frenchman named Martin utilized this furnace to melt steel in bulk in the hearth of the furnace, developing what was known for some years as Siemens-Martin steel, or open-hearth steel; the latter name has prevailed, and open-hearth steel is the fourth of the general kinds of steel mentioned in the beginning of this chapter.
At first open-hearth steel was made upon a specially prepared sand bottom, by first melting a bath of cast iron and then adding wrought iron to the bath until by the additions of wrought iron and the action of the flame the carbon and silicon of the cast iron were reduced until the whole became a mass of molten steel. Sometimes iron ore is used instead of wrought iron as the reducing agent; this is called the pig and ore process. Now in general practice wrought iron, steel scrap, and iron ore are used, sometimes alone and sometimes together, as economy or special requirements make it convenient.
It was found as in the Bessemer, so in the open-hearth, the sulphur and the phosphorus of the charge remained in the steel, making it necessary to see that in the charge there was no more of these elements than the steel would bear.
This is known now as the acid open-hearth process.