To persistent and intelligent iron-masters of Sweden must be given the credit of bringing the process of Bessemer to a commercial success, and so they gave to the world pneumatic or Bessemer steel, the latter name holding, properly, as a just tribute to the inventor, and this inaugurated the third general division:
BESSEMER STEEL.
Bessemer steel is made by pouring into a bottle-shaped vessel lined with refractory material a mass of molten cast iron, and then blowing air through the iron until the carbon and silicon are burned out. The gases and flame resulting escape from the mouth of the vessel.
The combustion of carbon and silicon produce a temperature sufficient to keep the mass thoroughly melted, so that the steel may be poured into moulds making ingots of any desired size.
In the beginning, and for many years, the lining of the vessel was of silicious or acid material, and it was found that all of the phosphorus and sulphur contained in the cast iron remained in the resulting steel, so that it was necessary to have no more of these elements in the cast iron than was allowable in the steel. The higher limit for phosphorus was fixed at ten points (.10%), and that is the recognized limit the world over. When Bessemer pig is quoted, or sold and bought, it means always a cast iron containing not more than ten phosphorus.
In regard to sulphur, it was found that if too much were present the material would be red-short, so that it could not be worked conveniently in the rolls or under the hammer, and that when the amount of sulphur present was not enough to produce red-shortness it was not sufficient to hurt the steel.
As red-short material is costly and troublesome to the manufacturer, it was not found necessary to fix any limit for sulphur, because the makers could be depended upon to keep it within working limits.
Later investigations prove this to be a fallacy, as much as ten or even more sulphur has been found in broken rails and shafts, the steel having made workable by a percentage of manganese. (See the results of Andrews’s investigation given in [Chap. X].)
During the operation of blowing Bessemer steel the flame issuing from the vessel is indicative of the elimination of the elements, and it is found that while the combustion is partially simultaneous the silicon is all removed before the carbon, and the characteristic white flame towards the end of the blow is known as the carbon flame; when the carbon is burned out, this flame drops suddenly and the operator knows that the blow is completed. Any subsequent blowing would result in burning iron only. During the blow the steel is charged heavily with oxygen, and if this were left in the steel it would be rotten, red-short, and worthless. This oxygen is removed largely by the addition of a predetermined quantity of ferro-manganese, usually melted previously and then poured into the steel.
The manganese takes up the greater part of the oxygen, leaving the steel free from red-shortness and easily worked.