An experienced blower can judge through every period of the operation of the condition of his metal and just how things are progressing.
After some minutes the flame begins to waver and later “drops”; i.e., there is scarcely a flame at all. This signal, which is very definite to an experienced man, cannot be lightly disregarded. Oxygen has affinity for iron as well as for the metalloids and it is only because of its greater love for silicon, manganese and carbon that it has thus far largely neglected the iron. With the metalloids mentioned out of the way, as they are when the drop occurs, the iron will begin to burn. Were the “blowing” continued we would shortly have no iron left, but in its place a mass of iron oxide and slag.
Thus we see that during the first minutes of the blow, more than one-half of the silicon and manganese are burned. The remainder of these and all of the carbon are removed in the subsequent five or six minutes. At the end of this short blowing period we have practically pure iron.
Two Converters in Operation and a Third Pouring
The metal is not yet in condition to pour well, however, largely because of the dissolved air and gases which it holds. Something akin to the “killing” of the steel which we observed in the crucible process must be accomplished or ingots from it will be spongy. And, having practically no carbon, it is not yet “steel.”
Bessemer, knowing that the finished steel should contain carbon, tried to stop the blow long enough before the drop of the flame to leave exactly the desired amount of this element. He found this difficult to do and therefore uncertain. It was found to be far better to blow until the drop of the flame and then put back sufficient carbon to give the proper composition.
An English metallurgist named Mushet discovered that addition of manganese ridded the metal of injurious gases and oxides and what is known as “red-shortness.” After a period of difficulty without it Bessemer acknowledged the necessity of manganese and adopted its use. It had before this been used in crucible steel.
Upon turning down the converter at the drop of the flame, the blast is turned off and a smaller ladle is run in on a track above. This brings a molten mixture of irons, usually known as “spiegel” or “spiegeleisen” which contains just enough carbon, manganese and silicon to give to the whole of the molten metal in the converter the metalloids needed to make of it steel of the composition desired. This addition also accomplishes the “deoxidation” of the metal. By deoxidation we mean that the iron is relieved of the oxygen and gases which have remained as a result of the blast. This is necessary in order to give proper fluidity for pouring and the best physical properties to the finished steel.
After “recarburization,” as this addition of manganese-silicon-carbon metal is called, the steel and slag are quickly poured out into a ladle waiting below from which the steel is “teemed” (i.e., poured), through a “nozzle” or hole in the bottom into ingot molds arranged on trucks on the railroad track which runs through the building.