In their proper sphere they are highly satisfactory, but they are “inelastic” in that they must be run continuously day and night and should not be allowed to cool until extensive repairs are imperative.
It was mentioned that in the open-hearth process the furnace is always hotter than the metal which it contains and that the heat which can be put into the steel is limited by the ability of the refractories of which the roof and side walls are made to withstand melting. In the Bessemer process the metal is hotter than the furnace because the heat is generated by combustion of certain of the metalloids contained in the metal itself. As metal for castings must be very hot and fluid the Bessemer process is very satisfactory for the making of steel for castings.
A 30–ton Basic Open-Hearth Furnace Tapping
The overflow from the ladle into the pit is slag.
It has, also, the advantage of “elasticity.” The supply of metal is practically continuous and one furnace can make from one to eighteen or even more heats on day turn only and be shut down for the night turn or longer and then started again without such loss as would result from the shutting down of an open-hearth furnace with regenerators.
For the making of metal for steel castings, very small-sized Bessemer converters are used which make from one to three tons of metal per blow. Some converters of as little as one-half ton capacity are being used. While some are of the “bottom-blown” type already described, the majority are what are called “surface-blown” or “side-blown.” In these, from four to eight round tuyères, about one and one-half inches in diameter each, pierce the brick or ganister lining just above the surface of the bath. They slope downward a little toward the bath so that when the converter is tipped to its upright or blowing position the air blast will strike the adjacent edge of the metal and blow across its surface. This three or four pounds per square inch of air blast keeps the metal in circulation, meanwhile burning out its silicon, manganese, and carbon, just as it does in the larger bottom-blown converters. Surface-blown give hotter metal than do bottom-blown converters and very fine steel castings are made from their metal. For these converters, which are practically all acid-lined (i.e., with silica or clay brick or ganister), metal low in phosphorus and sulphur is regularly drawn from a cupola specially run for the purpose.
Small Side-Blown Converter Making Steel for Castings
The remaining recognized type of furnace for steel for castings is the comparatively new electric furnace.
Commercial melting of metals by the electric current has been sought for half a century. In 1879 the first furnaces of promise were patented by Sir William Siemens, one of the Siemens brothers who became so well known through their great work with the open-hearth furnace, the gas producer and many other things metallurgical. While Siemens melted as much as twenty-two pounds of iron per hour in his furnace, the cost of the electric current at that time was so high as to be practically prohibitive for the manufacture of steel in competition with the open-hearth, Bessemer and crucible processes.