At about the time Bessemer was developing his process, other men were perfecting an improved reverberatory furnace. Steel-making temperatures had been obtained in small coke-fired furnaces, but were found impossible in large furnaces until Sir William Siemens tried the pre-heating of gas and air before allowing them to combine over the hearth of the furnace. Pre-heating the air, by the alternated passing of the waste gases and incoming air through a fire-brick checkerwork, made possible the attaining of a temperature entirely new for refractory furnaces. This is called regenerative heating; the checkerworks are called regenerators. The new furnace of course was developed largely with the idea of making steel in it, and it soon came into successful commercial operation.
The open-hearth furnace, with its accessories, has been brought to a high state of adaptability and efficiency. It can be heated with any sort of combustible gas, oil, or tar; the manual labor has been reduced to the minimum by all sorts of mechanical appliances; and the tonnage capacity has been continually increased - many furnaces now are able to put through 300 tons in 24 hours. It is by far the most important method for making steel which the world has today. Its drawbacks are in its inability to change its temperature range quickly, and a rather necessary slowness in burning out carbon, as the gas produced by a too rapid burning from solid or liquid ingredients in such a large bath would make the metal boil out of any reasonably sized furnace. Although it is such a well-established process at the present time, its combination with other processes, as mentioned in the section on Bessemer Steel, has more than pretensions for such possible future usefulness.
Fig. 24 is a section through an open-hearth furnace of a European plant. It is a tilting furnace moved by the hydraulic piston as indicated. The furnace is charged from the working platform, and the metal is poured from the spout on the opposite side. The checkerwork for heating the gas and air is indicated in the center of the section and below the furnace level. The dust chamber is at the end of the flues directly down from the end of the furnace; a passage leads to the checker chamber. From the checkers ducts lead to the right to the gas and air inlets and to the stack; the valves are located here. In this figure all dimensions are in millimeters.
Fig. 25. Perspective Diagram of Open-Hearth Furnace Courtesy of American Institute of Mining Engineers.
A perspective giving an excellent idea of the relative positions of the furnace, ducts, checker room, and dampers is shown in Fig. 25.
This furnace is oil-fired, so only the air has to be pre-heated. The diagram shows how all the dampers are reversed when the waste gases are switched from one checker room to the other. The student should follow the actions of all the valves which result from reversing the air cylinder and from adjusting the air-regulating screws. The draft through the stack is depended upon to pick up the gases from the hearth and to pull them through the regenerators.
The open-hearth furnace may be either stationary or tilting; its hearth refractory may be either siliceous or basic; the walls and roofs commonly will be of silica brick; the ports at each end of the hearth which admit the heated air and pass the burned gas suffer much from the high temperature and now usually are made sectional so that they can be replaced without interrupting operations except for a few minutes. Flues lead from the ports down to the checkerwork where the air and gas will be pre-heated. If either oil or tar is used, pressure steam will blow the liquid into the furnace through a nozzle, and only air will have to pass through the checker-work. Sufficient valves are provided in the flues for reversing the currents and for regulating the draft. It is not unusual now to find waste-heat boilers beyond the checkerwork so that the gases coming up the stack will have given up most of their available energy.
In the process using a basic bath, the pig iron used is preferably rather high in manganese so as to remove as much sulphur as possible during the process. Scrap material is a common ingredient of the charge; iron ore also is used; while varying amounts of lime will be added to combine with the phosphorus present as it is oxidized and removed as slag. After some hours, the phosphorus having been slagged away sufficiently and removed, and the carbon being reduced to about the right percentage, the metal is tapped into a large ladle, deoxidized with metallic aluminum, and the manganese brought to the right specification by the addition of ferromanganese, and then, after standing a few minutes, the steel is teemed from the ladle into the ingot molds.
The process can be hastened by certain modifications, which gives rise to the special processes known as the Talbot process, the Monell process, the Campbell process, or to the use of a converter to finish the metal after the phosphorus has been eliminated.