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290                    METALLURGY OF CAST IRON.
which are generally used for large castings which permit of refining, testing, and changing the character of the mixture somewhat before the metal is tapped into ladles. The Siemens-Martin acid open-hearth furnace is now being very successfully employed for heavy castings. These furnaces are much hotter than air furnaces. The temperature of metal in th.em rises, possibly, to 3,500 to 4,000 degrees F. This permits the practice of using much steel scrap in with the low silicon iron to lower the total carbon slightly, which is a desirable point in making malleables as it gives a metal, after annealing, softer and tougher on account of the lower total carbon than is practicable with air furnace or cupola irons. Small quantities of iron ore are also sometimes added to the charge during the process of melting to assist in reducing the carbon. One disadvantage of furnaces over cupolas lies in the loss of iron, as the former often causes a loss of 12 per 'cent, of the iron charged by reason of scintillation and oxidation of the metal's surface when exposed to the flame.
The process of annealing is one that varies greatly with different firms. One firm may anneal similar thicknesses of castings in half the time another will take. The changes effected by annealing are chiefly in lowering the total carbon in the skin and turning the combined that remains into temper carbon, the silicon, sulphur, manganese, and phosphorus remaining practically the same. The time occupied in annealing ranges from one to seven days, with castings packed in boxes, etc. This wide difference is due to different customs and the character of castings to be treated. The ovens used are of simple construction in small ladles to the moulds, than that coming from furnacesthat would stand the tests of the United States Government for gun carriage work rightfully belongs to Messrs. Robert Poole & Son Co. of Baltimore, Md., and Muirkirk pig iron made by me. This was in 1893. The War Department at first refused to accept cupola iron as gun iron, but when it was fully demonstrateduois Iron Co., Illinois Steel Co., Jefferson Iron Co., Kittan-ning Iron & Steel Co., C. A. Kelly Plow Co., Lebanon Furnace, Longdale Iron Co., Lackawanna Iron & Steel Co., Logan Iron........................   16,720     "                                     t-