To promote solidity in steel castings it is the practice to add at the end of the refining and decarburizing operation certain amounts of ferro-silicon and ferro-manganese. The former may carry from 10 to 50 per cent of silicon and the latter usually about 80 per cent manganese. The combined action of the alloys is to thoroughly deoxidize the steel by uniting with the dissolved oxides, flux them so that they become separated from the liquid steel and enter the slag on top of the bath of metal and restore to the steel the desired percentage of carbon. Were the oxides not removed there would continue, so long as the metal remained liquid, a reaction between them and the small quantity of carbon always present in a bath of liquid steel, so that there would result a forma tion of gaseous carbon. Without a stoppage of that action the metal, if fractured when cool, would be found to be more or less honeycombed —an undesirable condition in any casting.
The value of a steel casting, physically con sidered, depends upon its elasticity, toughness and ability to resist heavy duty. The elasticity is controlled by the carbon content of the fin ished casting and that will vary with the re quirements of its service. Steel castings are divided into three grades: Soft, medium and hard. Standard specifications give the follow ing figures as the physical properties: Soft Medium Hard Tensile strength lbs. per sq. in 60.000 70,000 85,000 Yield point (elastic limit) lbs.
per sq. in 27,000 31,500 38,250 Elongation per cent. in two in. 22 18 15 Contraction of area per cent 30 25 20 Approximately the following will represent the ranges of chemical composition for the fore going: Soft Medium Hard Carbon, per cent... 0.17 -0.20 0.20 -0.30 0.30 -0.40 Silicon. per cent . . . 0.25 -0.35 0.25 -0.35 0.25 -0.35 Sulphur, per cent . . 0.015-0.05 0.015-0.05 0.015-0.05 Phosphorus, per cent 0.02 -0.04 0.02 -0.04 0.02 -0.04 Manganese, per cent 0.50 -0.75 0.50 -0.75 0.75 -1.00 In order to increase the ductility of castings, the ability to resist shock or impact without fracture, they are subjected to an annealing process, Which treatment consists of a careful heating to a temperature that releases original cooling stresses and refines the grain of the metal. In a steel casting cooled from casting temperature there is a tendency of the crystals to become enlarged and with that condition the metal may be of a doubtful value, since a coarse internal structure is more or less brittle, but by heating to a temperature to or about a full cherry red the coarse crystals become broken up and replaced by others of much finer mation. With that internal structure the ductil
ity of the metal is greatly improved by removing all previous brittleness. The refining tempera ture varies with the carbon content of the steel to be treated. The higher the carbon the nar rower the ranges of temperature for refining. The temperature need not be so closely observed when the percentage of carbon is low. It is a rule, however, that with the proper temperature being known, the heating below the refining range does not accomplish anything in chang ing the crystalline formations. Too high a heat ing will cause the crystals to increase in size. The length of time consumed in heating a steel casting to anneal, varies directly with the sec tion, but after the piece has reached the proper temperature the fire can be drawn.
The location of the refining range can best be determined by a pyrometer and a microscope to study the various degrees of crystalline struc ture which reflect the rates of cooling from various temperatures.
With the carbon content given to control the tensile strength and elasticity (usually one half of the tensile strength), the maximum de grees of toughness in any grade of steel cast ing can only be obtained by careful heat treat ment or annealing.
The other elements, silicon, sulphur, phos phorus and manganese, each have characteris tic properties influencing the ultimate quali ties of the product. An excess of any one tends to interfere with the best results in serv ice. Long practice, however, has established certain ranges of composition as being suited to both producer and consumer.
For the year 1914 the census of manufac tures reported 140 establishments in the United States engaged in making open-hearth steel, and employing in this industry a total of 864 hearths or furnaces. Of the whole, 99 establishments operating 706 furnaces made basic steel and the remainder, with 158 furnaces, made acid steel. The daily capacity of all the basic hearths combined (double run) was 85,471 tons; that of the acid hearths, 8,179 tons.
The rapid and steady growth in favor of basic steel is well illustrated by the compara tive figures furnished by the census, and tabu lated on preceding page, the figures expressing short tons. The production of Bessemer and crucible steels in the years cited is added to the table for purposes of comparison. See STEEL, THE BESSEMER PROCESS.