While there is little difference in the physi cal qualities of acid and basic steels, engineers are accustomed to specify acid steel for most structural work.
In a 30-ton furnace it is possible to produce 18 heats per week of six working days. In a 50-ton furnace 12 heats per week is an aver age, either on acid or basic practice.
Steel For marine, locomotive. freight and passenger car, electrical and gen eral machinery construction it is important to have certain parts of great strength and elas ticity. For such purposes steel-castings are pre ferred to gray-iron castings. In the United States the industry is increasing rapidly. The sources of supply are mainly divided between acid and basic open-hearth steel with moderate productions by small Bessemer, crucible and electric processes.
During the year 1914 the total production was as follows: Acid open hearth castings 228.742 tom Basic open hearth castings 301.206 tom Bessemer (Tropenas) castings, etc 43,437 tom Crucible castings, etc 8.920 tom Electric castings 6,386 tone Open-hearth castings generally enter into heavy parts weighing several hundredweights to tons or more each, but do not enter into parts weighing a few pounds. For lighter parts the Bessemer and crucible processes fill a special field.
So far as equality and reliability of output is concerned in open-hearth castings there is no choice between acid and basic steel. In the early history of the processes (open hearth) the acid held sway, but with the development of the basic process it has largely met the grow ing demand for steel castings in spite of the fact that at first basic steel was more uncertain in its qualities.
For the production of open-hearth castings the details of construction and operation are essentially the same as for ingot practice. The main differences are that direct molten metal is not used, the charges being made up of cold, raw material (pig-iron and scrap). The sizes of furnaces vary from 10 to 25 tons per heat.
In the manufacture of steel castings prob lems are encountered which are unknown in ingot practice. It is comparatively simple to pour liquid steel into such receptacles as ingot molds but when pouring it into sand molds de signed to cast shapes more or less intricate with various thicknesses of metal, greater or lesser length and irregularities of section than is found in the plain columnar lines of ingots the complexities become quite numerous. It is
necessary that the steel be more fluid and hence hotter. This condition is secured by starting with a siliceous iron. The common difficulties encountered are lack of solidity, liability of the castings to crack or separate when cooling, particularly in complicated designs where there may be different rates of cooling in sections of unequal thickness. These difficulties are not insurmountable, but their avoidance calls for constant care and watchfulness and a high de gree in skill in keeping conditions regular and uniform in the furnace melting practice and also in the molding and coring of designs.
For molding purposes a high-grade silica sand is preferred, one that is highly refractory and that will not fuse at the temperature of liquid steel— about 1600° C. An impure sand will stick to steel castings and make rough unsightly surfaces. The sand must he of such a formation that there will he voids between the grains to permit a free escape of the gases formed within the mold when the hot steel is poured into it. Silica sand is devoid of any bond, and, to allow it to retain the form of the pattern after the sand is rammed around it and thepattern withdrawn, a small amount of plastic fire clay is added to the sand with a little water, followed by thorough mixing to furnish the necessary bond or stability to the molding mass. Sometimes a mold is baked, or it may be used without any drying. The first is known as "dry sand° and the other a "green sand" practice.
When the mold is filled with the liquid steel the molding mass under the high temperature disintegrates or loosens and should allow a free movement of the casting which contracts when cooling. Should there be a resistance to such a movement in cooling the casting will be liable to crack at points when the resist ance and stresses are the greatest.