Flanges and Fittings of Cast Steel
Whatever the method of production of steel for castings the metal is poured into molds to receive its final shape. Because of the intensely high heat of the steel only sands of great refractoriness (resistance to heat) can be used as material for the mold. White silica sand is such a material and is generally used, mixed with enough clay and molasses-water to give it “bond.” While molds for some steel castings are made in “green” (i.e., undried or unbaked) sand, baked molds are preferred for fine finish and surest results. After the making of the molds in the usual way they are sprayed with very finely powdered white sand or quartz mixed with a little molasses-water. They are then thoroughly dried in an oven.
Cast steel shrinks during cooling even more than malleable iron and the pattern and mold must be made to allow for this. Upon the freezing of the surfaces of the casting with consequent attainment of rigidity, the interiors, which freeze last, may have cavities unless means for avoiding them is provided. For this purpose heavier pieces, which later can be cut off, are cast upon such parts of the casting as tend to have “shrink holes.” These may be likened to receptacles filled with fluid metal, which being larger than the parts of the castings which they “feed,” hold excess metal in fluid condition until the casting itself has become solid throughout. Such are usually called “risers” or, in Europe, “lost heads,” and the molten metal in them flows down into the interior of the casting and fills the shrink holes which are forming. Not only must the risers be large enough that the metal in them is the last to solidify but they must be built high enough above the casting that sufficient pressure is exerted on the steel entering the shrinking parts to make its entry sure.
Grain of Steel Castings as They Come from the Mold
Grain of Steel Castings after
Annealing
(Magnification 60 diameters)
Baked molds, of course, are comparatively rigid. As the risers which stand on top of the flanges and other high parts of castings aid in resisting the natural shortening of long castings during and after “setting” of the metal, there is great liability that the still red-hot casting will crack somewhere along its length. It is therefore necessary to loosen with bars the sand of the mold as soon as the metal of the casting has set, particularly between the risers, and to break out the sand of the core inside, around which the shrinking metal might crack were the sand left in its hard packed condition.