But while cast iron fittings gave satisfactory service up to temperatures, say, around 450° F., they faltered when forced to work under the new conditions which meant decidedly higher temperatures and pressures. And, too, the repeated heatings and coolings which were often necessary, disclosed a disadvantage previously unknown—a so-called “permanent growth” of the cast iron which was attended by loss of strength, and altogether it was soon found out that when superheated steam was to be used, higher types of materials were advisable than those which had been used under old conditions.
Steel Castings in the Annealing Oven
Superheated steam has rapidly come into general use. Some of the new locomotives and most of the modern power plants are now built for as much as 200° superheat, i.e., a total temperature of approximately 600° Fahrenheit.
Valves and fittings of cast steel not only are the articles “de luxe” for such service but they have come to be considered the necessary articles and their advantages have only fairly begun to be appreciated.
Though our most august scientific societies are proposing and debating upon systems of classification which shall include and satisfactorily define all of our ferrous metals, a satisfactory one has not yet been evolved, and, considering the intricacy of our ferrous metallurgy and the discoveries which are being made almost daily, the outlook for a strictly logical classification is not yet flattering.
With “Cast Steel” our metallurgical nomenclature is again faulty. Before what we now call the “steel casting” was known, crucible steel was poured into ingots, “forged” into tools just as it now is and often went under the name “Cast Steel” to distinguish it from the contemporaneous material, wrought iron. So to-day we buy many tools and implements which bear the name cast steel, which we know to have been forged in bringing them into their final shape.
But it is not these which we mean by the term, cast steel, but rather those steel products which get their final form by being “cast” from a fluid condition into a mold. These are what are rapidly coming to be understood when the term “cast steel” is used.
Satisfactory metal for steel castings may be made in any of three or four types of furnaces, but, as was suggested before, the making of molds for castings is a fine art, as is the preparation of the metal which is to go into them. Further, the making of that special class of castings which are to withstand water, steam or air pressure is a very different thing from the making of steel castings for other purposes, and this is too often forgotten.
For the former are necessary particularly close-grained castings, free from flaws or spongy spots. Under the great pressures applied such defects would certainly allow leakage.