CAST IRON, Malleable. Malleable cast iron is a grade of metal which has a special composition, such that when annealed for a continued period of time it becomes malleable, can be bent and twisted. The castings, when taken from the sand of the foundry, are very hard and brittle. The fracture is dead white (the ordinary iron casting appearing gray to black when freshly broken).
The tensile strength of a good malleable casting should run between 42,000 and 48,000 pounds per square inch, though for ordinary purposes 35,000 pounds is quite good enough. Castings have been made running up to 63,000 pounds per square inch, but these would not be soft enough for general use, being better adapted for conveyor chains and castings which must not stretch. The elongation of a malleable casting runs between 2.5 to 7 per cent measured in two inches. The transverse strength measured by a 1 inch square bar, placed on supports 12 inches apart, and load applied at the should be from 3,000 to 5,000 pounds for high quality material, and at least 2,500 for the ordinary product. The resilience of a malleable casting may be taken as eight times as high as a gray-iron one. Hence the great advantage. of using malleable castings to resist shock. In fact where the shocks are light and often repeated, this cast ing will stand up better than a steel one.
The malleablization of cast iron has been known since the early part of the 18th century. The first record we have is by Reaumur in 1722. He states that a hard casting, by being embed ded in ore and kept at a high temperature for a number of days, changed its structure and became soft and malleable. The process as then practised is still in vogue, and all attempts to hasten or otherwise modify it have not given continuously good results. The fundamental principle upon which the whole biped carbon in a white casting of a suitable process rests is the conversion of the corn composition to an amorphous form of carbon, which remains in the casting as a mechanical admixture. It is not crystalline like graphite, but in other respects behaves like it, and• is determined chemically in the same manner.
To understand this, let us consider the two great divisions of cast iron— the Gray and the White. In the former we haye nearly all the
carbon present in a mechanical admixture, as graphite flakes situate between the crystals of the iron proper. But little carbon is present in the combined state, and if less than 020 per cent, the casting is dead soft. If the combined carbon is as high as 0.80 per cent, the rest of about 3 per cent being in the form of graph ite, the casting will be a hard one. Now in the case of the white irons, the carbon is nearly all combined, and almost no graphite in mechanical admixture is present. Hence an exceedingly hard material is the result. It is the object of the malleablizing process to convert this chem ically combined carbon in a white iron, or in a hard gray one for that matter, to an amorphous form, to which the name temper carbon') has been given (from aTemperguss,) German for malleable casting). Any graphite present in the original casting is not changed, but with long continued heat gives an opportunity for the entrance of oxygen, with ruinous results to the casting. Hence the short annealing of gray castings to benefit them, while the white cast ing can be annealed for six days, and then re annealed again, without seriously hurting its strength.
There are two tendencies in the malleable casting industry, which result in different grades of metal. In Europe, from whence the process came, and where the irons used are not as pure, the annealing process is carried out longer than here, and hence much of the carbon present is removed from the casting by oxidizing it out. The result is a very ductile casting, but with a gray to white fracture (the fracture of a steel however, not of a hard white iron). The cast ings, moreover, are nearly all very light. In this country, with better irons, we have short ened the anneal to get just the conversion of per foot, but in annealing one-half of this is taken up by expansion, and hence the ordinary gray iron shrinkage is allowed for in making the patterns. For special work, however, it will pay to watch the action of the metal in the sand mold, and due allowances should be made in the pattern for abnormal contractions on the part of the casting, so that the dimensions of the annealed casting may come out all right.