Note to the Readers: Millers Falls Co. used large amounts of malleable iron to make their products. This article provides basic information of how malleable iron was produced as well as how tools or tool parts were made in cast iron and then converted to malleable iron. WK

"Malleable iron" is the term employed to designate those castings, the brittleness of which has been partly or entirely removed by the operation of "annealing," which consists in burning off the whole or a part of the carbon combined with the metal from which the castings were made.

Cast iron, disregarding certain other with it, is essentially a compound of iron and carbon, in which the carbon is partly combined with the metal, and partly mixed with it; in the latter case, it is said to exist in the "graphitic state."

Combined carbon, on account of its atomic state of division, is more easily removed from the metal, either by the action of oxidizing agents, such as metallic oxides, and the oxidizing flame of a puddling furnace, &c, or by readily combining with hydrogen and forming hydrocarbides, which we perceive when we dissolve cast-iron in sulphuric or hydrochloric acid, for instance. On the other hand, graphitic carbon is very hard to burn, and requires the protracted action of oxidizing influences.

From the states in which carbon exists in cast-iron, this metal has been classified into three principal subdivisions:

Gray metal, in which the light color is as it were concealed by a multitude of graphitic laminae [a thin plate, layer, or flake];

White metal, where the carbon is in the combined state and unseen;

Mottled cast iron, in which most of the carbon is combined, whereas that in the graphitic state gives to the metal the spotted appearance of the trout.

Gray metal is also called Foundry pig, and is generally preferred by the founders of ordinary castings, because it retains its carbon and fusibility longer than the other kinds. White metal is also called Forge pig, because it is preferred for puddling, since it loses its carbon more readily than the gray metal. The intermediate quality of mottled pig goes generally to the forge.

From what we have said about the two states in which carbon exists in cast iron, and the greater facility of its removal in one than in the other, we may rightly infer that white cast iron is to be preferred for malleable castings.

Another reason for doing so, is the appearance of the castings. Indeed, let us suppose an article made of gray metal, rich in graphitic carbon; if, after a protracted heating in contact with oxidizing substances we have succeeded in burning off the graphite, the place it occupied in the metal will be empty, and the article will be porous, and will show it.

On the contrary, the article cast from white metal, where the combined carbon is not visible, will appear with the same sharpness of shape and smoothness of surface after, as before the annealing process.

Therefore, and provided the metal employed contains sufficient combined carbon to insure the fluidity necessary for sharp castings, white pig iron is to be preferred to gray metal for the manufacture of "malleable iron" castings, because the decarburization is more complete and rapid, the appearance more pleasing, and the quality of the resulting metal better.

Carbon is removed from the cast-iron, by submitting it, at a certain temperature, to the action of substances holding oxygen, and the resulting combination will be carbonic oxide very possibly mixed with a certain proportion of carbonic acid. Air will cause the carbon to burn, but its action is too energetic, and is not well under control.

The substances preferred for the purpose are the magnetic scales of oxide of iron, produced by blacksmiths and at rolling-mills, and iron ores or peroxides of iron, which fulfill the requirements of cheapness, with regularity and facility of working.

We must, however, remark that these oxides should be, as far as practicable, free from silica and earths which, at the temperature of the annealing furnace, will fuse and form a slag or cinder, preventing the oxidizing action, especially if the castings should become coated with it. For this reason smithy scales are preferred, although they contain less oxygen than the ores; but the latter are with difficulty found entirely free from the above fluxing impurities.

There is, up to a certain point, an analogy in the mode of operation between cementing steel and annealing cast-iron.

In either case, the metals are submitted to a protracted heat in air-tight vessels, filled with the reacting substances, and the transformation takes place from the surface to the centre. But here the similarity ceases; in one case the carbon of the charcoal used penetrates the iron bar to form steel; in the other, the oxygen of the surrounding oxide penetrates the cast-iron, combines with its carbon, and escapes in the gaseous form.

It is easily understood that the thinner the casting, the more rapid will be its transformation into malleable metal. Thicker castings, if the heat has not been sufficiently high or protracted, will exhibit in their fracture a kind of gamut of the graduation of the transformation.

The external parts, which have been thoroughly decarburized, are gray, easily filed and drilled, and have lost their brittleness; and proceeding towards the centre (which we suppose not to be decarburized), we see the qualities of color, softness, &c, gradually diminishing, until we find the previous white metal.

For some reason, not well understood, it would appear that a temperature too high or prolonged will harden surfaces already softened. Possibly, this may be due to a superficial skin of magnetic oxide, hard and brittle, or to a coating of fluxed impurities.

At all events, castings not too thick, of a good metal, and thoroughly decarburized, may be considered chemically as iron without fiber, and a fiber may be imparted to them by rolling or hammering. Indeed, we have seen such malleable castings bent double, while cold, without breaking, and without any previous condensation under the hammer.

Their ring, or sound, very nearly approximates to that of wrought iron.