While heat alone effects the divorcing of the carbon and iron, which is the essential part of the annealing process, in the greater number of cases aid is given by what may be termed chemical means. Reaumur, who about 1722 discovered the annealing process, used iron oxide for the purpose. The white iron was packed in iron ore or mill scale. At the high temperatures employed the oxygen of the ore in some way not yet definitely known, gradually removed the greater amount of the carbon from the casting. It has always been a scientific conundrum how a solid, iron oxide, surrounding another solid, a piece of white iron, could remove from the latter its carbon when neither of them melts nor mingles with the other. Whether some of the oxygen from the ore penetrates the iron and burns out its carbon or whether the carbon of the casting itself migrates is not yet definitely settled. Certain it is that the carbon is gradually removed from the casting, from the surface first and with increasing length of time from greater depths.
No. 390b. Photomicrograph Showing Decarbonized Outer Layer
The photomicrograph also shows that this casting was not fully annealed.
In European practice malleable iron castings are still malleableized in this way, i. e., by burning out the carbon. The castings are made as thin as possible and the annealing in “packing” (iron ore or mill scale) is continued for from one to two weeks. At the expiration of this time the castings have a white, steely fracture which is entirely unlike the fractures of malleable iron castings which are made in this country. Photomicrographs of such malleable iron show few or none of the black spots which No. 35 exhibits, and analyses of castings annealed in this way give very low results for carbon.
Malleable Cast Iron in Which Practically All of the Carbon Has Been Removed by Reaumur Process Annealing
While in this country the Reaumur process of annealing is not followed, a “packing” of ore or scale is generally used. Some use an inert packing such as sand, and as first mentioned, some use no packing at all. Really, one of the main purposes of the “packing” as now used is the prevention of warping of the castings in the pots while annealing. The annealing temperature is not so high as in Europe nor is the annealing continued so long, but when packing is used for the shorter time only, some surface carbon is removed and the carbon throughout the center portions of the castings is precipitated in the coke-like form which is known as “temper carbon” to distinguish it from graphite which is shown in photomicrograph No. 35. To the eye, then, fractures of such castings show black centers and white rims. They are known as “black heart” castings and these form the bulk of the malleable cast iron made in this country.
Fracture of Black Heart Iron
Note the white rims and black coke-like interiors. The majority of American malleable iron is of this “black heart” variety.
We may say, then, that there are in general three varieties of malleable cast iron: the “all black” which is annealed without “packing,” the “black heart,” annealed in “packing” and the most common kind in this country, and in Europe, but very rarely here, the “whiteheart” from which practically all of the carbon has been burned during the “anneal.”