The remelting for the well-known cast iron is usually of such selected brands of pig iron and cast scrap as will produce cast iron best adapted to the purposes intended. The resulting alloy still contains 3 per cent or 3½ per cent of carbon.
No. 132 is interesting. It is a photomicrograph of a piece of cast iron which is approximately 200 years old. From the standpoint of the metallurgist it is the same as other photomicrographs of cast iron, the difference in appearance resulting probably from casting conditions, etc.
Malleable cast iron is made much as is gray cast iron except that it is brought to such a composition that sections of castings made from the melt show a white fracture when broken. Castings of gray iron of course show a gray fracture. The former are extremely hard and are as brittle as glass until they have gone through a careful long anneal or heat softening process. By maintaining them at cherry-red heat away from air for sixty hours or more and cooling slowly, they become “malleable”; i.e., not brittle at all but capable of considerable distortion without cracking.
No. 35. Malleable Cast Iron Annealed
From the viewpoint of malleability, malleable iron may be considered to occupy a position between gray cast iron and steel.
Photomicrographs Nos. 109 and 35 are those of malleable iron before and after the annealing treatment. The former shows the typical structure of white cast iron, while the latter plainly shows the minute lumps of pure carbon and the surrounding grains of pure iron metal, the two having become divorced through the annealing process. Note that the large amount of carbon in this, the “temper carbon” form, does not make the alloy brittle through cutting of the grains as does the crystalline graphite form of carbon.
The above gives very briefly the most essential points in the classification of the irons and steels from the structural standpoint. True it has not entered into the vast complications of the higher carbon or tool steels which are those which will take a “temper”; i.e., the tool steel’s quality of hardening by quenching in water from a cherry red heat. The simpler points of these will be taken up later. But enough has been given that we now understand something of the relative positions of the great main products, some reasons therefor, and their general structures.
No. 8. Bar Rolled from Scrap; Contains Both Wrought Iron and Steel