Gray Cast Iron Containing Combined Car Gray cast iron is seldom free from com bined carbon. It generally contains from 0.10 to 1 per cent and even more of carbon in the combined condition, and it is well known that this percentage of combined carbon depends chiefly upon the amount of silicon present and upon the rate of cooling of the casting. Vari ous proportions of combined carbon, and, there fore, of graphitic carbon impart different ap pearances to the fracture of cast iron, which was, and still is to a considerable extent, classi fied accordingly into the various grades °No. 1, 2 and 3 omottlecto etc.
The presence of some combined carbon in the iron must necessarily imply the presence of some pearlite in its structure. The metallic matrix which in perfectly gray iron consists of ferrite now possesses the structural character istics (and therefore the properties also) of a low carbon steel. Gray cast iron containing a small amount of combined carbon may be con sidered as a low carbon steel whose continuity is destroyed by the presence of numerous plates of graphite. Such an iron would correspond to a No. 1 Foundry.
The presence of a larger proportion of com bined carbon must necessarily result in a cor respondingly greater amount of pearlite in the structure.
By the percentage of combined carbon in cast iron. we convert its metallic matrix into steel of a correspondingly higher carbon content. We must necessarily reach a point, therefore, when the matrix will become a mass of pearlite, or, in other words, when it will assume the character of a saturated or eutectoid steel. This will occur when the metal contains about 0.70 per cent of combined car Classified by its fracture, a cast iron of this character would correspond to a No. 3 or 4 Foundry.
A further increase of combined carbon would result in the presence of some free cementite in the structure. Such cast iron is known, from the appearance of iu fracture, as bon. It will be remembered that in the case of steel some 0.85 per cent of carbon was required to convert it into a mass of pearlite. In the case of ,'act iron. however, the amount of iron to be saturated is less (because of the space occupied by the graphite) and requires a corre spondingly smaller amount of carbon, namely, some 0.70 per cent.
!mottled') iron. The structure of gray cast iron is illustrated in Fig. 15; it consists of a matrix of pearlite, numerous plates of graphite and of a white constituent which in iron con taining considerably less than some 0.70 per
cent combined carbon would correspond to ferrite, while in iron containing a considerably greater percentage of combined carbon it would indicate the presence of cementide.
Relation Between the Structure of Cast Iron and the Percentage of Combined Car relation between the structural com position of cast iron and the proportion of com bined carbon may be dearly illustrated by the diagram of Fig. 16, in which the percentage of tatalzarbon (graphitic carbon plus combined car bon) has been assumed to be 4 per cent and to remain constant. The plan followed is the same as that used to illustrate the changes of struc ture in steel and will therefore be readily under stood. By comparing this diagram with that of Fig. 11, the close analogy between the constitu tion of steel and that of cast iron is strikingly brought out. That the properties of steel and cast iron, in spite of this similarity, differ so much is due to the presence of graphitic car bon which by breaking up the continuity of the steel mass greatly decreases its strength and ductility, and deprives it of its malleability.
The structure of iron and steel briefly described in the foregoing paragraphs is that assumed by these metals oa zloty and. Undis turbed cooling from a sufficiently high tem perature. It may be called the normal struc ture, time and having been given for the formation of the final and, generally, stable constituents. The treatments, thermal and mechanical, to which iron and steel are so frequently subjected, however, in the manu facture of finished implements may alter these structures very profoundly, by preventing, in part or in all, the formation of the normal constituents and causing the formation and re tention of new constituents, thereby imparting to the metals widely different properties. In deed it is this sensitiveness of steel to thermal treatments that makes possible the manufacture of implements varying so widely in properties —from extreme hardness to great softness, from great strength combined with little ductility to moderate strength combined with great tough ness and ductility, etc.
Metallograplucal studies include also the construction and interpretation of thermal curves and equilibrium diagrams, the application of the phase rule to metallic alloys, as well as other closely allied scientific determinations.