The changes in the structural composition of steel, due to changes in the percentage of carbon, may be represented graphically as shown in Fig. 11.
This diagram clearly illustrates the fact that if no carbon be present the whole of the metal is made up of ferrite, and that by introducing increasing amounts of carbon, pearlite is formed in increasing quantity, while ferrite decreases correspondingly. With 0.85 per cent of carbon the whole mass consists of pearlite. Further addition of carbon results in the introduction of cementite in the structure, which then in creases in amount with the carbon, while the proportion of pearlite decreases.
In our description of the structural compo sition of steel we have purposely ignored the effect of impurities, because while their influ ence is not to be overlooked, quantitive infor mation on this point is lacking The Microstructure of Cast Iron.— Cast iron may be sharply divided into two classes : (1) White cast iron and (2) Gray Cast Iron, that is, into cast iron free from graphitic car bon and cast iron containing graphitic carbon. The microstructure of these two varieties of cast iron will be considered separately.
The Microstructure of White Cast Iron.— Like steel, white cast iron is free from graphitic carbon ; it contains the whole of its carbon in the combined condition, that is, as the carbide Fc3C or cementite. NN'e should, therefore, ex pect to find the microstructure of white cast iron very similar to that of high carbon steel, that is, made up of pearlite and a large excess of free cementite. In Fig. 12 is shown under a magnification of 56 diameters the structure of a sample of a white cast iron, and it will be seen that, as just anticipated, it is composed of pearlite and cementite, roughly in equal pro portions. This iron contained about 3.50 per cent of combined carbon, which theoretically would call for about 45 per cent of free cemen tite. A higher power would as usual be re quired to resolve the structure of the pearlite areas.
The close analogy which exists between the structure of steel and that of white cast iron is, therefore, evident. Indeed it is not possible
to distinguish high carbon steel from white cast iron by the examination of the structure of these metals; nor is this possible by any other test. To distinguish between high carbon steel and white cast iron, we must necessarily adopt an arbitrary rule, consisting, for instance, in calling steel, all carburized iron containing less than 2 per cent carbon and cast iron, those prod ucts which are more highly carburized. Start ing with carbonless iron and gradually increas ing the carbon content, we produce, in succes sion, low carbon steel, medium high carbon steel, high carbon steel, and finally white cast iron, without any suggestion as to when the metal ceases to be steel and becomes cast iron. The changes of structure and of properties caused by this gradual increase of carbon are contin uous and do not indicate any abrupt transforma tion. As the carbon increases the strength of the metal increases until the carbon content reaches about 1 per cent and then it decreases the hardness increases and the ductility and weldability decrease continuously; the malle ability decreases and finally disappears, but these changes occur gradually as the carbon increases and not abruptly upon reaching the composition of white cast iron.
To sum up, white cast iron may be consid ered as forming the most 'highly carburized member of the steel series.
The Microstructure of Gray Cast Iron— Gray Cast Iron free from Combined Carbon. —Perfectly gray cast iron does not contain any combined carbon. It should, therefore, be made up of a mass of carbonless iron (ferrite) and of a certain amount of graphitic carbon. The graphite occurs as numerous plates irregular in shape and size, disseminated through the iron, and breaking up its continuity. (See Fig. 14). It is because of this breaking up of the continuity of the metallic mass that the original ductility and malleability of the iron is so completely, destroyed by the presence of a sufficient amount ofgraphite.