steel in which is encased a skeleton of graphite plates, besides some very fine scattered particles of graphite.
Next let us imagine that, in a series of cast irons all containing 4% of carbon, the graphite of the initial skeleton changes gradually into cementite and thereby becomes part of the matrix, a change which of course has two aspects, first, a gradual thinning of the graphite skeleton and a decrease of its continuity, and second, a gradual introduction of cementite into the originally pure ferrite matrix. By the time that 0.4% of graphite has thus changed, and in changing has united with 0.4 × 14 = 5.6% of the iron of the original ferrite matrix, it will have changed this matrix from pure ferrite into a mixture of
| Cementite | 0.4 + 5.6 = 6.0 |
| Ferrite | 96.0 − 5.6 = 90.4 |
| —— | |
| 96.4 | |
| The residual graphite skeleton forms | 4 − 0.4 = 3.6 |
| —— | |
| 100.0 |
But this matrix is itself equivalent to a steel of about 040% of carbon (more accurately 0.40 × 100 ÷ 96.4 = 0.415%), a rail steel, because it is of just such a mixture of ferrite and cementite in the ratio of 90.4 : 6 or 94% and 6%, that such a rail steel consists. The mass as a whole, then, consists of 96.4 parts of metallic matrix, which itself is in effect a 0.415% carbon rail steel, weakened and embrittled by having its continuity broken up by this skeleton of graphite forming 3.6% of the whole mass by weight, or say 12% by volume.
As, in succeeding members of this same series of cast irons, more of the graphite of the initial skeleton changes into cementite and thereby becomes part of the metallic matrix, so the graphite skeleton becomes progressively thinner and more discontinuous, and the matrix richer in cementite and hence in carbon and hence equivalent first to higher and higher carbon steel, such as tool steel of 1% carbon, file steel of 1.50%, wire-die steel of 2% carbon and then to white cast iron, which consists essentially of much cementite with little ferrite. Eventually, when the whole of the graphite of the skeleton has changed into cementite, the mass as a whole becomes typical or ultra white cast iron, consisting of nothing but ferrite and cementite, distributed as follows (see fig. 2):—
| Eutectoid ferrite | 40.0 |
| Eutectoid cementite | 6.7 |
| —— | |
| Eutectoid Interstratified as pearlite | 46.7 |
| Cementite, primary, eutectoid and pro-eutectoid | 53.3 |
| —— | |
| 100.0 | |
| Total ferrite | 40.0 |
| Total cementite | 60.0 |
| —— | |
| 100.0 |
Fig. 28.—Physical Properties and assumed Microscopic Constitution of Cast Iron containing 4% of
carbon, as affected by the distribution of that carbon between the combined and graphitic states.
The constitution and properties of such a series of cast irons, all containing 4% of carbon but with that carbon shifting progressively from the state of graphite to that of cementite as we pass from specimen to specimen, may, with the foregoing picture of a skeleton-holding matrix clearly in our minds be traced by means of fig. 28. The change from graphite into cementite is supposed to take place as we pass from left to right. BC and OH give the proportion of ferrite and cementite respectively in the matrix, DEF, KS and TU reproduced from fig. 3 give the consequent properties of the matrix, and GAF, RS and VU give, partly from conjecture, the properties of the cast iron as a whole. Above the diagram are given the names of the different classes of cast iron to which different stages in the change from graphite to cementite correspond, and above these the names of kinds of steel or cast iron, to which at the corresponding stages the constitution of the matrix corresponds, while below the diagram are given the properties of the cast iron as a whole corresponding to these stages, and still lower the purposes for which these stages fit the cast iron, first because of its strength and shock-resisting power, and second because of its hardness.
115. Influence of the Constitution of Cast Iron on its Properties.—How should the hardness, strength and ductility, or rather shock-resisting power, of the cast iron be affected by this progressive change from graphite into cementite? First, the hardness (VU) should increase progressively as the soft ferrite and graphite are replaced by the glass-hard cementite. Second, though the brittleness should be lessened somewhat by the decrease in the extent to which the continuity of the strong matrix is broken up by the graphite skeleton, yet this effect is outweighed greatly by that of the rapid substitution in the matrix of the brittle cementite for the very ductile copper-like ferrite, so that the brittleness increases continuously (RS), from that of the very grey graphitic cast irons, which, like that of soapstone, is so slight that the metal can endure severe shock and even indentation without breaking, to that of the pure white cast iron which is about as brittle as porcelain. Here let us recognize that what gives this transfer of carbon from graphite skeleton to metallic matrix such very great influence on the properties of the metal is the fact that the transfer of each 1% of carbon means substituting in the matrix no less than 15% of the brittle, glass-hard cementite for the soft, very ductile ferrite. Third, the tensile strength of steel proper, of which the matrix consists, as we have already seen (fig. 3), increases with the carbon-content till this reaches about 1.25%, and then in turn decreases (fig. 28, DEF). Hence, as with the progressive transfer of the carbon from the graphitic to the cementite state in our imaginary series of cast irons, the combined carbon present in the matrix increases, so does the tensile strength of the mass as a whole for two reasons; first, because the strength of the matrix itself is increasing (DE), and second, because the discontinuity is decreasing with the decreasing proportion of graphite. With further transfer of the carbon from the graphitic to the combined state, the matrix itself grows weaker (EF); but this weakening is offset in a measure by the continuing decrease of discontinuity due to the decreasing proportion of graphite. The resultant of these two effects has not yet been well established; but it is probable that the strongest cast iron has a little more than 1% of carbon combined as cementite, so that its matrix is nearly equivalent to the strongest of the steels. As regards both tensile strength and ductility not only the quantity but the distribution of the graphite is of great importance. Thus it is extremely probable that the primary graphite, which forms large sheets, is much more weakening and embrittling than the eutectic and other forms, and therefore that, if either strength or ductility is sought, the metal should be free from primary graphite, i.e. that it should not be hyper-eutectic.
The presence of graphite has two further and very natural effects. First, if the skeleton which it forms is continuous, then its planes of junction with the metallic matrix offer a path of low resistance to the passage of liquids or gases, or in short they make the metal so porous as to unfit it for objects like the cylinders of hydraulic presses, which ought to be gas-tight and water-tight. For such purposes the graphite-content should be low. Second, the very genesis of so bulky a substance as the primary and eutectic graphite while the metal is solidifying (fig. 5) causes a sudden and permanent expansion, which forces the metal into even the finest crevices in its mould, a fact which is taken advantage of in making ornamental castings and others which need great sharpness of detail, by making them rich in graphite.
To sum this up, as graphite is replaced by carbon combined as cementite, the hardness, brittleness and density increase, and the expansion in solidification decreases, in both cases continuously, while the tensile strength increases till the combined carbon-content rises a little above 1%, and then in turn decreases. That strength is good and brittleness bad goes without saying; but here a word is needed about hardness. The expense of cutting castings accurately to shape, cutting on them screw threads and what not, called “machining” in trade parlance, is often a very large part of their total cost; and it increases rapidly with the hardness of the metal. On the other hand, the extreme hardness of nearly graphiteless cast iron is of great value for objects of which the chief duty is to resist abrasion, such as parts of crushing machinery. Hence objects which need much machining are made rich in graphite, so that they may be cut easily, and those of the latter class rich in cementite so that they may not wear out.
116. Means of controlling the Constitution of Cast Iron.—The distribution of the carbon between these two states, so as to give the cast iron the properties needed, is brought about chiefly by
