the primary austenite now split up into kernels of pearlite surrounded by envelopes of pro-eutectoid cementite, (2) the eutectic of cementite plus austenite, the latter of which has in like manner split up into a mixture of pearlite plus cementite. Such a mass is shown in fig. 4. Here the black bat-like patches are the masses of pearlite plus pro-eutectoid cementite resulting from the splitting up of the primary austenite. The magnification is too small to show the zebra striping of the pearlite. In the black-and-white ground mass the white is the eutectic cementite, and the black the eutectic austenite, now split up into pearlite and pro-eutectoid cementite, which cannot here be distinguished from each other.
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| Fig. 4.—The constitution of hypo-eutectic white or cementitiferous cast iron (washed metal), W. Campbell. The black bat-like areas are the primary austenite, the zebra-marked ground mass the eutectic, composed of white stripes of cementite and black stripes of austenite. Both the primary and eutectic austenite have changed in cooling into a mixture of pearlite and pro-eutectoid cementite, too fine to be distinguished here. |
25. As we pass to cases with higher and higher carbon-content, the primary austenite which freezes in cooling across region 2 forms a smaller and smaller proportion of the whole, and the austenite-cementite eutectic which forms at the eutectic freezing-point, 1130° (aB), increases in amount until, when the carbon-content reaches the eutectic ratio, 4.30%, there is but a single freezing-point, and the whole mass when solid is made up of this eutectic. If there is more than 4.30% of carbon, then in cooling through region 3 the excess of carbon over this ratio freezes out as “primary” cementite. But in any event the changes which have just been described for cast iron of 2.50% of carbon occur in crossing region 7, and at Ar1 (PSP′).
Just as variations in the carbon-content shift the temperature of the freezing-range and of the various critical points, so do variations in the content of other elements, notably silicon, phosphorus, manganese, chromium, nickel and tungsten. Nickel and manganese lower these critical points, so that with 25% of nickel Ar3 lies below the common temperature 20° C. With 13% of manganese Ar3 is very low, and the austenite decomposes so slowly that it is preserved practically intact by sudden cooling. These steels then normally consist of γ-iron, modified by the large amount of nickel or manganese with which it is alloyed. They are non-magnetic or very feebly magnetic. But the critical points of such nickel steel though thus depressed, are not destroyed; and if it is cooled in liquid air below its Ar2, it passes to the α state and becomes magnetic.
26. Double Nature of the Carbon-Iron Diagram.—The part played by graphite in the constitution of the iron-carbon compounds, hitherto ignored for simplicity, is shown in fig. 5. Looking at the matter in a broad way, in all these carbon-iron alloys, both steel and cast irons, part of the carbon may be dissolved in the iron, usually as austenite, e.g. in regions 2, 4, 5 and 7 of Fig. 1; the rest, i.e. the carbon which is not dissolved, or the “undissolved carbon,” forms either the definite carbide, cementite, Fe3C, or else exists in the free state as graphite. Now, just as fig. 1 shows the constitution of these iron-carbon alloys for all temperatures and all percentages of carbon when the undissolved carbon exists as cementite, so there should be a diagram showing this constitution when all the undissolved carbon exists as graphite. In short, there are two distinct carbon-iron diagrams, the iron-cementite one shown in fig. 1 and studied at length in §§ 22 to 25, and the iron-graphite one shown in fig. 5 in unbroken lines, with the iron-cementite diagram reproduced in broken lines for comparison. What here follows represents our present rather ill-established theory. These two diagrams naturally have much the same general shape, but though the boundaries of the several regions in the iron-cementite diagram are known pretty accurately, and though the relative positions of the boundaries of the two diagrams are probably about as here shown, the exact topography of the iron-graphite diagram is not yet known. In it the normal constituents are, for region II., molten metal + primary austenite; for region III., molten metal + primary graphite; for region IV., primary austenite; for region VII., eutectic austenite, eutectic graphite, and a quantity of pro-eutectoid graphite which increases as we pass from the upper to the lower part of the region, together with primary austenite at the left of the eutectic point B′ and primary graphite at the right of that point. Thus when iron containing 2.50% of carbon (v. fig. 1) solidifies, its carbon may form cementite following the cementite-austenite diagram so that white, i.e. cementitiferous, cast iron results; or graphite, following the graphite-austenite diagram, so that ultra-grey, i.e. typical graphitic cast iron results; or, as usually happens, certain molecules may follow one diagram while the rest follow the other diagram, so that cast iron which has both cementite and graphite results, as in most commercial grey cast iron, and typically in “mottled cast iron,” in which there are distinct patches of grey and others of white cast iron.
Though carbon passes far more readily under most conditions into the state of cementite than into that of graphite, yet of the two graphite is the more stable and cementite the less stable, or the “metastable” form. Thus cementite is always tending to change over into graphite by the reaction Fe3C = 3Fe + Gr, though this tendency is often held in check by different causes; but graphite never changes back directly into cementite, at least according to our present theory. The fact that graphite may dissolve in the iron as austenite, and that when this latter again breaks up it is more likely to yield cementite than graphite, is only an apparent and not a real exception to this law of the greater stability of graphite than of cementite.
Slow cooling, slow solidification, the presence of an abundance of carbon, and the presence of silicon, all favour the formation of graphite; rapid cooling, the presence of sulphur, and in most cases that of manganese, favour the formation of cementite. For instance, though in cast iron, which is rich in carbon, that carbon passes comparatively easily into the state of graphite, yet in steel, which contains much less carbon, but little graphite forms under most conditions. Indeed, in the common structural steels which contain only very little carbon, hardly any of that carbon exists as graphite.
27. Thermal Treatment.—The hardening, tempering and annealing of steel, the chilling and annealing of cast iron, and the annealing of malleable cast iron are explained readily by the facts just set forth.
28. The hardening of steel consists in first transforming it into austenite by heating it up into region 4 of fig. 1, and then quenching it, usually in cold water, so as to cool it very suddenly, and thus to deny the time which the complete transformation of the austenite into ferrite and cementite requires, and thereby to catch much of the iron in transit in the hard brittle β state. In the cold this transformation cannot take place, because of molecular rigidity or some other impediment. The suddenly cooled metal is hard and brittle, because the cold β-iron which it contains is hard and brittle.
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| Fig. 5.—Graphite-austenite or stable carbon-iron, diagram. |
The degree of hardening which the steel undergoes increases with its carbon-content, chiefly because, during sudden cooling, the presence of carbon acts like a brake to impede the transformations, and thus to increase the quantity of β-iron caught in transit, but probably also in part because the hardness of this β-iron increases with its carbon-content. Thus, though sudden cooling has very little effect on steel of 0.10% of carbon, it changes that of 1.50% from a somewhat ductile body to one harder and more brittle than glass.
29. The Tempering and Annealing of Steel.—But this sudden cooling goes too far, preserving so much β-iron as to make the steel too brittle for most purposes. This brittleness has therefore in general to be mitigated or “tempered,” unfortunately at the cost of losing part of the hardness proper, by reheating the hardened steel slightly,
