FIC. r3.--(Stoughton.) Meshes of pearlite in a network of cementite from hyper-eutectoid steel.
These are cementite, a definite iron carbide, Fe 3 C, harder than glass and nearly as brittle, but probably very strong under gradually and axially applied stress; and ferrite, pure or nearly pure metallic a-iron, soft, weak, with high electric conductivity, and in general like copper except in colour.
In view of the fact that the presence of 1% of carbon implies that 15% of the soft ductile ferrite is replaced by the glass-hard cementite, it is not surprising that even a little carbon influences the properties of the metal so profoundly.
But carbon affects the properties of iron not only by giving rise to varying proportions of cementite, but also both by itself shifting from one molecular state to another, and by enabling us to hold the iron itself in its unmagnetic allotropic forms, 0and 7-iron, as will be explained below.
Thus, sudden cooling from a red heat leaves the carbon not in definite combination as cementite, but actually dissolved in (3and 7-allotropic iron, in the conditions known as martensite and austenite, not granitic but glass-like bodies, of which the " hardened " and " tempered " steel of our cutting tools in large part consists.
The Cementite-Austenite or Metastable form.
On cooling into region 6 or 8 austenite should normally split up into ferrite and cementite, after passing through the successive stages of martensite, troostite and sorbite, Fe 0 C= Fe 3 C +Fe(i 3).
Eutectic here freezes 7 Austenite+Cementite Pro-eutectoid Cementite forms progressiuely 882930 1200 0 U a 5 6 ?
Pearli te T v 3 A here splits up '(and cementite 140 C C P Ferrite ?
Eutectic,a earIitei-Cementit .(/a primarg, Oxide, Cementite(t 400 300 20 even if rich in austenite, is strongly magnetic because of the very magnetic a-iron which inevitably forms even in the most rapid cooling from region 4.
Beta (13) iron, an unmagnetic, intensely hard and brittle allotropic form of iron, though normal and stable only in the little triangle GHM, is yet a state through which the metal seems always to pass when the austenite of region 4 changes into the ferrite and cementite of regions 6 and 8.
Ferrite and cementite, already described in § 10, are the final products of the transformation of austenite in slow-cooling.
Nearly pure a-iron) with austenite for the space Mhsp, cementite with austenite for region 7, and a-ferrite and cementite jointly for regions 6 and 8.
I 1), in the ratio of about 6 parts of ferrite to I of cementite, and hence containing about 0.90% of carbon.
The percentage of pearlite and of free ferrite or cementite in these products is shown in fig.
Measures the percentage of the excess of ferrite or cementite for hypoand hyper-eutectic steel and white cast iron respectively.
The Ratio of Ferrite to Cementite, of certain typical Steels.
3 shows how, as the carbon-content rises from O to 4.5%, the percentage of the glass-hard cementite, which is 15 times that of the carbon itself, rises, and that of the soft copperlike ferrite falls, with consequent continuous increase of hardness and loss of malleableness and ductility.
The tenacity or tensile strength increases till the carbon-content reaches about 1.25%, and the cementite about 19%, and then in turn falls, a result by no means surprising.
The presence of a small quantity of the hard cementite ought naturally to strengthen the mass, by opposing the tendency of the soft ferrite to flow under any stress applied to it; but more cementite by its brittleness naturally weakens the mass, causing it to crack open under the distortion which stress inevitably causes.
The fact that this decrease of strength begins shortly after the carboncontent rises above the eutectoid or pearlite ratio of o 90% is natural, because the brittleness of the cementite which, in hypereutectoid steels, forms a more or less continuous skeleton (Alloys, Pl., fig.
13) should be much more effective in starting cracks under distortion than that of the far more minute particles of cementite which lie embedded, indeed drowned, in the sixfold greater mass of ferrite with which they are associated in the pearlite itself.
The large massive plates of cementite which form the network or skeleton in hyper-eutectoid steels should, under distortion, naturally tend to cut, in the softer pearlite, chasms too serious to be healed by the inflowing of the plastic ferrite, though this ferrite flows around and Steel White Cast Iron 100 75 K 0 ?
Immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.
The distortion which rails undergo in manufacture and use is incomparably less than that to which rivets are subjected, and thus rail steel may safely be much richer in carbon and hence in cementite, and therefore much stronger and harder, so as to better endure the load and the abrasion of the passing wheels.
Thus the typical carbon-content may be taken as about o 05% for rivets and tubes, 0.20% for boiler plates, and 0.50 to 0.75% for rails, implying the presence of o 75% of cementite in the first two, 3% in the third and 7.5% to 11.25% in the last.
Carbon-Content of Hardened Steels.-Turning from these cases in which the steel is used in the slowly cooled state, so that it is a mixture of pearlite with ferrite or cementite, i.e.
The heat evolved by this process of solidification retards the fall of temperature; but after this the rate of cooling remains regular until T (750°) on the line Sa (Ar 3) is reached, when a second retardation occurs, due to the heat liberated by the passage within the pasty mass of part of the iron and carbon from a state of mere solution to that of definite combination in the ratio Fe 3 C, forming microscopic particles of cementite, while the remainder of the iron and carbon continue dissolved in each other as austenite.
This formation of cementite continues as the temperature falls,- till at about 690° C., (U, called Ar 2 _ 1) so much of the carbon (in this case about 0.10%) and of the iron have united in the form of cementite, that the composition of the remaining solid-solution or " mothermetal " of austenite has reached that of the eutectoid, hardenite; i.e.
The cementite which has thus far been forming may be called " pro-eutectoid " cementite, because it forms before the remaining austenite reacnes the eutectoid composition.
As the temperature now falls past 690°, this hardenite mother-metal in turn splits up, after the fashion of eutectics, into alternate layers of ferrite and cementite grouped together as pearlite, so that the mass as a whole now becomes a mixture of pearlite with cementite.
The passage of this large quar`it-y of carbon and iron, 0.90% of the former and 12.6 of the latter, from a state of mere solution as hardenite to one of definite chemical union as cementite, together with the passage of the iron itself from the y to the a state, evolves so much heat as actually to heat the mass up so that it brightens in a striking manner.
This change from austenite to ferrite and cementite, from the y through the # to the a state, is of course accompanied by the loss of the " hardening power," i.e.
13, the slowly cooled steel now consists of kernels of pearlite surrounded by envelopes of the cementite which was born of the austenite in cooling from T to U.
Again, as the temperature in turn falls past Ar l this hardenite mother-metal splits up into cementite and ferrite grouped together as pearlite, with the resulting recalescence, and the mass, as shown in Alloys, Pl., fig.
In short, from Ar 3 to Ar t the excess substance ferrite or cementite, in hypoand hyper-eutectoid steels respectively, progressively crystallizes out as a network or skeleton within the austenite mothermetal, which thus progressively approaches the composition of hardenite, reaching it at Ar t, and there splitting up into ferrite and cementite interstratified as pearlite.
At this point selection ceases; the remaining molten metal freezes as a whole, and in freezing splits up into a conglomerate eutectic of (1) austenite of about 2.2% of carbon, and therefore saturated with that element, and (2) cementite; and with this eutectic is mixed the " primary " austenite which froze out as the temperature sank from v to v'.
The reason for its birth, of course, is that the solubility of carbon in austenite progressively decreases as the temperature falls, from about 2.2% at 1130° (a), to 0.90% at 690° (Ar 1), as shown by the line aS, with the consequence that the austenite keeps rejecting in the form of this pro-eutectoid cementite all carbon in excess of its saturation-point for the existing temperature.
Here the mass consists of (1) primary austenite, (2) eutectic austenite and cementite interstratified and (3) pro-eutectoid cementite.
This formation of cementite through the rejection of carbon by both the primary and the eutectic austenite continues quite as in the case of 1.00% carbon steel, with impoverishment of the austenite to the hardenite or eutectoid ratio, and the splitting up of that hardenite into pearlite at Ari, so that the mass when cold finally consists of (1) 1 Note the distinction between the " eutectic " or alloy of lowest freezing-point, 1130°, B, with 4.30% of carbon, and the " eutectoid," hardenite and pearlite, or alloy of lowest transformation-point, 690° S, with 0.90% of carbon.
Here the black bat-like patches are the masses of pearlite plus proeutectoid cementite resulting from the splitting up of the primary austenite.
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.
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.
The carbon which is not dissolved, or the " undissolved carbon," forms either the definite carbide, cementite, Fe C, or else exists in the free state as graphite.
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.
5 in unbroken lines, with the iron-cementite diagram reproduced in broken lines for comparison.
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.
I) solidifies, its carbon may form cementite following the cementite-austenite diagram so that white, 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 Fe C = 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.
That to which the hardened steel is thus reheated, the more is the molecular rigidity relaxed, the farther on does the transformation go, and the softer does the steel become; so that, if the reheating reaches a dullred heat, the transformation from austenite into ferrite and cementite completes itself slowly, and when now cooled the steel is as soft and ductile as if it had never been hardened.
Hastening its cooling by casting it in a cool mould, favours the formation of cementite rather than of graphite in the freezing of the eutectic at aBc, and also, in case of hyper-eutectic iron, in the passage through region 3.
In the former case, the objects are heated only to the neighbourhood of Aci, say to 730° C., so that the 0-iron may slip into the a state, and the transformation of the austenite into pearlite and cementite may complete itself.
The joint effect of such chilling and such annealing is to make the metal much harder than if slowly cooled, because for each 1% of graphite which the chilling suppresses, 15% of the glass-hard cementite is substituted.