even if rich in austenite, is strongly magnetic because of the very magnetic α-iron which inevitably forms even in the most rapid cooling from region 4. Only in the presence of much manganese, nickel, or their equivalent can the true austenite be preserved in the cold so completely that the steel remains non-magnetic.
13. Beta (β) 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. Though not normal below MHSP′, yet like γ-iron it can be preserved in the cold by the presence of about 5% of manganese, which, though not enough to bring the lower boundary of region 4 below the atmospheric temperature and thus to preserve austenite in the cold, is yet enough to make the transformation of β into α iron so sluggish that the former remains untransformed even during slow cooling.
Again, β-iron may be preserved incompletely as in the “hardening of steel,” which consists in heating the steel into the austenite state of region 4, and then cooling it so rapidly, e.g. by quenching it in cold water, that, for lack of the time needed for the completion of the change from austenite into ferrite and cementite, much of the iron is caught in transit in the β state. According to our present theory, it is chiefly to beta iron, preserved in one of these ways, that all of our tool steel proper, i.e. steel used for cutting as distinguished from grinding, seems to owe its hardness.
14. Martensite, Troostite and Sorbite are the successive stages through which the metal passes in changing from austenite into ferrite and cementite. Martensite, very hard because of its large content of β-iron, is characteristic of hardened steel, but the two others, far from being definite substances, are probably only roughly bounded stages of this transition. Troostite and sorbite, indeed, seem to be chiefly very finely divided mixtures of ferrite and cementite, and it is probably because of this fineness that sorbitic steel has its remarkable combination of strength and elasticity with ductility which fits it for resisting severe vibratory and other dynamic stresses, such as those to which rails and shafting are exposed.
15. Alpha (α) iron is the form normal and stable for regions 5, 6 and 8, i.e. for all temperatures below MHSP′. It is the common, very magnetic form of iron, in itself ductile but relatively soft and weak, as we know it in wrought iron and mild or low-carbon steel.
16. Ferrite and cementite, already described in § 10, are the final products of the transformation of austenite in slow-cooling. β-ferrite and austenite are the normal constituents for the triangle GHM, α-ferrite (i.e. nearly pure α-iron) with austenite for the space MHSP, cementite with austenite for region 7, and α-ferrite and cementite jointly for regions 6 and 8. Ferrite and cementite are thus the normal and usual constituents of slowly cooled steel, including all structural steels, rail steel, &c., and of white cast iron (see § 18).
17. Pearlite.—The ferrite and cementite present interstratify habitually as a “eutectoid”[1] called “pearlite” (see Alloys, Pl., fig. 11), in the ratio of about 6 parts of ferrite to 1 of cementite, and hence containing about 0.90% of carbon. Slowly cooled steel containing just 0.90% of carbon (S in fig. 1) consists of pearlite alone. Steel and white cast iron with more than this quantity of carbon consist typically of kernels of pearlite surrounded by envelopes of free cementite (see Alloys, Pl., fig. 13) sufficient in quantity to represent their excess of carbon over the eutectoid ratio; they arc called “hyper-eutectoid,” and are represented by region 8 of Fig. 1. Steel containing less than this quantity of carbon consists typically of kernels of pearlite surrounded by envelopes of ferrite (see Alloys, Pl., fig. 12) sufficient in quantity to represent their excess of iron over this eutectoid ratio; is called “hypo-eutectoid”; and is represented by region 6 of Fig. 1. This typical “envelope and kernel” structure is often only rudimentary. The percentage of pearlite and of free ferrite or cementite in these products is shown in fig. 2, in which the ordinates of the line ABC represent the percentage of pearlite corresponding to each percentage of carbon, and the intercept ED, MN or KF, of any point H, P or L, measures the percentage of the excess of ferrite or cementite for hypo- and hyper-eutectic steel and white cast iron respectively.
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| Fig. 2.—Relation between the carbon-content and the percentage of the several constituents of slowly cooled steel and white cast iron. |
18. The Carbon-Content, i.e. the Ratio of Ferrite to Cementite, of certain typical Steels.—Fig. 3 shows how, as the carbon-content rises from 0 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 copper-like 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 carbon-content rises above the eutectoid or pearlite ratio of 0.90% is natural, because the brittleness of the cementite which, in hyper-eutectoid 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 immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.
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| Fig. 3.—Physical properties and assumed microscopic constitution of the pearlite series, graphiteless steel slowly cooled and white cast iron. By “total ferrite” is meant both that which forms part of the pearlite and that which is in excess of the pearlite, taken jointly. So with the “total cementite.” |
As the carbon-content increases the welding power naturally decreases rapidly, because of the rapid fall of the “solidus curve” at which solidification is complete (Aa of fig. 1), and hence of the range in which the steel is coherent enough to be manipulated, and, finally, of the attainable pliancy and softness of the metal. Clearly the mushy mixture of solid austenite and molten iron of which the metal in region 2 consists cannot cohere under either the blows or the pressure by means of which welding must be done. Rivet steel, which above all needs extreme ductility to endure the distortion of being driven home, and tube steel which must needs weld easily, no matter at what sacrifice of strength, are made as free from carbon, i.e. of as nearly pure ferrite, as is practicable. 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. Indeed, its carbon-content is made small quite as much because of the violence of the shocks from these wheels as because of any actual distortion to be expected, since, within limits, as the
- ↑ A “eutectic” is the last-freezing part of an alloy, and corresponds to what the mother-liquor of a saline solution would become if such a solution, after the excess of saline matter had been crystallized out, were finally completely frozen. It is the mother-liquor or “bittern” frozen. Its striking characteristics are: (1) that for given metals alloyed together its composition is fixed, and does not vary with the proportions in which those metals are present, because any “excess metal,” i.e. so much of either metal as is present in excess over the eutectic ratio, freezes out before the eutectic; (2) that though thus constant, its composition is not in simple atomic proportions; (3) that its freezing-point is constant; and (4) that, when first formed, it habitually consists of interstratified plates of the metals which compose it. If the alloy has a composition very near that of its own eutectic, then when solidified it of course contains a large proportion of the eutectic, and only a small proportion of the excess metal. If it differs widely from the eutectic in composition, then when solidified it consists of only a small quantity of eutectic and a very large quantity of the excess metal. But, far below the freezing-point, transformations may take place in the solid metal, and follow a course quite parallel with that of freezing, though with no suggestion of liquidity. A “eutectoid” is to such a transformation in solid metal what a eutectic is to freezing proper. It is the last part of the metal to undergo this transformation and, when thus transformed, it is of constant though not atomic composition, and habitually consists of interstratified plates of its component metals.
