In the former case there is no later chance to remove sulphur, a minute quantity of which does great harm by leading to the formation of cementite instead of graphite and ferrite, and thus making the cast-iron castings too hard to be cut to exact shape with steel tools; in the latter case the converting or purifying processes, which are essentially oxidizing ones, though they remove the other impurities, carbon, silicon, phosphorus and manganese, are not well adapted to desulphurizing, which needs rather deoxidizing conditions, so as to cause the formation of calcium sulphide, than oxidizing ones.
If this carbon is all present as graphite, so that in cooling the graphite-austenite diagram has been followed strictly (§ 26), the constitution is extremely simple; clearly the mass consists first of a metallic matrix, the carbonless iron itself with whatever silicon, manganese, phosphorus and sulphur happen to be present, in short an impure ferrite, encased in which as a wholly distinct foreign body is the graphite.
The iron of the original ferrite matrix, it will have changed this matrix from pure carbon (more accurately 0.40 X I oo --96 4 = 0.415%), a rail steel, because it is of just such a mixture of ferrite and cementite in the But this matrix is itself equivalent to a steel of about 0.40% of ratio of 90.4:6 or 94% and 6%, that such a rail steel consists.
2) 100.0 The constitution and properties of such a series of cast irons, all containing 4% of carbon but with that carbon shifting pro o v,,3 950 R portion of ferrite and cementite respectively in the matrix, DEF, KS and TU reproduced from fig.
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.
(Sectional Elevation.) heat for some hours in order to settle out 'the ferric oxide which it always contains, and which becomes insoluble (through the destruction of the sodium ferrite) only at high temperatures.
It was, however, found that the behaviour of this alloy was in part due to a layer of pure iron (" ferrite ") averaging o 1 mm.
Whites, ferrite; blacks, carbide.
Whites, ferrite; blacks, carbide.
Whites, ferrite; blacks, carbide.
- (Stoughton.) Meshes of pearlite in a netv.-ork of ferrite, from hypo-eutectoid steel.
Whites, ferrite; blacks, carbide.
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.
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).
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.
Martensite, Troostite and Sorbite are the successive stages through which the metal passes in changing from austenite into ferrite and cementite.
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 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.
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 ?
By " total ferrite " is meant both that which forms part of the pearlite and that which is in excess of the pearlite, taken jointly.
Of as nearly pure ferrite, as is practicable.
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.
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 iron thus liberated, as the ferrite of this pearlite, changes simultaneously to a-ferrite.
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.
Its further cooling undergoes three spontaneous retardations, one at K' (Ar 3 about 820°), at which part of the iron begins to isolate itself within the austenite mother-metal in the form of envelopes of 0-ferrite, i.e.
At the second retardation, K" (Ar2, about 770°) this ferrite changes to the normal magnetic a-ferrite, so that the mass as a whole becomes magnetic. Moreover, the envelopes of ferrite which began forming at Ar 3 continue to broaden by the accession of more and more ferrite born from the austenite progressively as the temperature sinks, till, by the time when Ar t (about 690°) is reached, so much free ferrite has been formed that the remaining mother-metal has been enriched to the composition of hardenite, i.e.
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.
Further, any ferrite liberated at Ar 3 changes there from -y to a, and any present at Ar 2 changes from (3 to a.
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.
Would have consisted chiefly of graphite with pearlite and ferrite (which are all relatively soft bodies), if thus chilled and annealed consists of cementite and pearlite.
The molecular freedom which this high temperature gives enables the cementite to change gradually into a mixture of graphite and austenite with the result that, after the castings have been cooled and their austenite has in cooling past Aci changed into pearlite and ferrite, the mixture of cementite and pearlite of which they originally consisted has now given place to one of fine or " temper " graphite and ferrite, with more or less pearlite according to the completeness of the transfer of the carbon to the state of graphite.
Magnetite, Fe304, may be regarded as ferrous ferrite, FeO-Fe2O3.
Calcium ferrite, magnesium ferrite and zinc ferrite, ROFe203(R=Ca, Mg, Zn), are obtained by intensely heating mixtures of the oxides; magnesium ferrite occurs in nature as the mineral magnoferrite, and zinc ferrite as franklinite, both forming black octahedra.
When concentrated the solution is nearly black, and on heating it yields a yellow solution of potassium ferrite, oxygen being evolved.
Steel differs in many ways from iron in respect of atmospheric corrosion; the heterogeneous nature of steel gives occasion to a selective rusting, ferrite is much more readily attacked than the cementite and pearlite; moreover, the introduction of other elements may retard rusting; this is particularly the case with the nickel-steels.
Now this matrix itself is equivalent to a very low-carbon steel, strictly speaking to a carbonless steel, because it consists of pure ferrite, which is just what such a steel consists of; and the cast iron as a whole is therefore equivalent to a matrix of very low-carbon content greater than r so%lest its 111E brittleness should be excessive, yet -(1 cast iron with be U H c ° ?
BC and OH give the prothe cast iron the properties needed, is brought about chiefly by ferrite into a mixture of adjusting the silicon-content, because the presence of this element favours the formation of graphite.