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DROPS OF SLAG

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Originally appearing in Volume V14, Page 828 of the 1911 Encyclopedia Britannica.
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DROPS OF SLAG. - . cast iron with be- 0o s i E o FC ~cr. r eo -^^—MO000 ..rp s0000 t of ,0050 ee'0,- `.wood C t 20 °f~~ !'•._-- wP `~' ~•: r/11 in which is encased a skeleton of graphite plates, besides some portion of ferrite and cementite respectively in the matrix, DEF, cry fine scattered particles of graphite. KS and TU reproduced from fig. 3 give the consequent properties Next let us imagine that, in a series of cast irons all containing ¢ % of the matrix, and GAF, RS and VU give, partly from conjecture, carbon, the graphite of the initial skeleton changes gradually the properties of the cast iron as a whole. Above the diagram are to cementite and thereby becomes part of the matrix, a change given the names of the different classes of cast iron to which different 0400 of course has two aspects, first, a gradual thinning of the stages in the change from graphite to cementite correspond, and aphite skeleton and a decrease of its continuity, and second, a above these the names of kinds of steel or cast iron. to which at the introduction of cementite into the originally pure ferrite corresponding stages the constitution of the matrix corresponds, atrix. By the time that o•4 % of graphite has thus changed, while below the diagram are given the properties of the cast iron as nd in changing has united with o•¢XI¢=5.6%of~ the iron of the a whole corresponding to these stages, and still lower the purposes ,46400501 ferrite matrix, it will have changed this matrix from pure for which these stages fit the cast iron, first because of its strength rrite into a mixture of and shock-resisting power, and second because of its hardness. Cementite 115. Influence of the Constitution of Cast Iron on its Properties. Ferrite 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 brittle- ness should be lessened somewhat by the decrease in the extent to loo o which the continuity of the strong matrix is broken up by the ut this matrix is itself equivalent to a steel of about o 40°~ of graphite skeleton, yet this effect is outweighed greatly by that of arbon (more accurately 0•¢0X400=q6.4=o•¢t5%), a rail steel, the rapid substitution in the matrix of the brittle cementite for the ecause it is of just such a mixture of ferrite and cementite in the very ductile copper-like ferrite, so that the brittleness increases ado of 90 4: 6 or 94% and 6 %, that such a rail steel consists. The continuously (RS), from that of the very grey graphitic cast irons, mass as a whole, then, consists of 96 4 parts of metallic matrix, which which, like that of soapstone, is so slight that the metal can endure self is in effect a o•¢t5 % carbon rail steel, weakened and embrittled severe shock and even indentation without breaking, to that of the y having its continuity broken up by this skeleton of graphite pure white cast iron which is about as brittle as porcelain. Here 0,5,4006 3.6 ° of the whole mass by weight, or say I2 % by volume. let us recognize that what gives this transfer of carbon from graphite As, in succeeding members of this same series of cast irons, more skeleton to metallic matrix such very great influence on the proi the graphite of the initial skeleton changes into cementite and perties of the metal is the fact that the transfer of each i % becomes part of the metallic matrix, so the graphite skeleton of carbon means substituting in the matrix no less than 15 % of ecomes progressively thinner and more discontinuous, and the the brittle, glass-hard cementite for the soft, very ductile ferrite. matrix richer in cementite and hence in carbon and hence equivalent Third, the tensile strength of steel proper, of which the matrix rst to higher and higher carbon steel, such as tool steel of I % consists, as we have already seen (fig. 3), increases with the carbon- content file steel of 1 So wire-die steel of 2 % carbon and then content till this reaches about 1.25 %, and then in turn decreases o white cast iron, which consists essentially of much cementite 1 (fig. 28, DEF). Hence, as with the progressive transfer of the with little ferrite. Eventually, when the whole of the graphite of carbon from the graphitic to the cementite state in our imaginary he skeleton has changed into cementite, the mass as a whole becomes series of cast irons, the combined carbon present in the matrix ypical or ultra white cast iron, consisting of nothing but ferrite and increases, so does the tensile strength of the mass as a whole for ementite, distributed as follows (see fig. 2) : two reasons; first, because the strength of the matrix itself is in- Eutectoid ferrite creasing (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 % 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 100.0 but the distribution of the graphite is of great importance. Thus it is extremely probable that the primary graphite, which forms large The constitution and properties of such a series of cast irons, sheets, is much more weakening and embrittling than the eutectic all containing } % of carbon but with that carbon shifting pro- 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 Bo 00 low resistance to the passage of liquids or gases, or in short they Interstratified as pearlite . Cementite, primary, eutectoid and pro-eutectoid Total ferrite Total cementite Lose carbon Medium carbon steel Very open gray eery 6,3004040 t iron High or burl 00551 time V White cast iron 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 II! 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 snaking 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 i °,o, and then in turn decreases. That strength is good and brittleness bad goes with-out 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 rr 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 of Cast Iron containing 4% of carbon, as affected by the distribution of great value for objects of which the chief duty is to resist of that carbon between the combined and graphitic states. 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 01,50310 and. 004011000 t0.1i1CCk Strength Brittleness Baran.. Orapbft. $,V O Om item d. Suited to uses in whim it rem hem to undergo et - 0.020[11 Haut Week Leah brittle toot owls s•o soft moderate the iron ie suited to 8800 088l8a.t11K parpb.ell roach maahioing 8850108.0. merino- in prepacatio0, mg 100 pttpantla0. but 111110 not excusing abrasion in use abrasion in use 1.5 0 Most erotic Hardest 80 shack unless 5,5,504 it strongly supported no machime3 in preparation, much abrasion gr m a steel gradual original ferrite matrix, it will have changed this matrix from pure b Q~ d „ cementite 0.4+5.6= 6o 96o-56=90.4 96'4 The residual graphite skeleton forms 4 -0'4= 3'6 tnereby !,011000 Name of matrix Name of the 0+00 1,00 I.e.,ol the wt,ett ComOlh.d Cdr6O0.s0 O.8 a•o 8.0 0.0 5trwyert 0.0 8•d 4.0 8.8 Harder 40.0 6.7 Mottled ust iron 46.7 53.3
End of Article: DROPS OF SLAG
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