Open main menu
This page needs to be proofread.

steel in which is encased a skeleton of graphite plates, besides some very fine scattered particles of graphite.

Next let us imagine that, in a series of cast irons all containing 4 % of carbon the graphite of the initial skeleton chan es raduall s g y

into cementite and thereby becomes part of the matrix, a change 1 which of course has two aspects, first, a gradual thinning of the graphite skeleton and a decrease of its continuity, and second, a gradual introduction of cementite into the originally pure ferrite matrix. By the time that o~4% of graphite has thus changed, and in changing has united with 0-4><14=5-6% off the iron of the original ferrite matrix, it will have changed this matrix from pure ferrite into a mixture of

Cementite . . . 0-4 +5-6 = 6-0

Ferrite . 96-O - 5-6 = 90-4


The residual graphite skeleton forms 4 °-O'4= 3-6 100-0

But this matrix is itself equivalent to a steel of about 0-40 % of carbon (more accurately 0-40><100+96-4=o-415 %), a rail steel, because it is of just sucfi a mixture of ferrite and cementite in the ratio of 90-4: 6 or 94% and 6%, that such a rail steel consists. The mass as a whole, then, consists of 96-4 parts of metallic matrix, which itself is in effect a o-415 'I/2, carbon rail steel, weakened and embrittled by having its continuity broken up by this skeleton of graphite forming 3-6 % of the whole mass by weight, or say 12 % by volume. As. in succeeding members of this same series of cast irons, more oi the graphite of the initial skeleton changes into cementite and tnereby becomes part of the metallic matrix, so the graphite skeleton becomes progressively thinner and more discontinuous, and the matrix richer in cementite and hence in carbon and hence equivalent first to higher and higher carbon steel, such as tool steel of I % carbon, file steel of 1-50 %, wire-die steel of 2% carbon and then to white cast iron, which consists essentially of much cementite with little ferrite. Eventually, when the whole of the graphite of the skeleton has changed into cementite, the mass as a whole becomes typical or ultra white cast iron, consisting of nothing but ferrite and cementite, distributed as follows (see fig. 2):-

Eutectoid ferrite ..... . 4O'0

cementite . . . 6-7

Interstratiiied as pearlite . . . ' . 46-7

Cementite, primary, eutectoid and pro-eutectoid . 53-3 loo-0

Total ferrite . . 40-0

Total cementite . . 60-0

4 100-o

The constitution and properties of such a series of cast irons, portion of ferrite and cementite respectively in the matrix, DEF, KS and TU reproduced from fig. 3 give the consequent properties of the matrix, and GAF, RS and VU give, partly from conjecture, the properties of the cast iron as a whole. Above the diagram are giyen the names of the different classes of cast iron to which different stages in the change from graphite to cementite correspond, and above these the names of kinds of steel or cast iron. to which at the corresponding stages the constitution of the matrix corresponds, while below the diagram are given the properties of the cast iron as a whole corresponding to these stages, and still lower the purposes for which these stages fit the cast iron, first because of its strength and shock-resisting power, and second because of its hardness. 115. Influence of the Constitirtion of Cast Iron on its Properties.-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 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 cop er-like fe1'rite, so that the brittleness increases continuously (RSS), 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. Here let us recognize that what gives this transfer of carbon from graphite skeleton to metallic matrix such very great influence on the properties of the metal is the fact that the transfer of each 1% of carbon means substituting in the matrix no less than 15%, of the brittle, glass-hard cementite for the soft, very ductile ferrite. Third, the tensile strength of steel proper, of which the matrix consists, as we have already seen (fig. 5), increases with the carbon content till this reaches about I-25 %, and then in turn decreases (fig. 28 DEF). Hence, as with the progressive transfer of the 1 carbon from the graplutic to tne cementite state in our imaginary series of cast irons, the combined carbon present in the matrix increases, so does the tensile strength of the mass as a whole for two reasons; first, because the strength of the matrix itself is increasing (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 I % 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 but the distribution of the graphite is of great importance. Thus it is extremely probable that the primary graphite, which forms large sheets, is much more weakening and embrittling than the eutectic all containing 4% of carbon but with that carbon shifting proand other forms, and therefore that, if either strength or ductility is sought, the metal should be free from primary graphite, i.e. Nmommu LW&? n' igggégg, ,, , s, ;&Z, ,, ,, , mw, ,, that it should not be hyper-eutectic. ='== c The presence of graphite has two further and very natural N, ,, ,, ,, ,, ,, ,, ,, ,, ,, gigmggyxc C'°;g*Y “°;gf' "'§§ , ' effects. First, if the skeleton which it forms is continuous, then “'~°'“'f "“°" wiifw 'fm ffm 'f°f~ its planes of Junction w1th the metallic matrix offer a path of M ' ' ., a, ,, ,, ,° low resistance to the passage of liquids or gases, or in short they B '~~l' E -3 make the metal so porous as to unht it for objects l1ke the sg °, P” Q cylinders of hydraulic presses, which ought to be gas-tight 215 - f ~- 41; . . moooof and water-tight. P or such purposes the graphite-content should § a§ Kfi'3=, ;, rs, ' ., ,, I, .-° U 31 be low. Second, the very genesis of so bulky a substance as the fig 4-:E - -"fx wif- H § primary and eutectic graphite while the metal is solidifying § :§§ ', =° m, ,, 'mugi " ~ B°°°°5 (fi. 5) causes a sudden and permanent expansion, which forces §§ f'~. . nw g°.{¢ '.-*' w,5 U' ' g is the metal into even the'finest crevices in its mould, a fact = gg ' , nf, '5g'.¢ 'K lj' "'¢, o 5, which IS taken advantage of 1n making ornamental castings and igigngg .' -' . ' llqq/f a Mo, '°°°° 3 others which need great sharpness of detail, by making them g ~ » ' ' a g rich in graphite.

° 1, g4§ 1:, :::: : '~ To sum this up, as graphite is replaced by carbon combined °°°”"'S.€°.2'r2a2» 22: .xt ts 21: 12: 1:2 21: 25 ° as.r3'f“;'“;.f'2S, .*“ff'1"@i-ii~.?.;t§ E;i."@§ ;.i;;i.;'@'if:%'..;'f.°'§§ 5§ ; an e x s in so 1 1 ', s

5*'="S"' . "'“* Y. "“'”°“' f ""”' . continuously;/ii, while the tensile strength increases till the com”'f'='=““ -€“"""“' . . "'3°'“"' bined carbon-content rises a little above I %, and then in turn

mn;;"““3 “uf” i°“ "mah "M" “pm decreases. .That itrength is good anchbéittéenesshbag goes

2, ,, ,, ¢, ,, ,, ,, ,, ,, wa u»¢rmu»u|¢¢4x» 25:5 out saying, but ere a word ls nee e a out ar ness. L -4 § , ;'§ §§ , ';§ u, §§ ", § 1§§ ', ¢, , r, ., ,.p°n.¢ expense 0 cutting castings accurately to shapexcutting on them 5 n umm: 1° ' nsumimmmnng " mhknu main.. * ina manning screw threads and what not, called ' machining in trade § i..f.¢.¢» “"i'<=='i:°i' '° Ki°rif€i:°°' mmgiigixm »2ff»:'iii'fa'L'i»' parlance, is often a very large part of their total cost; and it abrasion in au |5ns|cn In unc H U-W

increases rapidly with the hardness of the metal. On the other FIG. 28.—Physical Properties and assumed Microscopic Constitution of Cast Iron containing4%of carbon, as affected by the distribution of that carbon between the combined and graphitic states. gressively from the state of graphite to that of cementite as we pass from specimen to specimen, may, with the foregoing icture of a skeleton-holding matrix clearly in our minds be traced fiy means of fig. 28. The change from graphite into cementite is supposed to take place as we pass from left to right. BC and OH give the prohand, the extreme hardness of nearly graphite less cast iron is of great value for objects of which the chief duty is to resist 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.

1 16. Illerms of coutrollin the Constitution of Cast Iron.-The distribution of the carbon getween these two states, so as to give

T the mst iron the properties needed, is brought about chiefly by