Page:EB1911 - Volume 14.djvu/860

Rh carbon has been burnt away to leave just the carbon-content desired, and it is then tapped out and poured into the moulds.

Of the two the cupola is very much the more economical of fuel, thanks to the direct transfer of heat from the burning coke to the pig iron with which it is in contact. But this contact both causes the iron to absorb sulphur from the coke to its great harm, and prevents it from having any large part of its carbon burnt away, which in many cases would improve it very greatly by strengthening it. Thus it comes about that the cupola, because it is so economical, is used for all but the relatively few cases in which the strengthening of the iron by the removal of part of its carbon and the prevention of the absorption of sulphur are so important as to compensate for the greater cost of the air-furnace melting.

112. Cast iron for foundry purposes, i.e. for making castings of cast iron. Though, as we have seen in § 19, steel is rarely given a carbon-content greater than 1.50% lest its brittleness should be excessive, yet cast iron with between 3 and 4% of carbon, the usual cast iron of the foundry, is very useful. Because of the ease and cheapness with which, thanks to its fluidity and fusibility (fig. 1), it can be melted and run even into narrow and intricate moulds, castings made of it are very often more economical, i.e. they serve a given purpose more cheaply, in the long run, than either rolled or cast steel, in spite of their need of being so massive that the brittleness of the material itself shall be endurable. Indeed this high carbon-content, 3 to 4%, in practice actually leads to less brittleness than can readily be had with somewhat less carbon, because with it much of the carbon can easily be thrown into the relatively harmless state of graphite, whereas if the carbon amounts to less than 3% it can be brought to this state only with difficulty. For crushing certain kinds of rock, the hardness of which cast iron is capable really makes it more valuable, pound for pound, than steel.

113. Qualities needed in Cast Iron Castings.—Different kinds of castings need very different sets of qualities, and the composition of the cast iron itself must vary from case to case so as to give each the qualities needed. The iron for a statuette must first of all be very fluid, so that it will run into every crevice in its mould, and it must expand in solidifying, so that it shall reproduce accurately every detail of that mould. The iron for most engineering purposes needs chiefly to be strong and not excessively brittle. That for the thin-walled water mains must combine strength with the fluidity needed to enable it to run freely into its narrow moulds; that for most machinery must be soft enough to be cut easily to an exact shape; that for hydraulic cylinders must combine strength with density lest the water leak through; and that for car-wheels must be intensely hard in its wearing parts, but in its other parts it must have that shock-resisting power which can be had only along with great softness. Though all true cast iron is brittle, in the sense that it is not usefully malleable, i.e. that it cannot be hammered from one shape into another, yet its degree of brittleness differs as that of soapstone does from that of glass, so that there are the intensely hard and brittle cast irons, and the less brittle ones, softer and unhurt by a shock which would shiver the former.

Of these several qualities which cast iron may have, fluidity is given by keeping the sulphur-content low and phosphorus-content high; and this latter element must be kept low if shock is to be resisted; but strength, hardness, endurance of shock, density and expansion in solidifying are controlled essentially by the distribution of the carbon between the states of graphite and cementite, and this in turn is controlled chiefly by the proportion of silicon, manganese and sulphur present, and in many cases by the rate of cooling.

114. Constitution of Cast Iron.—Cast iron naturally has a high carbon-content, usually between 3 and 4%, because while molten it absorbs carbon greedily from the coke with which it is in contact in the iron blast furnace in which it is made, and in the cupola furnace in which it is remelted for making most castings. This carbon may all be present as graphite, as in typical grey cast iron; or all present as cementite, Fe3C, as in typical white cast iron; or, as is far more usual, part of it may be present as graphite and part as cementite. Now how does it come about that the distribution of the carbon between these very unlike states determines the strength, hardness and many other valuable properties of the metal as a whole? The answer to this is made easy by a careful study of the effect of this same distribution on the constitution of the metal, because it is through controlling this constitution that the condition of the carbon controls these useful properties. To fix our ideas let us assume that the iron contains 4% of carbon. 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 primary graphite (§ 26) generally forms a coarse, nearly continuous skeleton of curved black plates, like those shown in fig. 27; the eutectic graphite is much finer; while the pro-eutectoid and eutectoid graphite, if they exist, are probably in very fine particles. We must grasp clearly this conception of metallic matrix and encased graphite skeleton if we are to understand this subject.

—Graphite in Grey Cast Iron.

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