Word Finder - graphite

definition

An allotrope of carbon, consisting of planes of carbon atoms arranged in hexagonal arrays with the planes stacked loosely, that is used as a dry lubricant and in "lead" pencils.

definition

Short for graphite-reinforced plastic, a composite plastic made with graphite fibers noted for light weight strength and stiffness.

example

Modern tennis racquets are made of graphite, fibreglass and other man-made materials.

definition

A grey colour.

The massive graphite is very easily machined and is widely used for electrodes, dynamo brushes, lead pencils and the like.

Graphite and some silver ores have also been found.

It is a black crystalline powder, resembling graphite in appearance.

Wotton's letter of 1620, already noted, was not published till 1651 (Reliquiae Wottonianae, p. 141), but in 1658 a description of Kepler's portable tent camera for sketching, taken from it, was published in a work called Graphite, or the most excellent Art of Painting, but no mention is made of Kepler.

Its other mineral resources include graphite, copper, zinc, lead, salt, alum, potter's clay, marble and good mill and building stones.

The Cumberland graphite, which is especially suitable for pencils, contains about 12% of impurities.

Graphite is present in Buganda and Unyoro.

Graphite is black and opaque, whilst diamond is colourless and transparent; it is one of the softest (H= I) of minerals, and diamond the hardest of all; it is a good conductor of electricity, whilst diamond is a bad conductor.

Besides the above, the mineral resources of Mexico include coal, petroleum, asphalt, platinum, graphite, soda and marble.

Troost produced crystallized zirconium by fusing the double fluoride with aluminium in a graphite crucible at the temperature of melting iron, and extracting the aluminium from the melt with hydrochloric acid.

See "Graphite and its Uses," Bull.

The charge is completely melted in about half an hour, and it is then thoroughly mixed by stirring with a graphite rod.

Graphite is used for the manufacture of pencils, dry lubricants, grate polish, paints, crucibles and for foundry facings.

Deposits of copper, tin, iron and tungsten have been discovered, and a variety of other mineral products (graphite, mica, spodumene, coal, petroleum, &c.).

A little graphite is produced in Humboldt county.

These chemists electrolyse either pure calcium chloride, or a mixture of this salt with fluorspar, in a graphite vessel which servos as the anode.

The lustre is bright and metallic. In its external characters graphite is thus strikingly similar to molybdenite.

But beyond this are the very useful, because very fusible, cast irons with from 3 to 4% of carbon, the embrittling effect of which is much lessened by its being in the state of graphite.

As these flakes readily split open, when a piece of this iron is broken rupture passes through them, with the result that, even though the graphite may form only some 3% of the mass by weight (say to% by volume), practically nothing but graphite is seen in the fracture.

Hence the weakness and the dark-grey fracture of this iron, and hence, by brushing this fracture with a wire brush and so detaching these loosely clinging flakes of graphite, the colour can be changed nearly to the very light-grey of pure iron.

There is rarely any important quantity of graphite in commercial steels.

Double Nature of the Carbon-Iron Diagram.-The part played by graphite in the constitution of the iron-carbon compounds, hitherto ignored for simplicity, is shown in fig.

In it the normal constituents are, for region II., molten metal+primary austenite; for region III., molten metal+primary graphite; for region IV., primary austenite; for region VII., eutectic austenite, eutectic graphite, and a quantity of pro-eutectoid graphite which increases as we pass from the upper to the lower part of the region, together with primary austenite at the left of the eutectic point B' and primary graphite at the right of that point.

Though carbon passes far more readily under most conditions into the state of cementite than into that of graphite, yet of the two graphite is the more stable and cementite the less stable, or the ' metastable " form.

Thus cementite is always tending to change over into graphite by the reaction Fe C = 3Fe +Gr, though this tendency is often held in check by different causes; but graphite never changes back directly into cementite, at least according to our present theory.

The fact that graphite may dissolve in the iron as austenite, and that when this latter again breaks up it is more likely to yield cementite than graphite, is only an apparent and not a real exception to this law of the greater stability of graphite than of cementite.

Slow cooling, slow solidification, the presence of an abundance of carbon, and the presence of silicon, all favour the formation of graphite; rapid cooling, the presence of sulphur, and in most cases that of manganese, favour the formation of cementite.

For instance, though in cast iron, which is rich in carbon, that carbon passes comparatively easily into the state of graphite, yet in steel, which contains much less carbon, but little graphite forms under most conditions.

Indeed, in the common structural steels which contain only very little carbon, hardly any of that carbon exists as graphite.

The joint effect of such chilling and such annealing is to make the metal much harder than if slowly cooled, because for each 1% of graphite which the chilling suppresses, 15% of the glass-hard cementite is substituted.

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.

The reason is that the particles of temper graphite which are thus formed within the solid casting in its long annealing are so finely divided that they do not break up the continuity of the mass in a very harmful way; whereas in grey cast iron both the eutectic graphite formed in solidifying, and also the primary graphite which, in case the metal is hypereutectic, forms in cooling through region 3 of fig.

In carrying out this process the castings are packed in a mass of iron oxide, which at this temperature gradually removes the fine or " temper " graphite by oxidizing that in the outer crust to carbonic oxide, whereon the carbon farther in begins diffusing outwards by " molecular migration," to be itself oxidized on reaching the crust.

This removal of graphite doubtless further stimulates the formation of graphite, by relieving the mechanical and perhaps the osmotic pressure.

Thus, first, for the brittle glass-hard cementite there is gradually substituted the relatively harmless temper graphite; and, second, even this is in part removed by surface oxidation.

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.

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.

Of these several qualities which cast iron may have, fluidity is given by keeping the sulphur-content low and phosphoruscontent 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.

This carbon may all be present as graphite, as in typical grey cast iron; or all present as cementite, Fe 3 C, as in typical white cast iron; or, as is far more usual, part of it may be present as graphite and part as cementite.

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.

We must grasp clearly this conception of metallic matrix and encased graphite skeleton if we are to understand this subject.

Next let us imagine that, in a series of cast irons all containing 4% of carbon, the graphite of the initial skeleton changes gradually into cementite and thereby becomes part of the matrix, a change 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 0.4% of graphite has thus changed, and in changing has united with o 4 X14 =5.6% of!

The mass as a whole, then, consists of 96.4 parts of metallic matrix, which itself is in effect a 0.415% 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 of the graphite of the initial skeleton changes into cementite and thereby 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.

Above the diagram are given 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.

Influence of the Constitution of Cast Iron on its Properties.- How should the hardness, strength and ductility, or rather shockresisting 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 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.