There may be other halts in the cooling, both before and after complete solidification, due to evolution of heat in the mixture.
Liquids may be cooled below their freezing-point without solidification, the metastable (after W.
If, ignoring temporarily and for simplicity the fact that part of the carbon may exist in the state of graphite, we consider the behaviour of iron in cooling from the molten state, AB and BC give the temperature at which, for any given percentage of carbon, solidification begins, and Aa, aB, and Bc that at which it ends.
The frames into which hard soaps are ladled for cooling and solidification consist of rectangular boxes made of iron plates and bound and clamped together in a way that allows the sides to be removed when required; wooden frames are used in the case of mottled soaps.
The solidification is a very gradual process, depending, of course, for its completion on the size of the block; but before cutting into bars it is essential that the whole should be set and hardened through and through, else the cut bars would not hold together.
The transformation of polymorphs presents certain analogies to the solidification of a liquid.
The effect of chemical agents in producing coagulation are in consonance with what is known of other instances of polymeric or condensation changes, whilst the fact that the collection of globules separated by creaming after thorough washing, and therefore removal of all proteid, is susceptible of solidification into caoutchouc by a merely mechanical act such as churning, strongly supports the view that the character of the change is distinct from that of any alteration which may occur in the proteid constituents of the latex.
The globules in the latex, however, consist more probably of a distinct liquid substance which readily changes into the solid caoutchouc. The coagulation of the latex often originates with the " curding " of the proteids present, and this alteration in the proteid leads to the solidification of the globules into caoutchouc. The latter, however, is probably a distinct effect.
Perhaps all metals are crystalline, only the degree of visibility of the crystalline arrangement is very different in different metals, and even in the same metal varies according to the slowness of solidification and other circumstances.
But when solidification commences, the thermometer will cease to fall, it may even rise slightly, and the temperature will remain almost constant for a short time.
This halt in the cooling, due to the heat evolved in the solidification of the first crystals that form in the liquid, is called the freezingpoint of the mixture; the freezing-point can generally be observed with considerable accuracy.
1 The instructive case of the solidification of a solution of common salt in water is discussed in the article Fusion.
This process goes on until the state of the remaining liquid is represented by the point C. Now crystals of B begin to form, simultaneously with the A crystals, and the composition of the remaining liquid does not alter as the solidification progresses.
It is evident that every mixture except the eutectic mixture C will have two halts in its cooling, and that its solidification will take place in two stages.
The graphical representation of the properties of alloys can be extended so as to record all the changes, thermal and chemical, which the alloy undergoes after, as well as before, solidification, including the formation and breaking up of solid solutions and compounds.
It therefore expands on solidification; and as it retains this property in a number of alloys, the metal receives extensive application in forming type-metals.
Hitherto it had been felt as a great difficulty in casting specula that the solidification did not begin at one surface and proceed gradually to the other, the common sand mould allowing the edges to cool first, so that the central parts were subject to great straining when their time of cooling came, and in large castings this generally caused cracking.
As we have seen, the maxima of the various curve-branches at C, E, G, and J correspond with the melting points of the various hydrates at 37°, 32.5°, 56° and 73.5° respectively; and at these points melting or solidification of the whole mass can occur at constant temperature.
It will be noticed that in all these theoretical curves the points of initial fusion and solidification do not in general coincide; we reach a different curve first according as we approach the diagram from below, where all is solid, or from above, where all is liquid.
A familiar example is to be found in solutions of sodium sulphate, which may be cooled much below their saturation point and kept in the liquid state till a crystal of the hydrate Na 2 SO 4 IoH 2 O is dropped in, when solidification occurs with a large evolution of latent heat.
From this and other evidence it has been shown that the first thin shower in open vessels is produced by the accidental presence of tiny crystals obtained from the dust of the air, while the second dense shower marks the point of spontaneous crystallization, where the decrease in total available energy caused by solidification becomes greater than the increase due to the large surface of contact between the liquid and the potentially existing multitudinous small crystals of the shower.
Solidification of the first kind may be termed "setting," that of the second "coagulation."
But after solidification is complete and the metal has cooled to a much lower range of temperature, usually between 9 00 and 690 C., it undergoes a very remarkable series of transformations.
As the carbon-content increases the welding power naturally decreases rapidly, because of the rapid fall of the " solidus curve at which solidification is complete (Aa of fig.
The heat evolved by this process of solidification retards the fall of temperature; but after this the rate of cooling remains regular until T (750°) on the line Sa (Ar 3) is reached, when a second retardation occurs, due to the heat liberated by the passage within the pasty mass of part of the iron and carbon from a state of mere solution to that of definite combination in the ratio Fe 3 C, forming microscopic particles of cementite, while the remainder of the iron and carbon continue dissolved in each other as austenite.
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.
Steel castings have initially the extremely coarse structure due to cooling without mechanical distortion from their very high temperature of solidification; they are " annealed," i.e.
This enables it to take up enough silicon from the walls of the crucible to prevent the evolution of gas during solidification, and the consequent formation of blowholes or internal gas bubbles.
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 %, and then in turn decreases.
- In an early period of the solidification of a molten steel ingot cast in a cold iron mould we may distinguish three parts: (1) the outer layers, i.e.
Because this pipe is due to the difference in the rates of contraction of interior and exterior, it may be lessened by retarding the cooling of the mass as a whole, and it may be prevented from stretching down deep by retarding the solidification of the upper part of the ingot, as, for instance, by preheating the top of the mould, or by covering the ingot with a mass of burning fuel or of molten slag.
These numbers must be varied with the variations in other conditions, such as casting temperature, rapidity of solidification, &c.
- The solidification of an ingot of steel takes place gradually from without inwards, and each layer in solidifying tends to expel into the still molten interior the impurities which it contains, especially the carbon, phosphorus, and sulphur, which by this process are in part concentrated or segregated in the last-freezing part of the ingot.
31, has three advantages - (1) that the temperature is adjusted with absolutely no consumption of fuel; (2) that the waste of iron due to the oxidation of the outer crust of the ingot is very slight, because the little atmospheric oxygen initially in the pit is not renewed, whereas in a common heating furnace the flame brings a constant fresh supply of oxygen; and (3) that the ingot remains upright during solidification, so that its pipe is concentrated at one end and is thus removable.
The phenomenon absolutely corresponds to that of fusion and solidification, only that it generally takes place less quickly; consequently we may have prismatic sulphur at ordinary temperature for some time, as well as rhombic sulphur at loo°.
It may be skimmed off the underlye and placed direct in the frames for solidification; but that is a practice scarcely at all followed, the addition of resin soap in the pan and the subsequent " crutching in " of silicate of soda and adulterant mixings being features common to the manufacture.
It is a gas at ordinary temperature; when liquefied it boils at -63.5° C. and on solidification melts at -139° C. Water decomposes it into nitric and hydrofluoric acids.
Ruff and Curt Albert (Ber., 1905, 38, p. 53) by decomposing titanium fluoride with silicon chloroform in sealed vessels at 100 -120° C. It is a colourless gas which may be condensed to a liquid boiling at -80 2° C. On solidification it melts at about -110° C. The gas is very unstable, decomposing slowly, even at ordinary temperatures, into hydrogen,, silicon fluoride and silicon: 4SiHF 3 =2H 2 +3SiF 4 +Si.
Let us con sider a little more closely the solidification of the mixture represented by the vertical line Pqrs.
Neville proved that when one metal is alloyed with a small quantity of some other metal, the solidification obeys the law of F.
He next experimented with a highpressure hydrogen jet by which low temperatures were realized through the Thomson-Joule effect, and the successful results thus obtained led him to build at the Royal Institution the large refrigerating machine by which in 1898 hydrogen was for the first time collected in the liquid state, its solidification following in 1899.
The presence of the antimony in this alloy gives to it hardness, and the property of expanding on solidification, thus allowing a sharp cast of the letter to be taken.