Chemical Composition
It is evident that the chemical composition of a refractory material will affect, to a large extent, the observed melting point. While attempts have been made to determine a relationship between the melting point and composition of fireclays, no definite and complete connection has been found. In fact, no equilibrium diagram can be established for such complex and heterogeneous mixture as fireclays and firebricks; first, because of the large number of components, and, second, because of the inhomogeneity of the chemical constituents. For the same reasons it is difficult to determine empiric relations between the observed melting point (which is not an equilibrium temperature) and the composition. Nevertheless, chemical analyses will often indicate the relative refractoriness of different materials. Of course, where we have combinations of chemically pure oxides in which are formed definite chemical compounds, solid solutions or eutectics, the temperature versus composition or equilibrium diagrams have been established for a number of groups of oxides, such as for the combinations of lime, alumina, magnesia, and silica.
The addition of an impurity to a refractory material usually lowers its melting point. For instance, in fireclay substances, the addition of sodium, potassium, iron, titanium, calcium, or magnesium compounds produces a very marked depression of the melting point; the addition of silica to fireclay materials decreases the refractoriness while the addition of alumina increases it.
Size of Particles and Shape and Position of Body
It is well known that, within certain limits, the smaller the particles of a refractory material the lower may be its melting point. The softening of the surface of the particles takes place at a lower temperature than the softening of the whole body en masse; in other words, the particles sinter together before the body flows. It is readily apparent that the smaller the particles or the finer the texture, the greater is the surface area exposed to softening. The fine division of the particles also allows a wider and more thorough distribution of the fluxing agents; consequently, the vitrification will proceed more rapidly, the solution and reaction of the constituents will be facilitated, and the material will flow at a lower temperature. On the contrary, a finer division of the particles may produce a wider and more thorough distribution of the higher melting-point constituents to the extent of raising the melting point.
The total effect on the melting point of varying the size of the particles ordinarily is not large. For example, in the case of a large number of samples of coal ash, those specimens ground “to an impalpable powder tended to soften at a slightly lower temperature than ash that would pass a 100-mesh screen. The difference averaged 6° C. and in no test exceeded 40° C.” Experiments made in the pyrometry laboratory at the Bureau of Standards on the melting points of silica foundry sands of particles just passing a 10-mesh screen showed no differences in melting point larger than the experimental error when the particles were ground to pass an 80-mesh screen. Other experiments on a fireclay brick gave a melting point of 1655° C. when ground to pass an 80-mesh screen and 1640° C. when ground to pass a 200-mesh screen. The melting point of the unground brick was found to be 1630° C. In this case it appears that the grinding served to modify the distribution of the different constituents in addition to reducing their size. All of these experiments were made under the same conditions.
Because the melting of a refractory material is accompanied by a more or less gradual decrease in viscosity, the temperature of marked flow will be dependent on the original geometrical form and position of the substance. For example, pyrometric cones in the shape of a tetrahedron with the axes at various angles from the vertical will be subject to different bending moments while softening and falling over; thus when bent over, the degree of fluidity attained will not be the same. If the same substance were in the form of a short cylinder, it is probable that one would not be able to judge by the squatting of the cylinder the temperature at which the same degree of viscosity occurs as in the case of the cone; hence the melting point observed with a cylinder may be different from that observed with a cone. However, experiments made at this Bureau showed no difference between the melting points of a cone and cylinder of the same height placed vertically and heated under the same conditions. The cylinder measured 2.5 cm. in height and 1.2 cm. in diameter; the cone was in the shape of a tetrahedron, being 2.5 cm. high and having 8-mm. sides for the base.
Time and Rate of Heating
The process of vitrification and melting of refractories is a matter of time as well as of temperature. Obviously, the longer the time during which the substance is held within its vitrification range the greater the extent of sintering; that is, the softening, melting, solution, or reaction of the components. Thus, if a refractory is held for a long time within its vitrification range, its fusibility will be increased; if a refractory is kept for a long time below the vitrification range, the sintering will not be appreciable. The melting point may be increased by the occurrence of a chemical reaction that results in the formation of a compound with a higher melting point than either of the components; rapid heating would, in this case, arrest the formation of such a compound. In the case of some materials prolonged heating brings about volatilization of the more volatile constituents, such as alkali compounds, with a consequent increase of refractoriness.
The observed melting point will also vary markedly with the rate of heating. In accordance with the well-known principle of the increase in the rate of reaction with the rise in temperature, the speed of a vitrification is accelerated by raising the temperature; consequently, the faster the rise in temperature, the smaller is the total amount of sintering or vitrification. At the same time, by rapid heating the solution of the components and the formation of eutectics may be arrested considerably; thus some of the factors that can cause the material to flow are largely diminished in effectiveness.
It takes a long time for some pure refractory compounds to melt; thus the melting temperature will vary with the rate of heating, for the extent of superheating while melting will be different for every rate of heating. In practically all instances of impure refractory mixtures or compounds, the melting range will depend on the rate of heating also because of the time effect in melting. It is believed that the effect of a change of rate of heating is more marked, the closer one approaches the melting point.
As a general rule, and within certain limits, the faster the rise in temperature the higher is the apparent melting point. No better illustration of this can be found than with Seger cones, where the softening temperatures can be easily varied by 50°C. or more by changing the rate of heating. At the Bureau of Standards, no difference in melting point was found in the case of a firebrick heated to the melting point in 1 hr. and one heated for 5 hr. In the case of very rapid rates of heating, the large temperature gradient in the sample may play a part in causing a high value for the melting point.
Nature of the Surroundings
Several possible external conditions affect the melting point. The pressure of the atmosphere, per se, will have practically no effect; that is, it would take a pressure of many atmospheres to change the melting point even slightly. In an indirect manner, however, the melting point may be changed considerably in a vacuum; namely, the more volatile and fusible components, such as alkali and alkali earth compounds, may distill or sublime, thus causing a rise in melting point, and vice versa, those substances that go off at atmospheric pressure may not do so at higher pressures.
Due to chemical reaction with the gases in the atmosphere surrounding the refractory, its melting point, can be altered considerably. In the case of some materials, coal ash, for example, the nature of the atmosphere is the factor exercising the greatest influence on the melting point. The terms reducing, oxidizing, and neutral atmospheres are not sufficiently, definite and, when considering the nature of the atmosphere, the gases present should be indicated. For example, in a reducing atmosphere either carbon vapor and carbon monoxide or hydrogen and water vapor may predominate; and the effect of one atmosphere may be totally different from that of the other.
In a carbon and carbon-monoxide reducing atmosphere, many refractories are very strongly attacked, the extent being dependent on the chemical composition, the pressure, and the temperature. Under some reducing conditions, ferric oxides in fireclay substances or other refractories are reduced to the ferrous state and combine to form low-melting-point silicates, which very materially increase the fusibility. In very strongly reducing carbon atmospheres, all the iron oxides may be reduced to metallic iron, thus preventing reactions with the silicates. At high temperatures, silica and silicates are reduced by carbon forming, under certain conditions, various compounds of silicon, carbon, and oxygen. On the other hand, in oxidizing atmospheres, some substances may be oxidized, allowing or preventing them from reacting with the refractory and bringing about a change in the melting point.
Conditions in Use Affecting Apparent Melting Point
Substances coming in contact with the refractory, such as molten metals, slags, fluxes, and flue dust, often attack the refractory and may lower its melting point considerably. Since the temperature at which a refractory begins to flow is related to the degree of viscosity the material has attained, the application of a load will make the material deform faster and at a lower temperature. On account of the more intimate contact of the particles, the application of a load will allow a refractory to sinter at a lower temperature and the continued application of the force will result in the material softening or melting at a lower temperature. It also appears to be true that the larger the load applied, the lower is the temperature at which the material will soften and collapse. For instance, the softening point of a fireclay brick with no load was 1730° C. while with a load of 50 lb. per sq. in. it was 1200° C. A fireclay with a softening point of 1650° C. gave a softening point of 1435° C. with a load of 54 lb. per sq. in.; and one of 1380° C. with a load of 72 lb. per sq. in.
In general, so large a number of complex physico-chemical phenomena enter into the melting of a refractory material that it becomes impossible to predict in most cases in which direction the melting point will change by changing the factors and conditions under which the material is heated.
