US1491510A - Fused eutectic salt bath - Google Patents

Description

April 22, 1924.. 1,491,510

A. E. BELLIS FUSED EUTECTIC SALT BATH Filed Sept. 24. 1920 3 Sheets-Sheet 1 L/ou/o 5|0 6a COMPOSITION 3817.1. VUJd W31 vwemtoz 3 5 Gwen WW 2 Z Z April 22, 1924. 1,491,510

A. E. BELLIS FUSED EUTECTIG SALT BATH April A. E. BELLI S FUSED EUTBCTIC SALT BATH Filed Sept. 24 1920 3 SheetsSheet 3 LIQUID COMPOSITION L mums alo ONE

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ARTHUR E. BELLIS, 0F SPRINGFIELD, MASSACHUSETTS, ASSIGNOR, BY MESNE AS- SIGNMENTS, TO THE BELLIS HEAT TREATING COMPANY, OF NEW HAVEN, CON- NECTICUT, A. CORPORATION OF CONNECTICUT.

FUSED EUTECTIC SALT BATE.

' Application filed September 24, 1920. Serial No. 412,588.

To all whom it may concern:

Be it known that I, ARTHUR E. BELLIS, a citizen of the United States, and resident of the city of Springfield, county of Hampden, State of Massachusetts, have invented certain new and useful Improvements in Fused Eutectic Salt Baths, of which the fol lowing is a specification.

This invention relates to a fused eutectic salt bath, and contemplates the provision of a bath of this character which may be advantageously used in the heat treatment of metals and for other purposes.

The essential and distinguishing characteristic of an eutectic mixture of two or more components is recognized as that particular proportionate mixture of the components which possesses the lowest melting point, and is, therefore, the most fusible mixture of such components. 7 I My present. invention, therefore, has for its primary object to provide such an eutectic fused salt bath which, in addition to its low melting point, will have a relatively high-volatilization point and thus afford a relatively wide range of temperatures, in which the mixture of the components will have its greatest fluidity, and in which the components of such mixture will, at all times, remain in the original eutectic proportions in both solid and liquid phases. Owing to the fact that such maximum fluidity is obtained in the eutectic mixture, there is a rapid and uniform circulation of heat throughout all parts of the bath, so that the metal'object being treated will be uniformly heated throughout its structure. Also, the low viscosity of the bath minimizes the thickness of the bath film which adheres to the object when it is withdrawn, so that there will be a relatively small loss of-the bath material.

In order to illustrate the physical characteristics of an eutectic mixture within a variable temperature range as compared to the physical characteristics of hypo-eutectics and hyper-eutectics within a similar range, I have illustrated in the accompanying drawings equilibrium and constitutional diagrams of the respective mixtures. In these drawings:

Figure 1 graphically represents a mixture of two salts which do not form chemical compounds or conjugate liquids when in the liquid state. 7

Figure 2 is amixture of salt components which form conjugate liquids in the liquid state, but does not form chemical compounds in the liquid state, and,

Figure 3 represents a mixture of salt components which form both chemical compounds, and conjugate liquids in the liquid state.

Referring to the diagram, in Figure 1 of the drawings, which represents two salt components that are completely soluble in each other when the components are just liquid, and are partially soluble in each other, when the components are just solid, alpha, represented by the letter A denotes a solid solution of component b in the component a, not necessarily constant, and beta, B, denotes a solid solution of the component a in the component I), also not necessarily constant. The line 1, 3, 4, 5 is the solidus line, and the line 1, E, 5 is the liquidus line. The lines 3, 9, and 4E, 10 are the solid solubility lines. The liquid solubility curve is shown by the dotted line F below the solidus line. This curve indicates that below the melting point temperature of the eutectic mixture of the components, the liquid eutectic combines with the individual components alpha and beta in solid form. At the left-hand side of the diagram t represents the freezing temperature of component a, t the freezing temperature of component b, and t the eutectic freezing temperature. The mixture of the two components indicated by the line X will be entirely liquid at any temperature above the liquidus line, but at temperatures between this line and the solidus line, the mixture will be partly liquid and partly solid, while below the solidus line it will be entirely solid. The eutectic mixture of the two components is indicated by the line Z at the point where the solidus line merges with the liquidus line. At the temperature t which is the lowest melting temperature of any mixture of these components, the mixture will be entirely liquid at all temperatures above the eutectic temperature 6", and it will be entirely solid at all temperatures below the eutectic temperature. Thus it is seen that there is but one phase present in the eutectic salt bath at any temperature except the temperature t, where the two phases, solid and liquid, will be present.

At the lower side of the diagram 1 have illustrated the compositionscales A and B for the relative proportions of the respective components in the mixture. When the temperature is lowered below'the eutectic temperature, the eutectic bath solution is immediately converted into the solid phase of the fused components a and Z2, as indicated by the melting point diagram.

In Figure 2 of the drawings 1 have diagrammatically illustrated the equilibrium and constitutional characteristics of another system in which the components a and b are not completely soluble in all proportions when just liquid, but form conjugate liquid solutions between certain limits of temperature and composition, and which are partiall soluble in each other when just solid. In t lis diagram the line 1, 2, 3, at, 5 is the solidus line, the line 1, 7, 6, E, 5 is the liquidus line, 3, 9 and 4, 10 are the solid solubility lines, and 7, 8, 6 is the liquid solubility curve.

As in the diagram in Figure 1, alpha represents a solid solution of component b in component a, while beta represents a solid solution of component a in component I), such solutions not necessarily being constant. E represents the eutectic mixture of the components.

t is the freezing temperature of the component a, t is the critical liquid solution temperature, i is the freezing temperature of component b, and Z is the eutectic temperature.

The mixture indicated by the line at is liquid above the liquidus line at the temperature t and is both solid and liquid between this temperature and the temperature t Another mixture of the components of diilerent relative proportions, as indicated on the scales A, B, and indicated by the line Y is one liquid above the temperature and two non-miscible liquids between the temperatures i and t and is both liquid and olid between the temperatures t and t and below the temperature 25 is entirely solid.

A mixture of the ingredients in true eutectic proportions indicated by the line Z will be one liquid above the eutectic temperature t", and will be entirely solid below that temperature. Thus the mixture of the components in these last referred to proportions has the lowest possible melting point, and above or below this melting point such mixture has but a single phase, namely, entirely li uid or entirely solid.

11 Figure 3 of the drawings I have diagrammatically illustrated the equilibrium and constitutional characteristics of the two components a and b, which form a compound, which compound is partially soluble inertia Referring to the diagram in Figure 3, to I which coordinates similar to those in Figures l and 2 are applied, the line 1, 2, 3, a, 5, 6, 7 is the solidus line, the lines 1, E, 12, 13, E 7 is the liquidus line, the- lines 2, 8 and 3, 9 represent the solubility curve of the solid component a in the solid compound, while the lines 5, 10, and 6, 11 represent 'the solubility curve of the solid component b in the solid compound. The line 12 14, 13 is the liquid solubility curve of the components and the compound.

Alpha represents a solid solution of component Z) in component a, and beta represents a solid solution of component a in component 6, such solutions being not necessarily constant.

Gamma represents a solid solution of the compound in the respective components.

t isthe freezing point of component a, 23 the critical solubility temperature of the compound in the components, t the freezing temperature of the compound, i the eutectic temperature of the component a and the compound, I." the freezing temperature of component b, and t the eutectic temperature of component 6 and the compound. E is the eutectic of component a with the compound, and E is the eutectic of component '5 with the compound.

It will be apparent from this diagram that the second eutectic E has the lowest freezing point, and that this mixture of the components at all temperatures above t is a single liquid of one constantcomposition, and at all temperatures below the eutectic temperature the mixture is a. solid of the fused components.

From a consideration of the foregoing in connection with the diagrams it will be seen that the eutectic mixture of the components will solidify at a constant temperature, and on reheating the components will liquefy at a constant common temperature. This temperature, as seen from the diagrams, has the lowest melting point, and, therefore, is the most fusible mixture of the components. At the eutectic temperature the eutectic is nonvariant, and remains of a constant composition. The mixture will remain in the form of a single liquid at all temperatures between the eutectic temperature and the volatilization point, and at any temperature above the eutectic temperature the components in solution have the greatest fluidity at those temperatures of any mixture of the components. At any temperature above the eutectic temperature the bath consists of but a single liquid, the composition of which remains invariable under all tem erature changes within the predetermine range above the eutectic temperature.

In order to prepare an eutectic mixture of the selected components the melting point curves of the components in different relative proportions are plotted, as indicated by the diagrams, and from these plotted curves the eutectic proportions of the components having the lowest melting point and the greatest fluidity through a relatively wide range of temperatures may be determined. The eutectic mixture of the two components is then made in these proportions and thoroughly melted. The correctness of the eutectic may be confirmed by the use of pyrometers to determine that solidification of the fused mixture occurs at a constant temperature, which is below the melting temperatures of the individual components. This complete fusion of the components and the complete transformation from the liquid to the solid phase without change in temperature is the only absolute proof of the fact that an eutectic mixture has been obtained.

I have heretofore referred principally to an eutectic mixture consisting of two coinponents, but it is to be understood that such an eutectic may also be comprised of three or more components. In a characteristic eutectic salt bath, which I have used principally in the heat treatment of metals, a mixture of 39% of NaCl (sodium chloride) and 61% Na CO (sodium carbonate) has been employed. These are comn'iercial salts, and the proportions in which they are used will vary slightly with the degree of impurity which may be present in the salt. It has been determined that these salts when thoroughly melted and fused will always remelt at a constant temperature of approximately 1190 F. This temperature likewise will vary slightly in accordance with the purity of the connnercial salts. The volatilization temperature of such salts is considerably above 1600 F. At any temperature above the eutectic freezing point only the liquid phase will exist in the fused eutectic mixture and no non-miscible liquid will form, thus giving a bath wnich is free from scum or sediment.

As above noted, it is also a peculiarity of such a solut ion that at all temperatures above the eutectic temperature or melting point, the solution has the greatest fluidity of any mixture of the bath components at thattemperature. The eutectic proportions in which the salt components are mixed requires the least degree of heat for rem'eltin while such proportions will remain constant and invariable throughout the mass upon solidiiicat ion. Consequently the bath, whether in liquid or in solid form, is always of the same chemical composition.

I have found in practice that such an eutectic mixture possesses numerous advantages in the heat treatment of metals. Thus in the treatment of steel having a carbon content of .40 of 1% the eutectic salt bath is heated to a temperature of approximately 1500 F., which is slightly above the critical temperature of the steel. \Yhcn the cold steel is immersed in the bath solution some of the salts will solidify on the surfaces of the steel. As the temperature of the steel gradually rises, the salts thereon will remelt, and when the steel throughout its structure reaches the temperature of the bath, the steel will assume a color approaching that of the bath, and become only faintly visible. It will, therefore, be seen that the eutectic bath solution ail'ords an indication to the operator that the metal has reached its criti -al temperature, at which point the transforma tion in the physical structure takes place, and it passes from an aggregate state intothe state of a solid solution, so that on cooling the hardening and refining of the steel results. It is to be noted that at this critical temperature of the metal, the bath solution will not volatilize, so that harmful vapors will not be evolved, as in the case of the use of lead baths.

It is also to be observed that since this eutectic mixture possesses a very high degreeof fluidity, and, therefore, low viscosity, there will be a rapid and uniform circulation of the heat in all parts of the bath, and when the metal is withdrawn from the solution, the film of the bath on the metal surfaces will be exceedingly thin, so that there is a. minimum loss of the bath material.

If the metal is to be hardened after it has reached its critical temperature, it is removed from the eutectic salt bath, and immediately immersed in a quenching bath of oil, brine or other suitable fluid, preferably a second eutectic fused salt, that is liquid below a temperature of 400 F. l/Vhen removed from the heating bath, the film of the bath solution which covers the surfaces of the metal, protects the same against oxidization. As visual indication is afforded, whereby the time when the metal reaches the temperature of the bath, may be delinitely known, liability of the metal being heated in different portions of its structure to relatively diliercnt temperatures, or for a longer time than is necessary is obviated. It is in this particular that the very highly viscous lead bath, commonly used in the art of heat. treating metals meets with the greatest objection. Owing to high viscosity of lead and other metallic baths, it is exceedingly diilicult, if not impossible to obtain and maintain a uniform heat temperature in all portions of the bath. 'lhcrefore, it is apparent that it may and docs'frequently occur in the use of baths of this character that one portion of the metal ob lit) &

ject being treated will be heated to one temperature, while another portion of the article will be heated .to a relatively different temperature.

From the above it will be seen that I secure by means of my improved eutectic salt bath a uniform heating of the metal being treated to the desired temperature without oxidization, and as a result thereof a final product of high quality, and having the desired attributes of hardness, high tensile strength or ductility, as the case may be, will be obtained. Obviously such accurately definite results with the attendant advantages ttb0\6 enumerated cannot be attained by the use of the ordinary mullie furnace or lead bath, as heretofore employed in the art.

It is to be understood that the example of an eutectic salt bath mixture to which I have above made reference, is merely suggestive, and that it is possible, to devise numerous other mixtures in which the working temperature may vary through a considerable range, such tei'uperature ranges, of course, differing for the-different chemical components of the individual baths, and as may be determined from practical experience to be most desirable for the particular object in view. For instance, for most purposes I prefer to use a mixture of calcium chloride and sodium chloride. A eutectic mixture of these two salts consists of 71% of the calcium chloride and 29% of the sodium chloride and has a melting point temperature of 925 F The possible disadvantage in the use of carbonates at the higher temperatures, namely, that they may dissociate -and in the reaction decarburize the steel, 1 have found in practice can be obviated by using a mixture consisting of an alkali and an alkaline earth chloride, such as that last referred to.

It is accordingly to be borne in mind that I do not consider myself at all limited in the practical use of my present invention to an eutectic mixture of the character which I have mentioned, or to mixtures having the precise melting or freezing points referred to, since the nature of the bath components and the relative proportions thereof, which determines the eutectic temperature, are susceptible of more or less variation. There fore, the privilege is reserved of adopting all such legitimate modifications as may be fairly embodied within the spirit and scope of the invention as claimed.

I claim:

1. A metal heat treatment bath of fused chemical salts characterized by a single liquid phase at any temperature above the freezing point, and a single solid phase at any temperature below the freezing point.

2. A metal heat treatment bath of fused chemical salts characterized by the presence of both the liquid and solid phases only at one predetermined temperature, and a single liquid phase at all temperatures above the said predetermined temperature.

3. A metal heat treatment bath of fused chemical salts consisting of a plurality of chemical components, and characterized by the fact that said components are present in but a single liquid phase at all temperatures above a predetermined temperature and do not form conjugate liquids in the bath.

4. A metal heat treatment bath consisting of a plurality of fused chemical components, said bath having a freezing temperature below the freezing temperature of any one of its individual components, and said bath at a temperature above the freezing temperature thereof forming a single non-conjugate liquid.

5. A metal heat treatment bath consisting of a lurality of fused chemical components, said ath characterized by the presence of the compenents therein in the form of a single liquid phase at all temperatures above the freezing temperature of the bath, and at such temperatures possessing maximum fluidity and low viscosity.

6. A metal heat treatment bath consisting of a plurality of fused chemical components of such predetermined proportions that the bath is characterized by a single liquid phase at all temperatures above the freezing temperature of the bath, and which temperature is below the freezing temperatures of the individual bath components, whereby the bath will possess a maximum degree of fluidity with relatively low viscosity at all tempera tures above the freezing temperature thereof.

7. A fused salt bath for treatment of metals at high temperatures consisting of a plurality of chemical salts in such predetermined relative proportions as to produce the greatest fluidity of the bath solution at any temperature above the freezing temperature of a solution of any mixture of the same components at the same temperature.

8. A fused salt bath for treatment of metals at high temperatures consisting of a plurality of salt components, said bath having a freezing temperature below the freezing temperature of the individual components thereof, and at all temperatures above said freezing temperature of the bath constituting a single liquid of the same components or compounds, and further characterized by the solidification of the components or compounds, when the temperature of the bath is reduced to the freezing point, into the form of a single solid having a uniform aggregate physical structure.

In testimony that I claim the foregoing as my invention, I have signed my name.

A. E. BELLrs.

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