US2992088A - Slurry casting of high explosive content compositions - Google Patents

Description

5 Sheets-Sheet 1 .wfl

ATTORNEYS A. BURKARDT ETAL July 11, 1961 SLURRY CASTING OF HIGH EXPLOSIVE CONTENT COMPOSITIONS Filed Dec. 15, 1955 SOLIDS N S RNM ORE O O 1 A 5 6 5 N C E M VU. WES I O A R U H H 1 Wm m L LW 0 3m ww w T F I O M O M 1 T F 0 m.

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RV 6W ATTORNEYS July 11, 1961 L. A. BURKARDT ET AL SLURRY CASTING OF HIGH EXPLOSIVE CONTENT COMPOSITIONS Filed Deb. 15, 1955 5 Sheets-Sheet 3 INVENTORS LOHR A. BURKARDT WILLIAM S. MC EWAN ATTORNEYS y 1961 A. BURKARDT ETAL 2,992,088

SLURRY CASTING OF HIGH EXPLOSIVE CONTENT COMPOSITIONS Filed Dec. 15, 1955 5 Sheets-Sheet 4 J I jg INVENTORS LOHR A. BURKARDT WILLIAM S, MCEWAN ATTORNEYS July 11, 1961 1.. A. BURKARDT ETAL 2,992,088

SLURRY CASTING OF HIGH EXPLOSIVE CONTENT COMPOSITIONS Filed Dec. 15, 1955 5 Sheets-Sheet 5 INVENTORS LOHR A. BURKARDT WILLlAM 8- MC EWAN BY j/dd/QS.

ATTORNEYS 2,992,688 Patented July 11, 1961 2,992,088 SLURRY CASTING OF HIGH EXPLOSIVE CONTENT COMPOSITIONS Lohr A. Burkardt and William S. McEwan, China Lake,

Calif., assign-ors to the United States of America as represented by the Secretary of the Navy Filed Dec. 15, 1955, Ser. No. 553,395 2 Claims. (Cl. 5220) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to slurry casting, more particularly, it relates to a process for slurry casting eutecticforming mixtures.

The casting of single component liquid melts is subject to a number of disadvantages. For example, the castings formed by this procedure are subject to excessive shrinkage during cooling, with the result that cavitation is produced in the finished casting. This seriously alfects the physical properties of the casting. In addition, former casting techniques required long cooling times for the castings. A further disadvantage attendant to prior pro cedures is the fact that decided crystal orientation results in the castings, with resultant undesirable effects on the properties of the product.

Attempts to hasten the cooling time of the casting by the use of cold probes and other expedients to conduct heat away from it have not been satisfactory. The expedient of introducing solid pellets of the material being cast into the melt to accelerate crystallization has resulted in the format-ion of large crystals with resultant weakened castings.

It is, therefore, an object of this invention to provide a method for producing from fusible materials castings which are not subject to excessive shrinkage upon cooling, and which have a high density, random crystal orientation, and in which the crystals are of small size.

It is a further object of this invention to provide a process for slurry casting eutectic-forming mixtures at a state in which the solid to liquid ratio of the two-phase mixture is high.

By the term cast as used herein is meant the forming of a molten substance into a definite shape or state by allowing it to cool in a mold.

The invention is best described by reference to the following explanation taken in connection with the accompanying drawings hereby made a part of this application and in which:

FIG. 1 is a graph in which temperature is plotted against the contraction occurring from 80 to 25 C. in a casting made from a TNT-4 percent naphthalene slurry;

FIG. 2 is a graph showing the calculated percent heat remaining to be dissipated at various temperatures in TNT-4 percent naphthalene slu-rries;

FIG. 3 is a graph showing a plot of viscosity against solids content of a TNT-15.5 percent picric acid slurry as it cools;

FIG. 4 is a freezing-point diagram of the hypothetical system A+B;

FIG. 5 is a graph in which the solid-liquid ratio of a binary system of fixed initial composition of the components A and B of FIG. 4 is plotted against temperature;

FIG. 6 is a graph showing a plot of temperature against initial composition of components A and B of the system of FIG. 4 required to produce a certain ratio of liquid to solid in the system;

FIG. 7 is a showing of a casting made of trinitrotoluene liquid melt;

FIG. 8 is a showing of a casting made from a TNT- 15.5 percent picric acid melt according to the process of the invention;

FIG. 9 is a showing of a casting made from a TNT- 15.5 percent picric acid melt according to a modification of the invention;

FIG. 10 is a showing of a casting made from a TNT-4.2 percent naphthalene melt made by the process of the invention;

FIG. 11 is a showing of a casting made from a TNT-4.2 percent naphthalene melt by a modification of the process of the invention;

FIG. 12 is a showing of a casting made from a TNT-3.5 percent anthracene melt by the process of the invention;

FIG. 13 is a showing of a casting made from a TNT-4 percent trinitro-m-xylene melt by the process of the invention;

FIG. 14 is a showing of a casting made from a TNT-4.2 percent naphthalene melt which was not stirred during the casting; and

FIG. 15 is a showing of a broken edge of a composite casting comprising a TNT-naphthalene composition and a TNT-anthracene composition.

While the invention is applicable to the production of castings from any solids-liquid system of fusible eutecticforming components, it will be illustrated by its application to the casting of mixtures containing a major percentage of an explosive, in this case TNT (trinitrotoluene).

The advantages of slurry casting eutectic-forming mixtures 'at a state of high solid to liquid ratio is apparent from an examination of a representative TNT-additive system, as respects contraction and heat dissipation. The additive gains realized by the additive-slurry casting method are illustrated by a comparison of the volume change and heat content of an additive-TNT slurry with those of a liquid TNT30% solid-TNT slurry. The solid TNT in a 30% solid-TNT slurry has undergone 27% of the total contraction that occurs when an equal weight of liquid TNT at C. cools to 25 C. A TNT slurry containing 4% of naphthalene as the additive behaves as shown in FIG. 1. This slurry contains 44% of solids at 72 C. and, by virtue of this amount of solidification, 43% of the normal contraction of the system in cooling from 80 to 25 C. has taken place. The shrinkage of such a casting is 22% less than that of the liquid TNT-30% solid-TNT slurry casting.

With 30% of solid TNT in a liquid TNT slurry, the heat to be dissipated by the casting is reduced to 16% of the total heat to be dissipated by a completely liquid TNT slurry cast at 80 C. and cooled to 25 C.

80 (AH=n.AH.-nf o al) Where H =the change in heat, n=the number of moles, H =the heat of fusion, and C -=the heat capacity at constant temperature. The behavior of a TNT4% naphthalene system with respect to heat content is shown in FIG. 2. A casting made of this material will contain 70% of the heat contained by a liquid TNT casting at the freezing point of TNT. The heat to be dissipated by a TNT4% naphthalene casting at 72 C. is 8 3% of that of a liquid TNT30% solid TNT casting at 80 C. A corresponding reduction in the cooling time of such castings should be realized by using other TNT-additive systerns.

The advantages pointed out above of slurry casting eutectic-forming mixtures at a state of high solid to liquid ratio, emphasize the desirability of a reproducible process for controlling the solid to liquid ratio of a two phase system of a given percentage composition, so that can be effected at the most practicable solid to liquid ratio. Attempts to cast melts of a single component in a two-phase state have not been successful at high solids concentration because solidification proceeds at constant temperature so that temperature cannot be used as an indication of solids content and no other simple indication of solids content is available, making control of the solid to liquid relationship difficult. However, two-phase systems containing soluble additives do not solidify isothermally until the eutectic composition isreached. Up to this point, the solid to liquid relationship may be ascertained from measurement of the temperature and knowledge of the starting composition. It has been found that control of the temperature of such a system permits control of the solid to liquid ratio in the system. This stems from the fact that a true equilibrium is formed between the solid phase and the liquid phase at a given temperature above the eutectic temperature.

The advantages of using soluble additives in two-phase systems are not limited to the feature of controlling the solid to liquid ratio by temperature control in casting, as additives may be selected which enhance the properties of the cast product. For example, in the case of explosives, an additive may be selected which is itself an explosive so that the efficiency of the cast explosive is increased. Additives may be chosen which have a beneficial effect on physical properties of the final casting, such as grain size, grain growth, tensile strength, and modulus of elasticity.

Various factors will determine the choice of additives depending upon the material itself and the desired properties of the cast product. For example, in the case of explosives, a major factor in the choice of additives, for practical use of the additive-slurry technique, is the melting point of the explosive-additive eutectic. In a casting of explosive, the presence of a eutectic mixture with a melting point lower than possible storage temperatures may lead to exudation difiiculties during storage. This places a lower limit on the melting points of the eutectic mixtures formed by suitable additives. For possible exposure to high temperature storage, in the case of TNT systems, a lower limit of 70 C. (159.8 F.) on the melting point of eutectic mixtures would appear adequate to prevent liquid exudation.

Ordinarily, it is undesirable to reduce appreciably the concentration of explosive, since this causes corresponding changes in the explosive properties of the material. Therefore, the concentration of additive required for the formation of eutectic mixture should be low. It is desirable that the additive form a eutectic with the major explosive component in concentrations less than an approximate maximum of 10%. Ordinarily, the maximum amount of additives that would be employed would be half of the amount required by the eutectic composition, or 5% if additive is required for the eutectic mixture. Additives meeting this last requirement will generally have melting points considerably higher than that of the major explosive component, and will tend to yield eutectic mixtures having melting points close to the melting point of the major explosive component. This is particularly important in the case of TNT which has a melting point only 10 C. above the minimum service melting temperature. Additives that have explosive properties may be used in higher concentrations, proportional to their explosive power. Other factors which will influence the selection of additives for practical use are compatibility with containers, chemical and physical stability, availability, cost, and effect on such physical properties as tensile strength and modulus of elasticity. Except in cases where difiiculties such as abnormal viscosities may occur, the casting technique permits the substitution of one additive for another to meet the requirements of a given case. FIG. 3 illustrates a case in which the high viscosity of the supernatant liquid of a TNT-picric acid system influences the selection of a casting temperature. To meet a wide range of specifications it is desirable to have available the properties of a large number of possible additives, their freezing-point diagrams with the given explosive, and their eifect on the physical properties of castings of the explosive. Freezingpoint diagrams of some of the additive-TNT systems thus far studied have been reported in Technical Memorandum 965, U.S. Naval Ordnance Test Station, China Lake, California, Freezing Point Curves for Some Binary Systems Containing 2,4,6-Trinitro-toluene, by L. A. Burkardt, D. W. Moore, and W. S. McEwan, December 1952.

The concentration of additive in an optimum casting composition will be influenced by other factors also. In cases where the additive has a beneficial effect on other physical properties of the final casting, such as grain size, grain growth, tensile strength, and modulus of elasticity, the amount of additive used may be varied by the quantity required to produce this effect. If a minimum casting temperature with a given additive is the principal concern, the amount of additive used will approach the maximum that can be used without generating more eutectic material than is required for satisfactory casting of the slurry. If, on the other hand, a minimum of a given additive is desirable, the minimum will then be fixed by the highest casting temperature that will allow the production of satisfactory castings. The process of controlling the solid to liquid ratio of a eutectic-forming mixture through temperature control is applicable to all fus ible systems and may be examined in general terms for determining the optimum casting temperature, once the particular additive and its optimum concentration are determined. Consider a system, as is shown in FIG. 4, composed of two compounds, A and B, which forms a simple eutectic mixture. If a composition lying between 0 and 20% B, such as CD (5%), is permitted to cool from the molten state, solid A will begin to form when the point D on the curve AL is reached. Solid A will continue to form as the material cools further, and the composition of the liquid phase will change the temperature as indicated by the curve AL. On reaching the point L, the temperature will remain constant until the remaining liquid has solidified. If the temperature is held constant at some point along AL, the ratio of solid to liquid in the system is held constant. This ratio, at any point along the curve AL, may be determined from the simple relation: y=u/ v where y=the weight fraction at a given temperature, v=the weight fraction of additive in the mixture which begins to freeze at the given temperature, and u=the weight fraction of additive in the starting composition.

The solid-liquid ratios vs. temperature obtained for several initial compositions containing 5%, 10%, and 15% of B (CD, EF, and GH respectively in FIG. 1) are shown in FIG. 5. If these three compositions are cooled to 72 C., the 5% composition will contain 71.1% of solid; the 10% composition, 42% of solid; and the 15% composition, 13.3% of solid. If on the other hand, one considers a fixed solid-liquid ratio such as 45% of solid, this will occur at 76.8 C. for the 5% composition and at 71.4 C. for the 10% composition. With the 15% composition the mixture reaches the eutectic freezing point before the solid to liquid ratio rises to 45%; hence, this ratio cannot be obtained with this composition.

In the usual case, it is desirable to have the minimum amount of liquid present which is required for successful casting. With this amount of liquid determined and the amount of additive determined by consideration of its effects on the final casting, as set forth above, the optimum casting temperature may be obtained from a curve similar to that shown in FIG. 6, in which the starting composition in terms of the particular additive is plotted against the temperature at which the system contains the desired amount of liquid.

The following examples, with the exception of Examples 1 and 6, are illustrative of the invention but ture were melted together and cooledwhile being stirred vigorously. Casting was done when the slurry reached a temperature 1 to 3 degrees above the eutectic melting temperature. By maintaining the temperature of the slurry at the proper point of the casting could be delayed for as long as necessary, though excessive delay tended to increase the crystal size of the solid phase. According to the second method a quantity of TNT and additive, in the proper proportions to be liquid at the casting temperature, was melted and held in a constant temperature bath at 1 to 3 degrees above the eutectic melting point. Preheated, finely divided TNT, which had been screened to obtain uniform particle size, was added to the melt. After thorough mixing of the solid TNT and the melt, the casting was done in heated molds. The optimum solids content for casting for all of the illustrative examples was chonsen to be'about 45%. The freezing point diagrams for most of the TNT-additive systems used in the examples are given in the above referred to Technical Memorandum.

EXAMPLE 1 As a blank for comparative purposes, a liquid TNT casting was made in the same molds as the slurry castings. This casting is shown in FIG. 7. Examination of this photograph shows rather marked shrinkage effects and an oriented structure of large crystals.

EXAMPLE 2 TN T-picric acid castings The eutectic mixture of the TNT-picric acid system contains 31% picric acid. Half of this amount of picric acid, 15.5%, would yield a composition that would contain 50% solid at the eutectic melting point and somewhat less solid at a temperature slightly above the eutectic melting point. With the choice of a starting composition containing 15.5% picric acid and using a casting temperature of 60 C., which is three degrees above the eutectic melting point of 57 C., application of the formula previously given shows that the slurry will contain 43.6% solids at the casting temperature. A casting made as described is shown in FIG. 8. The casting has been broken to show the internal structure. Examination shows that the crystals are randomly oriented in a matrix of eutectic material.

In accordance with the second method of casting, fairly large crystals of TNT heated to 65 C. were added to an equal amount of molten TNT-picric acid eutectic at 65 C. After thorough mixing at 65 C., the slurry was cast. A broken casting made in this manner is shown in FIG. 9. The crystals present random orientation.

EXAMPLE 3 TN T-naphthalene casting The eutectic mixture for this system contains 8.4% of naphthalene by weight. Therefore, to obtain a TNT- naphthalene composition that will contain 50% solids at the eutectic temperature, 4.2% of naphthalene will be required. Since the eutectic mixture freezes at 70.5 C., casting of the slurry is done at about 73 C. A fractured edge of the casting produced is shown in FIG. 10. The crystals in the casting are randomly oriented.

Castings were made by the addition of solid TNT preheated to 74 C., to an equal weight of TNT-naphthalene eutectic held at 74 C. After stirring in the solid TNT, the resulting slurry was cast. A broken edge of one of this castings is shown in FIG. 11. No crystal orientation can be detected in this casting.

EXAMPLE 4 T NT-anthracene castings Examination of the freezing-point diagram of this sys- 6 tem indictes that the eutectic composition contains 7.1 wt. percent of anthracene. Therefore, 3.5% of anthracene is required to produce a composition containing 50% solids at the freezing point of the eutectic mixture (73 C.). The slurry was cast at about 75 C. A fractured edge of a casting made in this manner is shown in FIG. 12. The physical structure of the casting is the same as that attained by using picric acid or naphthalene as an additive.

EXAMPLE 5 T N T-trinitro-m-xylene castings sition containing 50% solids at the eutectic freezing point (75 C.). Casting of trinitro-m-xylene slurries was done at 77 C. A broken edge of one of the castings prepared as above is shown in FIG. 13. As in the previous systems, a randomly oriented fine-grain structure is obtained.

EXAMPLE 6 This example was performed to provide a blank for comparative purposes. The casting shown in FIG. 14 was made from a liquid-melt TNT-additive system containing 4.2% of naphthalene. This casting was made with no control of the solids content of the system and without stirring. It will be noted that except for the presence of areas of fine-grained eutectic material, this casting has the same features as those of the liquid-melt TNT casting shown in FIG. 8.

EXAMPLE 7 Composite casting In this example a composite casting was made by permitting a TNT-naphthalene composition to freeze, after which a TNT-anthracene composition was cast beside the first portion. A fractured edge of the resulting casting is shown in FIG. 15. All indications from handling and breaking this casting suggest that the bond between the two parts of the casting had the same strength as the two parts of the casting.

Densities of some of the castings prepared above were measured by displacement of water saturated with TNT. No attempt was made to control the amount of air present in the melts from which the samples were made. The densities of the additives vary among themselves and diflfer from TNT. To compare castings of TNT alone with those of TNT systems, it is necessary to take into account these differences. This may be done by comparing the experimental density with that obtained by calculation using the formula:

Observed Calculated Calculated Casting density, density, density,

g./cc. g./cc. percent TNT-15.5% picric acid 1. 619 1. 670 96. 94 TN I-3.6% Anthracene 1. 615 1. 635 98. 77 TN T-3.9% triuitro-m-xylene 1. 593 1. 652 96. 43 TNT-4% naphthalene 1. 582 l. 625 97. 35

As seen from the above results, castings made by the process of the invention have a randomly oriented, fine: grained structure. Since the process is. based purely on physical properties it may be applied to other fusible materials provided the necessary technical data. about the additive-material system is available. More complicated systems may be employed if phase diagrams of such systems are available. The method provides a convenient manner in which to introduce additives. for the control of the physical properties of systems incorporating explosives, as well as the typeexplosive produced. The additives thus may serve a dual, purpose.

The higher densities, randomly oriented finer-grain structure, reduction in shrinkage, and reduction in cooling time obtained by this process are not provided by prior processes. An important feature of the process is control over the solid-liquid ratio in the slurry by simple temperature control. Slurries may be held for some time if the proper temperature is maintained. The effectiveness of the present process is due largely to the fact that a true equilibrium between the solid phase and the liquid phase of a eutectic-forming mixture is formed at a given temperature. This is in contrast to two-phase, single component systems in which such an equilibrium does not form.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention'may be practiced otherwise than as specifically described.

Whatv is. claimed. is:

1. The. process of making castings. of trinitrotoluene which comprises forming a two-phase two-component mixture by melting together trinitrotoluene and a material from the class consisting of picric acid, naphthalene, anthracene and trim'tro-rn-xylene, the amount of said material in said mixture being up to about one-half the amount required to form a eutectic with trinitrotoluene; cooling said mixture to a temperature from about one to about three degrees above the eutectic point of the mixture; and casting the melt at said temperature.

2. The process of claim 1 in which a portion of the trinitrotoluene in solid form heated to said temperature is added prior to casting to a mixture of the remainder of the trinitrotoluene and material maintained at said temperature.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Air Force Technical Order TD. 39 B-.-13, June 1945; page 41.

Metal Industry, October 25, 1935; pages 411-413.

Claims (1)

1. THE PROCESS OF MAKING CASTINGS OF TRINITROTOLUENE WHICH COMPRISES FORMING A TWO-PHASE TWO-COMPONENT MIXTURE BY MELTING TOGETHER TRINITROTOLUENE AND A MATERIAL FROM THE CLASS CONSISTING OF PICRIC ACID, NAPHTHALENE, ANTHRACENE AND TRINITRO-M-XYLENE, THE AMOUNT OF SAID MATERIAL IN SAID MIXTURE BEING UP TO ABOUT ONE-HALF THE AMOUNT REQUIRED TO FORM A EUTECTIC WITH TRINITROTOLUENE; COOLING SAID MIXTURE TO A TEMPERATURE FROM ABOUT ONE TO ABOUT THREE DEGREES ABOVE THE EUTECTIC POINT OF THE MIXTURE; AND CASTING THE MELT AT SAID TEMPERATURE.

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