DE1257364B - Graphite coating for refractory molds and process for its manufacture - Google Patents

Description

GERMAN 'MrWWi' PATENT OFFICE

EDITORIAL

German KL: 31 b1 -1/06

Number: 1257 364

File number: G 43428 VI a / 31 b1

J 257 364 filing date: April 23, 1965

Open date: December 28, 1967

The invention relates to a graphite coating for the inner surfaces of porous, for precision casting, in particular for the casting of titanium and its alloys, suitable shell molds and a process to produce the graphite coating.

In casting technology, the use of graphite molds for relatively small, for example off Titanium, zirconium and hafnium existing reaction metals as well as for metals that are difficult to melt and their compounds, for example based on molybdenum, niobium, chromium, tungsten, tantalum and the like, known. Such graphite forms are made of powder or granular graphite and one Carbonisable binders are produced, which are molded together in a mold and then hardened will. In addition to ground graphite, carbon black is also used to line the molds, either is powdered onto the wall of the mold or painted in a suspension. It is also known the casting mold by machining out of graphite blocks or by pressing in of model shapes in the graphite. The mold surface was coated with carbon black also proposed to vaporize naphthalene and deposit it on the inner surface of the mold.

In all these cases, however, graphite with an isotropic structure is used, and not only relatively quickly eroded from the mold wall, but also easily dissolves and thus at least partially is fed to the molten metal. This property of isotropic graphite makes it special the production of precision castings impossible because the casting molds by the destruction of the Graphite coating lose their dimensional accuracy.

The object of the invention is to create a casting mold whose graphite lining is more durable. This is achieved in that the graphite coating is made from anisotropic, pyrolytic graphite. Crystalline, anisotropic graphite material with a high degree of orientation, called pyrolytic graphite, has an unusual strength at high temperatures and has a high erosion resistance as well as other excellent thermal and electrical properties. Although the pyrolytic graphite and the isotropic graphite used in the known casting molds are two carbon configurations of the same color, there are considerable differences in their physical and mechanical properties and their grain and crystal orientation. For example, the grain orientation ratio for ordinary graphite coating for refractory molds and
Process for its manufacture

Applicant:

General Electric Company,
Schenectady, Ν. Υ. (V. St. A.)

Representative:

Dipl.-Ing. Μ. Light, dr. R. Schmidt,
Dipl.-Wirtsch.-Ing. A. Hansmann
and Dipl.-Phys. S. Herrmann, patent attorneys,
Munich 2, Theresienstr. 33

Named as inventor:

Wilbur Horace Schweikert, Cincinnati, Ohio
(V. St. A.)

Claimed priority:

V. St. v. America May 6, 1964 (365 255)

commercial, isotropic graphite 3: 1. The anisotropic, pyrolytic graphite, however, possesses an orientation ratio between 100: 1 and 2000 and more to 1. Therefore, the pyrolytic Graphite strand-like crystals that are generally free from cracks and oriented in a single plane are. This graphite is made by dissociation of a hydrocarbon gas at very low Pressures - less than 1 atm - and produced at temperatures up to about 2500 ° C. The rate of precipitation depends on the type of hydrocarbon, the temperature and the level of vacuum present. The result of it on a surface The resulting precipitate consists of a series of needle lamellas that are essentially parallel to the surface on which they are deposited.

Their special orientation means that the heat is spread along the layers or lamellas moved many hundreds of times faster than through the layers. As a result, the pyrolytic Graphite has good thermal conductivity in one direction, while acting as an insulator in the other works. The crystals of ordinary isotropic graphite are e.g. B. by pressure or by a binder randomly placed so that thermal conductivity is a function of material thickness is. Pyrolytic graphite is considerably harder and

709 710/465

offers greater resistance to corrosion and erosion than ordinary graphite. These and other differences between pyrolytic graphite and ordinary graphite are in the Literature has been described in detail.

High quality precision casting molds made from refractory materials have long been used with the help of known wax or pressure casting technology. It was therefore considered desirable while the execution of this casting process to cover the mold surface with very fine graphite powder, in the Opinion that after the mold is fired, there will be enough graphite left for the molten titanium or other reactive metals poured into the mold, a non-reactive one Surface forms. In this precision casting technique, however, it is usually worked so that the raw form in a high temperature flame in air, e.g. B. at 982 ° C, is fired to ensure that all of the carbonaceous material has burned off the wax model. Under these Conditions, the graphite on the surface of the wet mold would be oxidized as well. That's why The invention enables the creation of a pyrolytic graphite shell on the inside a precision casting mold that reacts or difficult to melt metals in a practical, can be shed in an effective and economical way. It also has showed that the pyrolytic graphite compared to the ordinary isotropic graphite from the fused Metal is less attacked. Indeed, through the use of the known wasotropic Graphite molds when casting titanium objects cause severe carbon pollution established. It now emerged, what is important for the casting, that through the use of pyrolytic Graphite only a relatively small amount due to the metal compound poured into the mold Carbon is absorbed.

It has been found that the deposition of pyrolytic graphite on the inner surfaces of precision casting molds made, for example, by the lost wax process, can easily be achieved by means of known devices used in the invention, at pressures between 1 mm Hg and less than 1 atm and at temperatures between 704 and 1204 ° C. The pressure and temperature values depend on the dissociation temperature and the deposition rate of the hydrocarbon and also on the weakening point or the usable strength range of the mold binder. In the drawing, a type of apparatus which is relatively simple and has been used with success is shown schematically in partial section. A mold 10, which consists of a conventional bowl-shaped ceramic mold and surrounds a normal body of wax, stands on a graphite base plate 12. The cover 14 of the mold is ground flat to produce good contact with the base plate. A putty-like material, such as one made from a mixture of graphite and a resin binder, is puttied at 16 as a seal around the outside of the hood, creating a gas-tight fit so that the gas introduced through inlet 18 does not escape from the hood can escape and flow around the outside of the mold.

The mold is surrounded by a graphite support 20 which completely encloses the mold. The carrier 20 is located within an induction coil 22, and both form a device which serves to heat the mold and the gas by thermal radiation. A graphite cover 24 and an outlet opening 26 are arranged over the mold. The whole arrangement is located in a vacuum chamber, which is not shown and which is connected to a vacuum pump ίο . The mold 10, which is used to produce an object 28 shown in dashed lines, has channels which open towards the hood 14 and which form ventilation nozzles 30 .

In use, a hydrocarbon gas flows into the mold through the inlet opening 18 of the hood 14 and rises through the sprue 32 of the mold. From the pouring funnel, the gas can flow freely into the mold space and, before it exits again through the ventilation openings 30 , pyrolytic graphite is deposited on the entire surface of the mold. Unused gas then flows around the outside of the mold and back through the top vent 26 into the vacuum chamber.

example 1

The shell mold 10 shown was made by the wax process from a commercially available ceramic shell molding material, which consisted of silicon powder, silicon sand and a binder of sodium silicate. Such molding materials are commercially available. Often other binders, e.g. B. alginates or other silicates, e.g. B. ethyl silicate used. Other bowl shapes of the ceramic type, e.g. B. those based on aluminum or zirconium oxide are available and are generally used in the die casting process.

In this particular example, the shell shape, made using common, commercially available model wax, was dewaxed and dried at 871 to 982 ° C for 1 hour. Originally, the parts of the mold shown in the drawing as vents 30 were attached to the hood 14.

However, in order to provide a vent for the hydrocarbon gas that was later to be pumped through the mold during pyrolysis, this part of the mold was separated from its point of contact with the hood. The hood 14 was then sealed with the usual mold dipping jacket. In this way, the shown vents 30 resulted in the mold.

The complete shell mold was then placed in the vacuum furnace shown and a vacuum of 0.4 mm Hg was created after the chamber had been flushed with argon. The mold was then heated to about 1038 ° C., after which acetylene gas was introduced through the inlet opening 18 , which flowed through the mold and left it again through the vent openings 30 to be sucked through the outlet opening 26 into the vacuum chamber. The amount of acetylene gas flowing through was about 1 cbm / hour. at a pressure of 3 mmHg. After maintaining these conditions for about 1 hour, a pyrolytic graphite coating between 0.05 and 0.1 mm thick had uniformly deposited on the inner wall of the mold.

Claims (2)

It was interesting to observe that, because of the porosity of this type of refractory mold, the cracked gas initially flowed through the pores of the mold as a result of the low negative pressure outside the mold and permeated the mold material with a soot-like carbon. This fact was found by successively dividing the mold, and it shows that a porous mold is more suitable than a non-porous one. The porous shape allows a small amount of gas to flow through the pores of the mold until each surface section is sealed with soot, whereupon a closed jacket of pyrolytic graphite is deposited. As soon as some pores are sealed, the gas will find other pores or passageways until all pores of the surface are completely filled with soot. A uniform jacket of pyrolytic graphite is then produced in the manner described above. It is therefore preferable to use an at least somewhat porous mold as it is particularly advanced in terms of reliably producing a more uniform casing. The mold of this example was used for vacuum casting a melt of commercially available pure titanium. No reaction and no visible carbon uptake were found between the mold and the casting. Example 2 In this example, the same molding material and the same apparatus as are described in Example 1 were used, but instead of raw acetylene gas, mainly methane was introduced through the inlet opening 18. Because of the higher dissociation temperature required for the pyrolysis of this type of gas, it was necessary to raise the temperature of the mold to over 1177 ° C. The hourly amount of gas introduced was about 0.4 cbm at a pressure of about 3 mm Hg. After the mold had been kept at the stated temperature for about 1 hour, it was cooled and covered with a jacket of pyrolytic graphite of a thickness between 0.05 and 0.1 mm. For the pyrolytic deposition of anisotropic graphite, many types of hydrocarbon gases can be used, the limiting factors according to the invention on the one hand being the maximum temperature that the molding material, in particular the binder, can withstand and still having sufficient strength for use in casting , on the other hand in the temperature required for dissociation of the gas for pyrolytic purposes. Acetylene gas has been used successfully for pyrolytic graphite deposition up to 704 ° C and methane up to 899 ° C. The amount of deposition for each hydrocarbon gas changes with dissociation temperature and pressure. Gas pressures below 1 mm Hg have proven impractical for the manufacturing process because of the long deposition times. Recently, an improved pyrolytic graphite containing a small percentage of such elements as boron, which serve to increase strength, has been produced. It can be assumed that pyrolytic graphite jackets of this type can also be produced using the novel casting process. Patent claims: 1. Graphite coating over the inner surfaces of porous, for precision casting, especially for the Casting of titanium and its alloys, suitable shell molds, characterized in that that the graphite coating consists of anisotropic, pyrolytic graphite. 2. A method for producing the graphite coating according to claim 1, characterized in that that the shell mold is heated to a temperature between 700 and 1200 ° C under vacuum and at the same time a hydrocarbon gas under a pressure of 1 to 10 mmHg the interior of the mold is passed through, which dissociates and opens at this temperature pyrolytic graphite is deposited on the inner surfaces of the mold. Considered publications:
German Patent No. 641 029;
Swiss Patent No. 71 367;
British Patent No. 673,320;
Römpp: »Chemistry Lexicon«, 1962, Vol. II,
L 1987. 1 sheet of drawings 709 710/465 12.67 © Bundesdruckerei Berlin