Method Of Carbonizing Refractory Moulds

Abstract

Carbonizing refractory moulds by exposing the moulds to a carbonizing material while heated to a critical temperature range extending from the minimum temperature required for dissociation of the carbonizing material to a higher temperature which is below the lowest temperature at which the carbonizing material dissociates to form soot. The preferred mould structure is made by forming a slurry containing refractory material, a binder and a gelling agent which is then heated to form a crazed structure. The crazed structure is strengthened by the addition of a further binder and the strengthened structure is carbonized. Also disclosed are moulds formed from a mixture of a refractory material and a silicate solution which is hardened by means of carbon dioxide before being impregnated with the pyrolytic graphite. In another embodiment, a phosphoric acid bond is formed in addition to carbonizing by impregnating the mould with a furfuryl alcohol solution containing 2 percent by volume phosphoric acid and then heating to 1000.degree. C. or higher in an inert atmosphere.

Parent Case Text

This application is a continuation-in-part of my copending application Ser. No. 744,818 filed on July 15, 1968 and now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to moulds and cores (hereinafter referred to for convenience as moulds) that are used in casting, to the method of forming such moulds and to tools for electric discharge machining prepared therefrom.

It is known to use refractory materials for making moulds and in one widely-used process (known commercially as the Shaw process) wherein a slurry or slurry mixture containing refractory material, binders and gelling agents is poured into a pattern to form a gelled refractory mass. This mass is then stripped from the pattern and subsequently ignited to burn off all volatile constituents. The mould thus formed may then be heated in a high-temperature oven to drive off all remaining traces of moisture. It is found that a mould which is prepared as above described contains a microscopic network of cracks giving a crazed structure which offers a number of important advantages when the mould is used for casting. Thus, there is complete resistance to thermal shock (the cracks closing slightly to take up thermal expansion when molten metal is poured into the mould) so that hot metal can, if desired, be poured direct into cold moulds. Also, the mould possesses very good dimensional stability so that little or no change of size will occur during the casting operation. Moreover, a mould produced as above described will not emit any gas when the molten metal is poured (thus ensuring that the surface of a casting made in the mould will be free from gas holes or blemishes) whilst trapped air or gas emitted by the molten metal will be able to escape through the micro-cracks in the mould.

Another class of moulds is formed from a mixture of a refractory material and a sodium silicate solution which is hardened by means of a hardening agent such as carbon dioxide.

Some metals (such as for example titanium and titanium base alloys) are extremely reactive at high temperatures and in an attempt to mitigate the effects of such high reactivity it is also known to use moulds which are formed of graphite. Such moulds are, however, expensive or suffer from other disadvantages and accordingly the object of the present invention is to provide a relatively inexpensive mould which can be used in the manufacture of precision castings even though the latter are formed in highly reactive materials such as titanium and titanium base alloys.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a mould which is impregnated with pyrolytic graphite, that is a form of crystalline carbon, preferably by treatment in a carbonaceous atmosphere under conditions such that the rate of diffusion of the gas into and along the interstices of the mould and the rate of dissociation of the gas to form pyrolytic graphite are such that a deposit of pyrolytic graphite is formed on the surface and within the pores of the mould structure while leaving the latter pervious to gases trapped or evolved in the mould cavity during pouring and casting of the metal therein. The formation of substantial amounts of soot or amorphous carbon which would block the pores of the mould structure and render it impervious to air or gases is avoided.

In accordance with one preferred embodiment of the invention there is provided a mould which is formed from a slurry containing refractory material, a binder and a gelling agent and which is subsequently heated to form a crazed structure, the mould then being further strengthened by the application of a further binder and impregnated with pyrolytic graphite. Conveniently, the further binder is applied by dipping the mould into a liquid solution. The impregnation of graphite (in the form of pyrolytic graphite) is preferably achieved by placing the mould in a furnace into which a gaseous hydrocarbon is admitted. Additional graphite impregnation may be obtained by incorporating finely divided carbon, which can be pyrolytic, in the liquid binder solution. Conventional binders, e.g., organic silicates, are employed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more particularly described with reference to the accompanying drawing wherein;

FIGS. 1A and 1B are sectional elevations which depict one way of forming a mould,

FIGS. 2A, 2B, and 2C are sectional elevations which show successive stages of a modified method of forming a mould, and

FIG. 3 is a sectional elevation illustrating one method of impregnating a mould with pyrolytic graphite.

DETAILED DESCRIPTION

The moulds of the present invention are prepared by providing a pattern which may be formed of wax or of wood or metal or any other suitable material, the pattern having a shape similar to the finished form of the article or part which is to be machined. To said pattern is applied a binder such as ethyl silicate, a filler (which may comprise at least one refractory oxide or powdered graphite or a mixture of at least one refractory oxide and powdered graphite) and a gelling agent and conveniently these substances may be applied in turn by dipping the pattern into containers which contain the substances, the process being repeated until a layer of material of sufficient thickness is built up on the pattern. After a sufficient thickness has been built up as aforesaid, the pattern is removed and the resulting mould is fired to form a crazed structure in which is formed a network of small cracks. The crazed mould is then impregnated with graphite in the form of pyrolytic crystalline carbon and in one example this may be done by passing a mixture of nitrogen and benzene through a container in which the mould is placed and which is heated. The benzene then permeates the mould and by a process of pyrolysis deposits a coating of pyrolytic graphite on the surfaces and within the crazed structure of the mould. Instead of using benzene, use may be made of other substances such as furfuryl alcohol (which may contain finely divided carbon in solution) which has been refluxed for some time with dilute hydrochloric acid, the mixture being impregnated into the mould by immersing the latter in the liquid. The impregnated mould is then heated in air at a relatively low temperature to resinify the impregnant and after this the mould is heated in a vacuum or an inert gas to over 1000.degree. C. to carbonize the resin. Yet another carbonizing material can be formed by passing a mixture of air and propane gas over a suitable catalyst as will be described in detail hereinafter.

Instead of building up the mould as above described, the mound can be produced by casting a slurry in or around a pattern, the slurry containing a binder, a filler and a gelling agent. The proportion of gelling agent will control the setting time of the slurry and after the latter has set, the pattern is removed from the mould and then fired to form a crazed structure. The mould is then further strengthened by the application of a further binder and subsequently impregnated with graphite as above.

Referring firstly to FIG. 1, a mould is prepared from a suitable pattern 10 by mounting the pattern on a base 11 which is enclosed by a peripheral wall 12 and a slurry or slurry mixture containing refractory material (such as refractory oxides, or silicates, or carbon or a mixture of these substances), a binder and a gelling agent is poured over the pattern as illustrated in FIG. 1A. After gelling has taken place, the pattern, base and peripheral wall are removed to produce a mould 13 as shown in FIG. 1B and the mould is then fired to produce a crazed structure as previously referred to. To further strengthen it, the mould is now dipped in any convenient liquid binder solution such as a solution of phosphorus pentoxide in alcohol.

The mould can also be formed as will now be described in connection with FIG. 2. Initially, there is provided an oversize pattern 14 which is mounted on a base 15 inside a peripheral wall 16 and over this pattern is poured or rammed a mixture of a refractory material and a sodium silicate solution which is subsequently hardened by carbon dioxide to form a backing layer 17 which is made of relatively inexpensive materials, said layer being formed with a plurality of apertures 18 which may be provided by suitable cores or projections on the pattern 14. The oversize pattern 14, base 15 and peripheral wall 16 are then removed and the hardened backing layer 17 is placed over a second pattern 19 of "finished" size. A slurry or slurry mixture similar to that already described in connection with the method illustrated in FIG. 1 is then poured through the apertures 18 to form an inner layer 20, the pattern 19 being removed after gelling has taken place so that a composite mould 21 is formed as shown in FIG. 2C. Said mould is then fired to produce a crazed structure which is further strengthened by the addition of a further binder by dipping in any convenient liquid binder as before. The composite mould may, if desired, be washed after firing with hydrofluoric acid in order to remove any traces of silica from the mould surface.

Finally, the mould as prepared by either of the above-described methods is impregnated with pyrolytic graphite. FIG. 3 illustrates one preferred method of achieving this. Thus, the apparatus shown in FIG. 3 includes a furnace 22 having a base 23, said base having an inlet 24 for a carbonizing liquid, vapor or gas. The furnace 22 has an outlet 25 for said carbonizing substance and there is also provided a cap 26 which is formed of heat resisting steel and which is placed over moulds 27 (formed as above described) after the latter have been placed on the base 23, said cap projecting around its lower edge into a liquid gas seal 28 formed in the base 23. After said cap 26 has been placed in position, the furnace 22 is placed over the cap.

An inert gas, such as nitrogen is then passed through the apparatus until all the air inside has been ejected whilst the furnace (conveniently of an electrically powered kind) is energized to heat the moulds 27. The inert gas is then mixed with a carbonizing liquid, vapor or gas and when the moulds have reached a sufficiently high temperature (say 800.degree.-1100.degree. C.), the carbonizing substance will dissociate so that carbon will be deposited throughout the crazed structure of the moulds.

The minimum temperature required for dissociation of the carbonizing substance to form pyrolytic graphite is readily determined in practice for the specific material used. There is a critical range of temperatures upward from the minimum temperature required for the formation of the desired pyrolytic graphite, that is a crystalline form of carbon, in which the process of this invention is carried out and must not be exceeded. Just above the upper end of the critical temperature range, instead of dissociating to form graphite, the carbonizing material forms amorphous carbon in the form of soot which interferes with and substantially impairs the accuracy of the moulds in reproducing the desired castings. In addition, the formation of copious amounts of soot is not only objectionable for that reason but also because the soot tends to clog and close the pores of the mould thereby preventing the escape of trapped air or gas that may be evolved in the mould cavity during pouring and casting of the metal. Temperature gradients across the mould wall thickness which could lead to part of the mould structure being maintained above the threshold temperature for the formation of soot under the operating conditions are also to be avoided because the soot which forms in those parts of the mould tends to destroy its permeability. Particular care must be taken in the case of monolithic moulds that are heated from the outside that in spite of the temperature gradient that may exist across the mould wall, the temperature of the outer surface and that of the adjacent wall material does not exceed the critical temperature above which the objectionable soot is formed. In practice, the limiting temperature which must not be exceeded for any given set of operating conditions is readily determined by means of the substantial and readily recognized formations of soot.

In the operating range of the present invention above the minimum temperature required for the formation of pyrolytic graphite and below the higher temperature above which soot forms, the rate of diffusion of the carbonizing substance and the rate at which pyrolytic graphite is formed and deposited are such that crystalline carbon permeates the mould structure without blocking or destroying its permeability.

After sufficient carbon has been deposited, the supply of carbonizing substance is interrupted and the inert gas alone passes through the apparatus. The hot furnace is then removed and the moulds allowed to cool down to room temperature in the inert atmosphere inside the cap 26. Instead of using a furnace 22, the mould can be initially heated to a somewhat higher temperature (say 1000.degree.-1150.degree. C.) in air and then transferred to the base 23 while hot, the cap 26 being placed over the mould and a hydrocarbon gas such as methane passed through the cap and over the mould whilst the latter is cooling down. As was described, care is exercised in selecting the temperature to which the mould is heated to ensure that no part of the mould is at a temperature to cause the formation of unwanted soot.

In accordance with a further embodiment, impregnation by pyrolytic graphite may be carried out by placing the mould in a container which is connected by means of a two-way cock to a cylinder of nitrogen. Said cock is also connected to one flask of a pair of flasks which are connected together in series and which each contain benzene, tEe other flask being connected to the aforesaid container. Thus, the two-way cock can either be set so that nitrogen passes directly into the container bypassing the flasks, or alternatively so that nitrogen passes through the flasks (picking up benzene) before flowing into the container. Initially, the cock is set to pass nitrogen directly into the container so as to flush it, and the container is then charged into a furnace when the cock is turned to pass nitrogen through the benzene and thence into the container. The benzene deposits onto the mould and within its pores, by a process of pyrolysis, pyrolytic graphite and it is found that the mould thus produced is very well suited for the production of castings of titanium or titanium base alloys, it being understood that the pyrolytic graphite has unusually high temperature strength and erosion and corrosion resistance, and is substantially nonreactive with molten titanium.

In accordance with a further feature of this invention, a mixture of air and propane gas can be used to provide the carbonizing gas. A mixture made up of about seven parts by volume of air and one part propane gas is passed over a catalyst of nickel oxide (NiO.sub.2) on alumina (Al.sub.2 0.sub.3) at about 1000.degree. C. to produce a gas, the typical analysis of which is, in percent by volume, 25% CO, 28% H.sub.2, 2% CH.sub.4 and the balance N.sub.2 plus incidental impurities. To that gas is added propane gas in an amount equal to about 1 percent by volume of the final mixture which then forms the carbonizing atmosphere that is fed to the cap 26 as was previously described, the moulds treated being at a temperature high enough for the formation of pyrolytic graphite but below the temperature at which the undesired soot is formed.

In accordance with another embodiment of the invention, use may be made of a furfuryl alcohol solution containing about 2 percent by volume of phosphoric acid. After impregnating the mould with this solution, the mould is heated up to 1000.degree. C. or more in an inert atmosphere so as firstly to develop a phosphoric acid bond and secondly carbonize the furfuryl alcohol. As before the furfuryl alcohol/phosphoric acid solution may also contain finely divided graphite which may be pyrolytic.

As above mentioned, a solution of phosphorus pentoxide in alcohol may be used as a liquid binder solution but other solutions such as phosphoric acid in water or sodium silicate solution or hydrolized ethyl silicate solution may be used instead. Furthermore, finely divided carbon which can be pyrolytic may be incorporated into any of these solutions and when thin sections are to be cast in titanium such finely divided carbon will give a sufficient impregnation of graphite. When heavier sections are to be cast or when hot moulds are being used, however, impregnation of pyrolytic graphite is required in addition.

The process of the present invention does not require the high temperature and vacuum or pressure conditions of the prior art but provides for the impregnation of moulds and cores under substantially atmospheric pressure conditions. The moulds prepared by the process of the present invention have an even distribution of graphite impregnant which heretofore has not been obtainable.

In still a further embodiment, a modified form of mould is prepared from a mixture of a refractory material and a sodium or potassium silicate solution which is hardened by means of carbon dioxide or other hardening agent and then impregnated with graphite, e.g., pyrolytic graphite as described hereinabove.

Thus, the present invention will provide a relatively inexpensive mould which can be used for the casting of metals (such as for example titanium and titanium alloys) which are extremely reactive at high temperatures, the graphite impregnation mitigating the effects of such high reactivity. The moulds will of course be formed of relatively inexpensive material and will not provide the highest degree of accuracy or surface finish, but where such criteria are not required, then, as stated above, a mould formed in accordance with the present invention will provide an extremely effective and inexpensive casting aid.

Moulds prepared by the novel method of the present invention are particularly suitable for use as a tool or electrode in carrying out electric discharge machining.

Such machining makes use of the phenomenon of spark erosion in which erosion of metal takes place when a spark crosses between an electrode or tool and a workpiece, the electrode eroding is own shape into the workpiece. The electrode or tool to be formed in accordance with the method of the present invention can be used for example in machining a die or other metal parts which can either be cast or rough machined before being further machined by said tool or electrode.

The moulds of the present invention are thus provided with a network of finely divided particles of pyrolytic carbon and in some instances it will be found that the deposition of such pyrolytic carbon will be sufficient to form an electrically conducting structure which can be used as the aforesaid electrode or tool. In this case the aforesaid filler which is used together with the binder and gelling agent to build up the mould may be in the form of a ceramic material. As an alternative, however, and as previously mentioned particles of carbon can be used instead of the ceramic material so as to enhance the conductivity of the completed mould if the mould is to be used as a tool in electric discharge machining. It is also to be understood that the impregnation of the mould to provide a network of pyrolytic carbon as above described can be carried out in an convenient way which may vary from the methods indicated above.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

Claims

I claim:

1. The method of making a refractory mould having a deposit of pyrolytic graphite and substantially free of amorphous carbon in the form of soot, which comprises heating the refractory mould, while the temperature across the wall thickness of said mould is within a predetermined range exposing it to a carbonizing material long enough to impregnate said mould with pyrolytic graphite, said temperature range extending from a minimum temperature at and above which said carbonizing material dissociates to deposit pyrolytic graphite on said mould to a higher temperature which is below that at which said carbonizing material forms substantial amounts of soot on said mould.

2. The method set forth in claim 1 in which said mould is exposed to said carbonizing material at substantially atmospheric pressure, and said temperature range is further characterized by the fact that the rates of diffusion and of dissociation of said carbonizing material are such that pyrolytic graphite is deposited on said mould and in the pores thereof without sealing the latter.

3. The method set forth in claim 1 which comprises forming a slurry containing refractory material, a binder and a gelling agent, heating said slurry to form a crazed structure, and then strengthening said crazed structure by the addition of a further binder to form said refractory mould.

4. The method set forth in claim 3 which comprises heating said refractory mould in an enclosure into which the carbonizing material is introduced while said mould is at a temperature within said temperature range.

5. The method set forth in claim 4 in which said carbonizing material is a hydrocarbon.

6. The method set forth in claim 1 in which a carbonaceous material is entrained in a carrier gas selected from the group consisting of an inert gas and air to form said carbonizing material.

7. The method set forth in claim 1 in which the carbonizing material is made up of a hydrocarbon and phosphoric acid, and said mould is impregnated with said carbonizing material, whereby a phosphoric acid bond is formed in addition to the deposit of pyrolytic graphite.

8. The method set forth in claim 7 in which said hydrocarbon is furfuryl alcohol.

9. The method set forth in claim 1 in which the carbonizing material is benzene.

10. The method set forth in claim 1 in which said mould is prepared by mixing a refractory material with a silicate solution, shaping the thus formed mixture, and treating the same with carbon dioxide to harden it.

11. The method set forth in claim 1 in which the carbonizing material comprises the pyrolyzable reaction product of air and propane at an elevated temperature.

12. The method of forming a refractory mould impregnated with pyrolytic graphite and substantially free of amorphous carbon in the form of soot, which comprises forming a slurry containing refractory material, a binder and a gelling agent, heating said slurry to form a crazed structure, further strengthening said crazed structure by the application of a further binder to form a mould structure, and then impregnating said mould structure with pyrolytic graphite.

13. The method as set forth in claim 12 in which said mould structure is exposed to a carbonizing material comprising furfuryl alcohol to impregnate the same with pyrolytic graphite.

14. The method as set forth in claim 12 in which said mould structure is impregnated with graphite by exposing it to a carbonizing material which comprises the pyrolyzable reaction product of air and propane at an elevated temperature.

15. The method set forth in claim 12 in which said mould structure is heated to a temperature within a predetermined range and while at said temperature is exposed to a carbonizing material long enough to form a deposit of pyrolytic graphite on said mould and in the pores thereof without sealing the latter, said temperature range extending from a minimum temperature at and above which said carbonizing material dissociates to form said pyrolytic graphite to a higher temperature which is below that at which said carbonizing material forms substantial amounts of amorphous carbon.

16. The method as set forth in claim 15 in which said carbonizing material is benzene.

17. The method set forth in claim 12 in which in impregnating said mould structure with said graphite a carbonizing material is used which is made up of a hydrocarbon and phosphoric acid, whereby a phosphoric acid bond is formed in addition to impregnating said mould structure with graphite.