IE902105A1

IE902105A1 - A method of and an apparatus for degassing and maintaining a low hydrogen content in liquid aluminium alloys while they are being carried in ladles

Info

Publication number: IE902105A1
Application number: IE210590A
Authority: IE
Inventors: Gerard Laslaz; Pierre Laty; Alain Marguier
Original assignee: Pechiney Aluminium
Priority date: 1989-06-13
Filing date: 1990-06-12
Publication date: 1991-01-02
Also published as: IE902105L; EP0403406B1; DE69007343D1; FR2648154A1; ATE103012T1; AU626425B2; CA2018491C; ES2050411T3; EP0403406A1; IE67886B1; DE69007343T2; FR2648154B1; AU5697790A; CA2018491A1

Abstract

Process and apparatus for degassing and maintaining a low hydrogen content in liquid aluminium alloys during their transport in ladles. The process is characterised in that a stream of gases which are inert towards aluminium is circulated in the ladle at a low and controlled flow rate and the said gas(es) is (are) then removed from the ladle. The apparatus consists of at least one ladle (1) and at least one bottle (4) containing a gas under pressure, these being joined together by a circuit (5) comprising a pressure reducer (6), a flow- meter (7) and a gas injection system, the said ladle being equipped in its upper part with a controlled gas leakage system. This invention makes it possible to reduce and avoid the gassing of metal during its transport and to provide the users with an aluminium alloy which is suitable for moulding articles which are metallurgically healthy. <IMAGE>

Description

Α METHOD OF AND AN APPARATUS FOR DEGASSING AND MAINTAINING A LOW HYDROGEN CONTENT IN LIQUID ALUMINIUM ALLOYS WHILE THEY ARE BEING CARRIED IN LADLES The present invention relates to a method of and an apparatus for degassing and for maintaining a low hydrogen content in liquid aluminium alloys while they are being carried in ladles.

It is known to transport aluminium alloys in the liquid state in closed metal ladles coated on the inside with a refractory material from the place where they are prepared to the place where they will be poured into moulds to produce parts of predetermined form. Generally, these ladles have a capacity of several tonnes and are carried in twos on lorries. Sometimes, transportation takes several hours, during the course of which the metal may come in contact with traces of water emanating either from the surrounding damp atmosphere which enters the ladles through faulty seals or from refractory linings which have become impregnated with moisture after draining of the ladle and which have dried insufficiently prior to being filled, or from slag which accumulates during the course of transportation in ladles which have not been properly cleaned.

The effect of this moisture is to produce a phenomenon referred to as gassing, that is to say of reacting with the aluminium and of giving rise to the formation of hydrogen dissolved in the liquid metal in the form of gas bubbles of relatively small dimensions which will persist in the member as porosity. - 2 Well, a man skilled in the art knows that the high content of dissolved hydrogen adversely affects the metallurgical well-being of members produced therefrom and results especially in low levels of resistance to mechanical fatigue and to low ductility.

Where parts are being produced which require a sound metallurgical well-being, it is therefore necessary to ensure that the liquid metal is not allowed to come in contact with moisture while it is being transported.

Indeed, attempts have been made to avoid the presence of humidity in ladles. For example, prior to filling» there has been a more intense drying of the inside of the ladles with systematic removal of slag. Furthermore, greatly improved sealing-tight systems have been produced and a non-renewed argon atmosphere has been created above the metal.

However, no-one has successfully resolved /the gassing problem. Indeed, it has been found that a metal which has undergone extensive degassing prior to filling, that is to say one which ..has a hydrogen content of less than 0.12 ppm, might well at random contain up to 0.3 to 0.5 ppm of hydrogen after being carried.

This gassing appears to be more frequent in summer without anyone being able to establish any correlation between the quantity of hydrogen and the hygrometric level of the atmosphere.

Faced with this problem, the Applicants have sought and found a solution which resides in a method whereby it is possible to alleviate the regaining of moisture originating from the refractory lining or from residual slag by the liquid aluminium alloys while they are being - 3 transported in closed ladles over more or less long distances, characterised in that throughout transportation and until such time as the metal is poured, a flow of a gas which is inert vis-a-vis the aluminium is caused to circulate in the ladle at a low and controlled rate, the gas being simultaneously evacuated from the ladle after it has been in contact with the metal.

Thus, the invention resides in introducing into the ladle a gas which is inert in respect of aluminium, that is to say a gas which is not of itself capable of reacting with aluminium. Preferably, it may be nitrogen or argon. Obviously, it is essential that the gas be without of moisture so that it cannot be a gassing vehicle. This gas is conveyed to the ladle and circulates between an inlet and an outlet according to a low and controlled flow rate. Preferably, this rate of flow is comprised between 0.02 and 0.2 Nra^ per tonne of alloy and per hour, because a lesser rate of flow has an inadequate effect on the degassing, while a greater rate of flow proves to be superfluous.

During the course of its passage through the ladle, the gas penetrates the metal bath but it is possible to have a simDle sweeDine of the surface because by virtue of the vibrations experienced by the ladle while it is being transported, there is renewed contact between the gas and the metal which makes it possible to ensure an almost complete exchange with the mass of metal.

Under these conditions, the hydrogen which is capable of forming on account of the random presence of moisture is entrained with the flow of gas and is discharged to the outside. - 4 Thus, a relatively low level of gassing is maintained and in any case this level is compatible with the desired quality of metal.

Furthermore, the possibility of improving during transportation the quality of a metal which was imperfectly degassed at the outset hae likewise been established.

Indeed, the practice of degassing is well-known in industrial installations. But it is carried out in fixed ladles using rates of flow which are far greater and in any case in excess of 0.5 Nm^ per tonne and per hour.

The invention likewise relates to an apparatus for carrying out the above-described method.

This apparatus is characterised in that it consists of at least one ladle and at least one bottle of pressurised gas, the two being connected by a circuit comprising an expansion valve and a flow meter, the said ladle being equipped in its upper part with a controlled gas leakage system.

Thus, the apparatus according to the Invention results from the combination of a ladle and a gas distributing system comprising a source: the bottle and means of ensuring circulation of the 3aid gases at a relatively low pressure and according to a controlled and initially low rate to the interior of the ladle and then to the exterior.

The ladle is of a type identical to that normally used on road-going vehicles, that is to say a steel casing of generally cylindrical or cylindrical-conical form lined on the inside with a refractory material and having an - 5 aperture towards the top on which a relatively sealingtight cover is fitted. Combined with this ladle is at least one bottle containing a pressurised gas which may be liquefied or compressed and which is fixed to the ladle or on the ladle mounting frame. From this bottle, a circuit emerges which comprises an expansion valve which makes it possible preferably to obtain a gas pressure of a few hundredths of MPa, a safety valve which makes it possible to avoid any overpressure in the ladle and a flow meter which makes it possible to monitor rates of flow comprised preferably between 0.1 and 1 Nm^ per hour. This circuit is connected to the ladle in various ways: - either it comprises a tube which discharges into the ladle through the cover and the end of which is at least 3 cm from the surface of the metal. Thia arrangement makes it possible for the ga3 to sweep the metal which is maintained in agitation by the vibrations of the vehicle; - or it comprises a tube which is immersed in the metal so that the gas bubbles through the metal and makes it possible to make up for any insufficient agitation caused by the vibration alone.

But it is also possible to place one or a plurality of porous stoppers on the side walls or in the bottom of the ladle, and to which the said circuit is connected.

Similarly, the gas circuit may also be connected to a hollow shaft rotor such as that described in French Patent No. 2512067 which is immersed in the metal and which disperses the gas within it.

Whatever may be the manner of introducing the gas, the ladle is equipped with a controlled system of discharging - 5 the introduced gag which may possibly be charged with hydrogen. This system may, for example, be a piping system, possibly calibrated, and possibly provided with a valve having an adjustable aperture.

This apparatus functions as follows: as soon as the ladle has been filled and degassed by means of conventional means of injecting neutral gas at a high rate of flow (lance or rotor), the bottle is opened, the expansion valve is regulated and the flow meter is adjusted to obtain the desired pressure and rate of flow. The quantity of gas available has been previously determined with a view to maintaining the flow of gas throughout the entire period of transportation and the time needed to supply the pouring installations.

The invention will be more clearly understood from the attached sheet of drawings which comprises a Fig. 1 diagrammatically showing in section the upper part of a ladle equipped with a tube which discharges above the level of the metal and a Fig, 2 with a same illustration of a ladle fitted with a tube which plunges into the liquid aluminium alloy.

More precisely, in Fig. 1 it is possible to see the ladle 1 containing the liquid aluminium alloy 2, closed at the top by a sealing-tight cover 3. Placed along the side wall of the ladle is a bottle 4 of compressed argon connected to the ladle by a circuit 5 comprising an expansion valve 6, a flow meter 7, a gas feed pipe 8 and a discharge pipe 9.

Fig. 2 shows the same elements as Fig. 1. - 7 The invention may be illustrated by means of the following examples of application: 100 deliveries of moulding aluminium alloys containing mainly silicon as an added element were carried out, half in prior art ladles and half in ladles according to the invention, of which likewise one half were fitted with a plunger tube while the others were fitted with a tube discharging into the ladle at 2.5 cm from the surface of the metal. These ladles each contained 6 tonnes of metal and the transportation time was close to 5 hours.

With the prior art ladles, it was found that 5% of the deliveries arrived in a gassy state, that is to say they contained more than 0.3 ppm hydrogen.

In contrast, in the ladles according to the invention and in which a flow of argon was caused to circulate at a rate of 0.15 Nm3. tonne! per hour!, and whatever’was the type of injection, none of the deliveries r arrived in a gassy condition, the hydrogen content being below 0.15 ppm Therefore, this invention makes it possible to reduce and avoid gassing of the metal during its transportation and to supply users with a product suitable for moulding parts having a good level of metallurgical soundness.

Claims

1. PATENT CLAIMS

1. A method which makes it possible to alleviate the regaining of moisture from the refractory lining or from residual slag by liquid aluminium alloys while they are being transported in closed ladles over more or less long distances, characterised in that throughout transportation and Until such time as the metal is poured, a flow of gas which is inert vis-a-vis the aluminium is caused to circulate at a low and controlled rate of flow, the said gas being simultaneously extracted from the ladle after it has been in contact with the metal.

2. A method according to claim 1, characterised in that the gas belongs to the group consisting of nitrogen and argon.

3. A method according to claim 1, characterised in that the rate of flow’is comprised between 0.02 and 0.2 Nm 3 per tonne of alloy and per hour,

4. A method according to claim 1, characterised in that the flow of gas is caused to circulate at the surface of the metal.

5. A method according to claim 1, characterised in that the flow of gas is caused to circulate in the liquid aluminium alloy.

6. An apparatus for carrying out the method according to claim 1, characterised in that it consists of at least one ladle (1) and at least one bottle (4) of pressurised gas - 9 which are connected to each other by a circuit (5) comprising an expansion valve (6) and a flow meter (7), the said ladle being equipped in its upper part with a controlled gas escape system.

7. An apparatus according to claim 6, characterised in that the circuit comprises a tube which discharges into the ladle above the level of the metal,

8. An apparatus according to claim 6, characterised in that the circuit comprises a tube which is immersed in the liquid aluminium alloy.

9. An apparatus according to claim 6, characterised in that the circuit comprises a porous stopper placed on the wall of the ladle.

10. An apparatus according to claim 6, characterised in that the circuit comprises a rotor with a hollow shaft.

11. Apparatus for degassing and for maintaining a low hydrogen content, in liquid aluminium alloys substantially as hereinbefore described with reference to the accompanying drawings.

12. A method for degassing and for maintaining a low hydrogen content, in liquid aluminium alloys substantially as hereinbefore described with reference to the accompanying drawings.