FR2648154A1 - METHOD AND DEVICE FOR DEGASSING AND MAINTAINING LOW HYDROGEN CONTENT IN LIQUID ALUMINUM ALLOYS DURING THEIR TRANSPORT INTO POCKETS - Google Patents

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.

Description

METHOD AND DEVICE FOR DEGASSING AND MAINTAINING A LOW CONTENT

  HYDROGEN IN LIQUID ALUMINUM ALLOYS DURING THEIR

TRANSPORT IN POCKETS

  The present invention relates to a method and a device for degassing and maintaining a low hydrogen content in alloys

  liquid aluminum during transport in pockets.

  It is known to transport aluminum alloys in the liquid state in closed metal pockets and internally coated with a refractory material from the place where they are made to where they will be cast in molds. to give pieces of predetermined shape. These pockets generally; a capacity of several tons and are transported by two on trucks. The transpocrt sometimes lasts several hours during which the meta. may come into contact with traces of water from either the surrounding humid atmosphere entering the pockets through the defective seal area or refractory lining which is wet-impregnated after draining of the bag and which have been insufficiently dried before filling is still dirt that accumulates during transport cycles in

my pockets cleaned.

  This moisture has the effect of producing a so-called gassing phenomenon, ie reacting with aluminum and giving rise to the formation of dissolved hydrogen in the liquid metal in the form of relatively small gas bubbles. that will remain in the form room

of porosities.

  However, one skilled in the art knows that a high content of dissolved hydrogen 25. deteriorates the metallurgical health of the parts that come from it and leads in particular to low resistance to mechanical fatigue and

reduced ductility.

  It is therefore necessary in the case of the development of parts requiring good metallurgical health to ensure that the liquid metal is not

  not brought into contact with moisture during transport.

  Certainly, actions have been taken to avoid the presence of moisture.

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  in the pockets. Thus, before filling, further drying of the inside of the pockets was carried out and systematically removing the dirt. Highly advanced sealing systems have also been developed and created a non-renewed atmosphere of argon above the metal. However, the gassing problem has not been solved. Indeed, it has been found that a metal which has undergone very thorough degassing before filling, that is to say having a hydrogen content of less than 0.12 ppm, could very easily contain after transport to 0 3

at 0.5 ppm hydrogen.

  This gassing seems to be more frequent in summer without any correlation being established between the quantity of hydrogen and the hygrometric degree of the atmosphere., Faced with this problem, the plaintiff has sought and found a solution which consists of a process for degassing and maintaining a low hydrogen content in liquid aluminum alloys during their transport in closed bags, characterized in that hydrogen is removed as its formation by circulating in the pocket a flow of inert gas with respect to aluminum following a flow rate

  weak and controlled, and then said gas removed from the pocket.

  Thus, the invention consists in introducing into the pocket an inert gas with respect to aluminum, that is to say which is not susceptible by itself.

  to react with aluminum. It can be preferably nitrogen or argon.

  It is obviously necessary that this gas is free of moisture so as not to be a gassing vehicle. This gas is fed to the bag and flows between an inlet and an outlet at a low and controlled flow rate. Preferably, this flow rate is between 0.02 and 0.2 Nm3 per ton of alloy per hour, because a lower flow rate has an insufficient effect on degassing.

  while a higher flow is superfluous ..

  During its passage in] a pocket, this gas enters the metal bath but it can be limited to a simple licking of the surface because because of the vibrations of the pocket Jors of its transport there is a renewed contact between I gas and the metal that allows for an exchange

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2648 154

  almost complete with the mass of metal.

  Under these conditions, the hydrogen likely to form due to the random presence of moisture is carried along with the gas flow and discharged to the outside. This maintains a relatively low gassing rate and in any case

  compatible with the desired metal quality.

  In addition, there was also the possibility of improving during the

  transport the quality of an imperfectly degassed metal at the start.

  Admittedly, the practice of degassing is known in industrial installations. But it is carried out in fixed pockets using much higher flow rates and in any case greater than 0.5 Nm per

ton and hour.

  The invention also includes a device for implementing the method

described above.

  This device is characterized in that it consists of at least one bag and at least one bottle of pressurized gas interconnected by a circuit comprising an expander and a flow meter, said bag being equipped at its upper part with a controlled gas leakage system. Thus, the device according to the invention results from the combination of a pocket and a gas distribution system comprising a source: the bottle and means for ensuring a circulation of said gases under a relatively low pressure and following a flow.controlled and weak first

  inside the pocket and outwards.

  The pocket is of a type identical to that usually used on road vehicles, that is to say a steel casing of generally cylindrical or cylindro-conical shape, lined internally with a refractory material having an opening upwards. on which is placed a relatively tight lid. This bag is combined with at least one bottle containing a pressurized gas which can be liquefied or compressed and which is attached to the bag or to the fixing frame of this bag.

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2648 1 5 4

  last. This circuit comprises a circuit comprising an expander making it possible to obtain a gas pressure of a few hundredths of a Pa, a safety valve to prevent any overpressure in the pocket and a flowmeter providing the pressure. Controlling flow rates preferably between 0.1 and 1 Nm3 per hour. This circuit is connected to the pocket in different ways: either i] comprises a tube which opens into the pocket through the lid and whose end is less than 3 cm from the surface of the meta]. to ensure licking by the gas of the metal kept agitated by the vibrations of the vehicle; - Or, it includes a tube which pgegege in the metal so that the gas bubbles in the metal and can provide for a possible

  insufficient agitation created by the only trepidations.

  But, o. can also place one or more porous plugs on the walls

  lateral or cu bottom of the pocket and which is connected said circuit.

  Similarly, the gas circuit can also be connected to a hollow shaft rotor such as that described in French Patent No. 2512067 which plunges into the

  metal and ensures a dispersion of the gas within it.

  Whatever the type of gas introduction, the bag is equipped with a controlled leakage system of the introduced gas and which may have been loaded with hydrogen. This system may be for example an optionally calibrated piping that can also be provided with an adjustable opening valve. This device operates in the following way: as soon as the bag has been filled and degassed using conventional high-volume neutral gas injection devices (lance or rotor), the bottle is opened, the regulator and the flow meter are adjusted to obtain the desired pressure and flow rate. The amount of gas available has been previously determined in order to maintain the flow of gas throughout the duration of transport and time.

  necessary to feed the casting plants.

  The invention will be better understood using the drawing board

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  attached, which comprises a figure 1 schematically showing a sectional view of the upper part of a ladle equipped with a tube which opens above the level of the metal and a figure 2 following the same representation of a pocket equipped with a tube which plunges into the liquid aluminum alloy. More specifically, in Figure 1. there is the pocket I containing the liquid aluminum alloy 2, closed on the top by means of a tight cover 3. Along the side wall of the ladle is placed a compressed argon bottle 4 connected to the ladle by a circuit 5 comprising an expander 6, a flow meter 7, a pipe 8

  gas introduction and exhaust piping 9.

  In Figure 2 we find the same elements as in Figure 1.

  The invention little. It is contained in the following examples of the application: Alumina alloys of molten alloy containing mainly silicon as additive have been made half in prior art bags. half in pohes according to the invention, which were also half equipped with a plunger tube, the others with a tube opening into the bag at 2.5 cm from the surface of the metal. These pouches each contained 6 tons of meta] and the duration of the

  Transportation was close to 5 hours.

  With prior art pouches, it was found that 5% of deliveries were gassed, that is, they contained more than 0.3 ppm of hydrogen. On the other hand, in the pockets according to the invention and in which

3 -1

  circulated a flow of argon at a rate of 0.15 Nm3on hour -l and what. whatever the type of injection, none of deliveries

  has not been gassed, since the hydrogen content is less than 0.15 ppm.

  This invention thus makes it possible to reduce and avoid gassing of the metal during its transport and to provide users with a product suitable for

  molding of parts having good health urgent]] metall.].

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Claims (10)

  1. A method of degassing and maintaining a low hydrogen content in liquid aluminum alloys during their transport in closed pockets, characterized in that I eliminate hydrogen as and when its formation by circulating in the pocket a flow of inert gas with respect to the aluminum at a low and controlled flow, and then   said gas is removed from the pocket.   2. Method according to claim 1 characterized in that the gas   belongs to the group consisting of nitrogen and argon.   3. Process according to claim 1, characterized in that the flow rate is   between 0.02 and 0.2 Nm7 per ton of feed and per hour.   4. Method according to claim 1 characterized in that one makes   circulate I flow of gas to the surface of the metal.   5. Method according to claim 1 characterized in that I made   circulating the gas flow in the liquid aluminum alloy.   6. Apparatus for carrying out the process according to claim 1, characterized in that it consists of at least one bag (1) and at least one bottle (4) of pressurized gas connected together. by a circuit (5) comprising a pressure reducer (6) and a flowmeter (7), said pocket being equipped at its upper part with a controlled gas leakage system. 7. Device according to claim 6 characterized in that] .e circuit   comprises a tube which opens into the pocket above the level of the meta.l.   8. Device according to claim 6 characterized in that] .e circuit   is a tube that dips into the liquid aluminum alloy.   9. Device according to claim 6 characterized in that the circuit   comprises a porous plug placed on the wall of the pocket. BAD ORIGINAL   10. Device according to claim 6 characterized in that the circuit includes a hollow shaft rDtor. BAD ORIGINAL