EP0916435B1 - Casting furnace for automatic molding - Google Patents

Description

Loading molds on automatic molding lines is mainly carried out by means of gas pressurized furnaces inert, and fitted with electric induction heating. Although there is great variety of systems, this type of oven presents a common problem, associated with the maintenance of inductors for wear or protrusion of the refractory in the coil area.

The frequency and severity of these problems is brought to its point maximum when it comes to casting nodular cast iron. This material has the particularity of incorporating into its composition a quantity magnesium, which reacts with certain elements present in the refractory and clean metal. This produces compounds with a melting point. which adhere to the walls of the refractory, especially at the points hotter and greater metal circulation.

The inductor and its adjacent areas suffer specially from this problem, since the loading and pouring pipes even arrive to clog up. It is then necessary to carry out an exhaustive cleaning of the nozzles and channels of the inductor. Such an operation is painful, delicate and very laborious, since it also involves the risk of damaging the refractory that protects the inductor.

Furthermore, the coupling of the inductors introduces a factor of complexity in the system which prevents the furnace from being switched off, otherwise by running big risks. For this purpose, once put into operation, we can only emptying of metal to demolish the refractory and, in the best cases, change the inductor. This requires keeping the heating connected in permanence, which supposes in addition to the problem mentioned above, an important additional energy cost.

Given that the inductor has the sole function of retaining casting at approximately the molding temperature, and faced with the problems described, which the applicant has deduced that they arise from the use of inductors, it is proposed in this application to replace the induction heating with a heating produced by a high-power thermal plasma. The transfer thermal occurs in this case by means of the plasma arc generated by the electric current and stabilized with the same inert gas (argon or nitrogen) which can be the same as that used to pressurize the oven.

US Patent 5,291,940 shows a tipping bucket in which the energetic contribution to the liquid mass of the metal is made by a plasma torch which operates in the open air, and which acts directly on the metal creating agitation currents which originate pollution from the point of view metallurgical.

The application of the invention shows a casting furnace in which the energy supply to the liquid mass of the metal is made by a plasma torch, which operates in a closed hood, with gas under pressure, which is both plasma-generating and inert with respect to cast metal (for example N ₂ ).

If nitrogen is used as the plasma gas, nitrogen in gaseous form (N ₂ ) breaks down into plasma N, which provides energy to the metal, so the action of the plasma torch on the metal is indirect.

The presence of plasma N under pressure induces stabilization of the plasma arc essential for the quality of the cast iron (without pollution).

The casting furnace also has means for introducing and keep the gas under pressure in the oven.

It is planned to install a multiple torch system which allows the indistinct use of a transferred or not transferred arc. The application of one or the other model will depend on the type of material that we pretend to pour.

a) Le transfert énergétique se réalise au moyen d'un arc au plasma thermique à haute puissance.

b) Le four peut se vider librement, ce qui permet de supprimer les périodes de maintenance.

c) Il n'est pas nécessaire de procéder au nettoyage quotidien du four lorsque l'on coule de la fonte nodulaire. Le fait qu'il n'y ait pas de chauffage à induction a pour résultat qu'il ne se produit pas d'excroissances localisées de silicates de magnésium dans les bouches des canaux de chargement et de coulée.

d) L'équipement nécessaire pour le chauffage au plasma ne fait pas monter le coût des installations actuelles, celui-ci pouvant même, dans certains cas, être sensiblement inférieur.

e) Le préchauffage du four à vide peut s'effectuer à l'aide d'une lance thermique (gaz, oxygène).

f) La torche au plasma peut s'installer sur les fours actuels sans qu'il soit nécessaire pour cela de procéder à des réformes ou modifications de conception.

g) Si l'installation de la torche se fait sur le couvercle du four, ceci donnera une plus grande liberté de conception, en optimisant le rendement énergétique.

The introduction of the plasma gives the casting furnace of the invention the following particularities:

a) The energy transfer is carried out by means of a high-power thermal plasma arc.

b) The oven can be emptied freely, which eliminates maintenance periods.

c) It is not necessary to carry out daily cleaning of the oven when pouring nodular cast iron. The fact that there is no induction heating results in no localized growths of magnesium silicates in the mouths of the loading and pouring channels.

d) The equipment required for plasma heating does not increase the cost of current installations, which in some cases may even be significantly lower.

e) Preheating the vacuum oven can be done using a thermal lance (gas, oxygen).

f) The plasma torch can be installed on current ovens without it being necessary for that to carry out reforms or modifications of design.

g) If the installation of the torch is done on the cover of the oven, this will give a greater freedom of design, by optimizing the energy efficiency.

The present invention recommends a casting furnace for molding automatic improved, including in particular a pouring-nozzle hole of loading of the metal in the pre-inoculation state, a pressurization gas in the bucket and, at the very least, a metal pouring and loading nozzle molding and characterized by the fact that above the level of the metal in the grab are arranged in the casting furnace, at the very least, a plasma torch thermal stabilized preferably using the same gas as that used to pressurize the oven and at least one gas inlet / outlet, said gas or gases being both plasma / s and inert / s with respect to the cast metal.

Figure 1 is a vertical sectional view of the molding furnace already known with a superimposition of a practical realization of the object of the invention, which makes it easier to understand.

A conventional oven consists of a tap hole (1) for the loading of metal, for example at 1430 ° C., already treated, for example, with ferro-silicon-magnesium, which descends through the loading nozzle (2) to the bucket (3) where an inductor (4) is provided which provides energy maintaining the casting at the desired temperature.

The bucket (3) is closed by the cover (5) in which an inlet / outlet (11) of a pressurization gas (g), for example N _{2, is made} .

Under the pressure of the pressurizing gas, the flow rises by the pouring nozzle (6) to the molds (7). All in a refractory sheath.

At the junction (8) of the loading nozzle (2) and the bucket (3), as well as in the channels (9) around the inductor (4), salts tend to accumulate, for example magnesium silicates which impede circulation of casting and thermal transmission.

The invention eliminates the inductor (9) with all its consequences and introduces one or more plasma torches as a fundamental element (10).

To avoid energetic concentrations on the surface (s) of the poured into the bucket (3) the torch (10) is given a movement so that the plasma jet moves on said surface (s).

The inert gas (N ₂ , argon, etc.) used with the plasma can be the same as that of pressurization.

To increase energy saving, it was planned that when the oven, for example a Monday morning, will be cold, preheat with a lance thermal to oxycombustion, that this phase can physically replace the torch in space (10), that is to say, which is introduced through the upper orifice (12).

1) Préchauffage de la benne. En partant d'un four froid on procède au chauffage de la benne jusqu'à une température appropriée pour recevoir le métal liquide, par exemple 1200÷1300°C, en évitant ainsi les problèmes dérivés de l'inévitable choc thermique qui se produit durant le chargement. Ce chauffage se réalise au moyen d'une lance thermique à oxycombustion (GLP et oxygène).

2) Chargement du four. Une fois terminé le préchauffage, on remplit le four du métal déjà traité et en état de pré-inoculation. Durant la phase de chargement, la vanne de décompression de la benne reste ouverte (11 ou 12).

3) Coulée. Lorsque l'on a procédé au chargement, on ferme la vanne de décompression et l'on initie l'injection de gaz de pressurisation sur la voûte (étanche) principale du four. La pression oblige le métal à monter par les tuyères de coulée et de chargement jusqu'à atteindre le niveau de bain approprié pour la première coulée. A partir de cet instant, l'unité est prête pour le chargement des moules.

3 bis) Dans cette phase de coulée et au cas où les capteurs de température (11), ou éléments équivalents, détecteraient un besoin d'apport énergétique, on chaufferait la coulée à la torche au plasma (10) en employant du gaz de pressurisation (g) à ce moment à l'état de plasma.

The operation of the unit in casting operation would be as follows

1) Preheating of the bucket. Starting from a cold oven, the bucket is heated to a suitable temperature to receive the liquid metal, for example 1200 ÷ 1300 ° C, thus avoiding the problems derived from the inevitable thermal shock which occurs during the loading. This heating is carried out by means of a thermal oxycombustion lance (GLP and oxygen).

2) Loading the oven. Once the preheating is finished, the furnace is filled with the metal already treated and in the pre-inoculation state. During the loading phase, the bucket pressure relief valve remains open (11 or 12).

3) Casting. When loading is complete, the pressure relief valve is closed and injection of pressurization gas is initiated on the main (watertight) roof of the oven. The pressure forces the metal to rise through the pouring and loading nozzles until reaching the appropriate bath level for the first casting. From this moment, the unit is ready for loading the molds.

3 bis) In this casting phase and in the event that the temperature sensors (11), or equivalent elements, detect a need for energy input, the plasma torch casting (10) would be heated using pressurizing gas. (g) at this time in the plasma state.

The lighting of the torch (10) can be delayed as desired.

The arrangement and number of plasma torches (10), their nature, for example with graphite electrode (13), the holes on the cover (5), gas inlets and outlets, may be of one kind or another, without altering provided the invention as defined by the appended claims.

Claims (5)

1. Casting furnace for automatic moulding that includes, at least, one melting orifice-loading nozzle for metal in preoinculation state, a pressurisation gas in the vat and, at least, one casting and loading nozzle for moulding metal, which is characterised by the fact that on the level of the metal in the vat there is at least one thermal plasma torch in the casting furnace, stabilised preferably using the same gas as that used to pressurise the furnace, this gas or these gases being in turn plasmageneous and inert with respect to the cast metal and, at least, one gas input/output.
2. Casting furnace for automatic moulding according to previous claim, which is characterised by the fact that the casting furnace has a thermal oxicombustion nozzle on the level of the metal in the vat.
3. Casting furnace for automatic moulding, according to the previous claims, which is characterised by the fact that the energy jet from the plasma torch moves on the level of the metal.
4. Casting furnace for automatic moulding according to previous claims, which is characterised by the fact that the plasma torch is placed on the top of the furnace.
5. Automatic moulding procedure, which is characterised by the fact that in a moulding furnace described in claims 1, 2, 3 and 4, it is foreseen that, bearing in mind that the casting furnace is cold, it will be pre-heated with the thermal oxi-combustion fire slice to an appropriate temperature to receive the liquid metal, proceeding, if necessary later with the metal now in the furnace, to the temperature maintenance energy intake by means of the thermal plasma torch and also having the means to introduce and maintain the gas under pressure in the furnace.

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