EP0100272B1 - Process and apparatus for the production of castings, and castings produced by this process - Google Patents

Description

The present invention relates generally to a new technique for heat treatment of molded parts and relates more particularly to a device for the manufacture of these parts.

It has been known for a very long time to manufacture metal or other molded parts, by introducing a molten material into a suitable mold where said material will form, after cooling, the molded part which is desired. More specifically, in the case where a metal alloy is used, the production of the molded parts from this alloy has hitherto been carried out in two stages, generally.

The alloy, in the molten state, is first cast in the mold. It solidifies there, then gradually cools there to room temperature, after which the demolding or unhooking operation is carried out.

Then, the molded part generally having to undergo a heat treatment, it is consequently advisable to reheat it at a temperature higher than room temperature to make it undergo an appropriate thermal cycle and which is a function of the alloy considered and the qualities of the part molded that we want to obtain, it being understood that the temperature range of the heat treatment adopted will always be between ambient temperature and the solidus temperature of the alloy considered.

However, such a two-step process for manufacturing a molded part from a metal alloy has many drawbacks. In fact, the step of cooling the alloy in the mold is almost never controlled or programmed. The cooling takes place naturally and in a more or less random manner, because it is in fact always linked to the thermal inertia of the mold, so that, in total, the parts thus molded by such random cooling, can already present structural or other defects.

• La pièce moulée doit être, après refroidissement, extraite du moule puis traitée par un appareillage séparé pour la soumettre au cycle thermique en question, ce qui, comme on le comprend, exige une manutention particulière et un temps de main d'oeuvre coûteux.

In addition, the heat treatment or reheating phase of the part in order to subject it to a chosen thermal cycle constitutes an additional operation which has many drawbacks, some of which will be listed below:

• The molded part must be, after cooling, extracted from the mold and then treated by a separate apparatus to subject it to the thermal cycle in question, which, as it is understood, requires special handling and costly labor time.

In addition, the heat treatment consisting in reheating the molded and cooled part naturally requires considerable heat energy since it is necessary to bring the part to the temperature at the start of the thermal cycle, that is to say to a temperature very high which, without necessarily reaching it, approaches the temperature which was at the end of solidification in the mold. As a result, we are far from meeting the energy saving requirements that we are looking for today.

In addition, the rise in temperature of the alloy as well as the thermal cycle in general require a relatively long time so that not only the heat energy to be supplied must be significant, but that the finished part remains expensive.

Methods of manufacturing steel blocks are already known as described for example in document DE-C - 764 264, and which consist in applying a heat flux on the steel in the liquid state in the mold, it is ie well before solidification in said mold. These methods cause a movement of the steel bath to homogenize it and remove the impurities, which has nothing to do with the heat treatment which must be carried out on the molded part as explained above.

Furthermore, there has been described in document US-A-1 417 638 a process for treating molded parts according to which the cooling of the part in the mold is controlled. The implementation of this process is carried out using a mold fitted with shells supplied with current and which are in direct contact with the molded part. Thus the molded part closes the electrical circuit and is fully heated with a uniform temperature distribution in said part.

Thus, as it is understood, it is not possible to carry out with such a mold a chosen and perfectly controlled heat treatment of the molded part or of such or such part of this part after it has solidified in the mold.

The present invention aims to solve this problem by proposing a new device for manufacturing molded parts, from a liquid called to solidify, such as for example any alloy, which device also allows savings to be made. considerable energy and manufacturing time, and to give the molded part all the qualities sought by the fact that one can choose and impose on this part or on one or more distinct parts thereof, in the mold, the or the most appropriate cooling laws that can easily be varied at will and within significant limits.

To this end, the invention relates to a device for the manufacture of molded parts and of the type comprising an electric heating means of a molded part and solidified in a mold in which this heating means is incorporated, characterized in that this the latter consists of at least one electric heating resistor or at least one inductor completely embedded or fitted inside the mold to impose on at least part of the solidified part in the molding space an appropriate heat treatment.

It is therefore understood that by acting on the inductor or the electrical resistance embedded in the wall of the mold and therefore without contact direct with the molded part, it is advantageously possible to vary at will and within significant limits the cooling law which it is desired to impose on one or more distinct parts of the part solidified in the mold and which will be heated by thermal conduction from of the surface of the solidified part in contact with the wall of the molding space.

A variant of this device is characterized in that it comprises an inductor arranged around the weight collar at the upper part of the mold, and / or at least one inductor provided at the level of the mold supply system, while a cooling circuit is arranged inside this mold.

According to another characteristic, the aforementioned electrical resistance or the inductor is arranged inside the mold, over the entire height of the latter or over only part of this height to affect for example a hollow or projecting part, molding space.

According to another characteristic, the electric heating resistance consists of conductive particles of current distributed uniformly or not in the mass of the mold according to a density capable of allowing the passage of a current supplied by a source of electrical power connected to said mold.

According to another characteristic, the aforementioned particles consist of metallic particles based on metals or alloys, oxide particles such as for example metallic oxides, metallic silicate particles, or else a mixture of the aforementioned particles.

According to yet another characteristic, the particles distributed in the mass of the mold can be more or less dispersed or else agglomerated together in a more or less compact form.

It will also be specified here that a mineral or organic binder can be added to the mass of the mold to maintain the structure of the particles in this mass.

However, other characteristics and advantages of the invention will appear more clearly in the detailed description which follows and which refers to the appended drawings, given solely by way of example, and in which:

Figures 1 to 3 show three temperature curves (T ° C) as a function of time (t) (the temperature and time scales being the same for the three curves), and in which: Figure 1 illustrates the cooling of an example of steel being solidified in a mold; FIG. 2 illustrates the heat treatment applied to this same steel, after the cooling illustrated in FIG. 1; and FIG. 3 illustrates the controlled heat treatment carried out according to the principles of the invention, this treatment being applied to the same steel as that of FIGS. 1 and 2 which illustrate the prior art.

Figure 4 is a schematic view of a mold equipped with means according to the invention.

Figures 5 and 6 are also schematic views illustrating two embodiments of the device according to the invention.

Figure 7 is a. schematic view of a mold with a particular electric heating resistance.

For the purpose of better understanding, we will first comment on the curves of FIGS. 1 and 2 respectively illustrating the solidification by cooling of a cast steel, and the heat treatment which is made to undergo this particular steel, it being understood that 'This is a non-limiting example.

As is known, a molten alloy is poured into a mold which is then at a temperature above about 1500 ° C. then, by cooling in this mold, the alloy solidifies, that is to say progressively passes from l liquid state in the solid state, as shown respectively in A to B in FIG. 1. Finally, the molded and solidified alloy completes its cooling down to room temperature, as shown in the portion curve C, the latter having, for the alloy considered, a bearing D which, in the case of the alloy considered, corresponds to a short-term heat release resulting from a transformation within this alloy.

When the cast steel part is almost completely cooled, it is then subjected to the heat treatment according to the curve in FIG. 2 which again represents only one example of the thermal cycle for the alloy considered. According to this example, the material is reheated as shown by the portion of curve E, to a temperature of 1100 ° C. Then the part is kept at this temperature for a certain time, as seen in F, before allowing it to cool, as can be seen in G.

If one applies the principles described to the alloy treated according to FIGS. 1 and 2, one then obtains the curve of FIG. 3 which one will comment on as follows. At the end of solidification, as shown in B, that is to say at a temperature in the region of 1300 ° C., a supply of heat energy is provided by means which will be described later. alloy in the mold, which keeps the alloy at the temperature of the start of heat treatment so that it can subsequently be subjected to controlled cooling in the mold. In other words, we will control the cooling law of the alloy in the mold, that is to say that we can for example, as before, maintain the alloy at the temperature of 1100 ° C and the allow to continue cooling thereafter. However, it can be seen here that the level H corresponds to a much shorter holding time (t _i ) compared to the level F (t ₂ ) with the previously known method. This is due to the fact that the alloy is already at temperature when a controlled cooling law is applied to it and that, consequently, the aforementioned holding time is minimized. Thus, the device described allows not only to avoid the loss of heat energy of the alloy during cooling in the mold (figure 1) and to avoid a considerable energy contribution for the reheating treatment (figure 2), but it also allows to treat the phases of the alloy (solid solutions, precipitates, etc.) from the start. On the other hand, according to the conventional two-stage thermal cycle (Figures 1 and 2), the phases had to be recreated or brought back into equilibrium at high temperature, which ultimately required a much longer time, significant calorific energy, and could be detrimental to the quality of the material, that is to say of the molded part.

It will be added here that the heat treatment can be implemented with ease, whether it is simple or complex, that is to say if it comprises one or more thermal cycles and / or comprises one or more operations in relation to this or these cycles.

Furthermore, the new way of operating can also be applied to the material before a possible hot unhooking operation itself preceding, if necessary, other operations.

We will now describe the means used, illustrated in FIG. 3.

According to an exemplary embodiment, and with particular reference to FIG. 4, it can be seen that a device essentially comprises a mold 1 with, at its upper part, a counterweight 2, this mold comprising a molding space 3 around which is arranged, according to a helical winding, an induction heating system 4 connected to a suitable supply 5. There is shown in 6 and 7 a chute and a mold supply duct, which duct communicates with the molding space 3 .

Thus, thanks to the inductor 4, it will be possible to provide the sufficient amount of heat energy at the end of solidification of the material, as explained previously, so as to be able to stabilize the temperature of this material when it is desired. and to be able to modulate the subsequent cooling. Of course, the passage time and the frequencies of the current applied to the inductor 4 can be chosen at will and will depend on the metal part or the part of this part which one wants to treat.

It should be noted here that the inductor 4 will preferably be constituted by a tubular element, as can be seen in FIG. 4, inside which a cooling fluid can circulate as shown by the arrows F in said figure. Thus, it is possible to accelerate the cooling of the part in the molding space 3 by imposing on this part a cooling law such that the heat flux extracted from this part is greater than the natural heat evacuation carried out by the mold. In short, the device shown in FIG. 4 remains particularly flexible in that it makes it possible to carry out all the possible thermal treatments, such as: isothermal maintenance, slow cooling, rapid cooling and stepped quenching, for example.

It is also possible to provide a cooling circuit independent of the means 4 of induction heating.

It is also possible to use as a heating means, in place of the inductor 4 or in combination with it, an electric heating resistor (not shown) and suitably arranged in the mold so as to be able to heat the molded part or such and such a part of it.

Thus, the inductor 4 and / or the electric heating resistance can be arranged over the entire height and the entire periphery of the molding space 3, as seen in FIG. 4, or over the entire height of this space and only over part of its periphery, as seen in FIG. 5, or even over a small part of the height of the molding space, for example in a cavity 8 formed in this space, as shown see in Figure 6. In this figure, we can also see that an inductor 4a has been provided in the cavity 8, and another inductor 4b which surrounds the upper part of the molding space 3. It can of course be understood imagine an infinity of variants of shapes, numbers and locations of inductors and / or heating resistors, this of course being a function of the conformation of the molding space 3 and also of the heat treatments which it is desired to subject to to the material at such or such place to be treated.

In the same order of ideas, the nature and the constitution of the mold 1 can be arbitrary. Thus, the mold 1 can be a non-metallic mold, for example a sand mold as shown in FIG. 4, or even a metallic mold in which the inductor will be fixed or fitted, by any suitable means. 4 and / or the electric heating resistance.

Returning to FIG. 4, an inductor has been shown at 9 which surrounds the flyweight neck 2 at the upper part of the mold 1. Thus, thanks to this advantageous arrangement, the neck of the flyweight will be kept in the liquid state so that said neck does not become a "cold spot" leading to premature solidification before solidification is completed in the mold, which of course would give the molded part defects.

Also, the use of induction heating at the neck of the counterweight will make it possible to maintain this part in the liquid state at a temperature determined according to the alloy considered, and it will thus be possible to modulate the cooling of the neck of the flyweight, including solid state cooling. In addition, the heating of the counterweight 2 by the inductor 9 will advantageously make it possible to significantly reduce the volume of said flyweight located above the neck.

The inductor 9 may be arranged above the mold 1, as seen in Figure 4, but it could very well be arranged in the mold around the part 2a of the neck (see Figure 4). It will be added here that inductors can be provided at other parts of the device, such as for example on the trough 6 and / or the supply duct 7.

In FIG. 7, a mold 1 has been shown containing around the molding cavity 3, current-conducting particles 10 embedded in the mass of said mold and allowing the transfer of heat energy to the molding cavity 3 to heat it appropriately and depending on the particle distribution. In other words, a thermal gradient control can be obtained by the distribution, the particle size and the density of the particles embedded in the mold 1.

These particles 10 are distributed uniformly or not in the mass of the mold which may consist of sand or any other suitable material. As can be seen in FIG. 7, the particles 10 can be more or less dispersed or agglomerated together in a more or less compact form, and this so as to allow the passage of a current which is supplied for example to the using electrodes 11 integral with an insulating wall 12 surrounding the mold 1, said electrodes being connected to a source of electrical power not shown.

Thus, the more or less dispersed or agglomerated conductive particles 10 constitute as many points or zones of transfer of electrical energy making it possible to control the heating of the molding cavity 3, this heating obviously being a function of the particle size and of the distribution density.

The particles 10 may be particles based on suitable metals or alloys, or alternatively particles of oxides, such as for example metal oxides, or particles of metallic silicates. It is also possible to use particles either sintered or composite.

A binder can be added to the mass of the mold 1 to maintain the structure of particles in said mass, this binder can be mineral or organic.

Finally, it will be added here that the device of the invention can be applied to all moldable materials and in particular to all heat treatments of cast metal alloys, resulting in a structural evolution of these alloys, therefore in general the appearance of new characteristics. . And, by structural evolution, it is necessary to hear any evolution of the metallographic structure having appealed either to a mechanism known as "diffusional", or to a mechanism known as "of shearing", or to the association of these two mechanisms.

There is therefore produced according to the invention a device for manufacturing molded parts which make it possible to achieve substantial energy savings, which make it possible to produce molded parts quickly, and which are very flexible in use, which their confers possibilities of application to the treatment of very diverse materials and alloys.

Claims (7)

1. Device for the manufacture of castings and of the kind comprising a means for electrically heating a part moulded and solidified in a mould (1) into which this heating means is incorporated, characterized in that the latter consists of at least one electrical heating resistor or of at least one inductor (4) fully embedded or nested inside of the mould (1) for imposing a suitable thermal treatment on at least one portion of the solidified part within the moulding space (3).

2. Device according to claim 1, characterized in that it comprises an inductor (9) arranged about the feeding neck (2) at the upper portion of the mould (1) and/or at least one inductor provided at the mould running and feeding system (6, 7) whereas a cooling circuit is disposed inside of this mould.

3. Device according to claim 1 or 2, characterized in that the aforesaid electrical resistor or the inductor (4) is arranged inside of the mould (1) throughout the height thereof or overone portion only of this height to for instance affect one hollow portion (8) or projecting portion of the moulding space (3).

4. Device according to claim 1, characterized in that the electrical heating resistor consists of current conducting particles (10) distributed uniformly or not within the body of the mould (1) according to a density apt to allow the passing of a current supplied by an electrical supply source (11) connected to the said mould.

5. Device according to claim 4, characterized in that the aforesad particles (10) consist of metal particles based on metals or alloys, oxide particles such for instance as metallic oxides, metallic silicate particles or a mixture of the aforesaid particles.

6. Device according to claim 4 or 5, characterized in that the particles (10) distributed within the body of the mould (1) may be more or less scattered or agglomerated with each other into a more or less compact shape.

7. Device according to one of claims 4 to 6, characterized in that one adds to the body of the mould (1) a mineral or organic binder for retaining the fabric of the particles within this body.

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