FR2868346A1 - MOLD FOR CASTING LIQUID METAL AND CORRESPONDING METHOD - Google Patents

Abstract

The mold for casting a liquid metal comprises a shaped surface (24) corresponding to the surface of the product (16) to be made by casting and intended to come into contact with the liquid metal. The mold comprises a heat dissipating portion (10) made of a material having a thermal conductivity greater than 150 W / mK.Application to the casting of cast iron pipes.

Description

2868346 1

  The present invention relates to a mold for casting a liquid metal, of the type comprising a shaped surface corresponding to the surface of the product to be made by casting and intended to come into contact with the liquid metal.

  It applies in particular to installations for manufacturing pipes by casting.

  Molds are known for the manufacture of cast iron pipes by casting. These molds have a general hollow cylinder shape about an axis of rotation and comprise an end corresponding to the plain end of the pipe and an end corresponding to the interlocking end of the pipe. The inner surface of the mold forms the outer surface of the manufactured pipe.

  In the manufacture of pipes, the mold is rotated about its axis, and a liquid metal is poured on the inner surface of the mold. The metal is successively cast from the end of the socket end to the end of the spigot end.

  The known molds are made of chromium low alloy steel to withstand the temperature of the molten steel.

  Due to the high temperature of the liquid metal and the temperature variation of the mold during the successive casting of several pipes, cracks appear on the inner surface of the mold. When pouring liquid metal enters these cracks. The metal in the cracks prevents the extraction of the pipe from the mold and leads to scratches on the outer surface thereof.

  When the cracks exceed a predetermined size, the mold must be repaired by machining the inner surface or by filling the cracks by welding. However, these operations modify the interior surface such that the pipes manufactured after repair may no longer meet the required tolerances.

  The life of known molds is therefore limited.

  Known steel molds have another disadvantage. When casting the first pipes, the steel mold deforms plastically significantly. As a result, the mold is manufactured with dimensions that take into account this initial deformation. In order to bring the mold to its desired dimensions, several pipes, for example six, are cast. These pipes do not respect the required tolerances and must be discarded. The need to manufacture these "lost" pipes leads to a decrease in the efficiency of the casting installation.

  The present invention aims to provide a mold for casting liquid metal which has a long life while allowing the casting of low tolerances parts.

  For this purpose the invention relates to a mold of the aforementioned type, characterized in that it comprises a heat removal portion made of a material having a thermal conductivity greater than 150 W / mK.

  According to particular embodiments, the invention comprises one or more of the following features: the material has a thermal conductivity greater than 300 W / mK; the material has a thermal conductivity greater than 325 W / mK; the heat removal portion is made of a copper-based alloy, especially comprising more than 50% by weight of copper; the copper-based alloy comprises chromium, especially between 0.25% and 2% by weight; the alloy comprises zirconium, in particular between 0.05% and 0.25% by weight; the heat evacuation portion forms at least a portion of an outer surface of the mold; the heat evacuation portion forms at least a portion of the shaped surface; the heat dissipating portion extends continuously between the shaped surface and the outer surface; the mold comprises a coating forming at least a portion of the shaped surface and extending from the shaped surface to the heat removal portion; the coating and the heat removal portion have thermal expansion coefficients such that the thermal expansion of the coating is greater than the thermal expansion of the heat removal portion under the effect of the cast metal; - The coating is a material having a hardness greater than the hardness of the heat removal portion, in particular chromium or nickel or an alloy based on chromium and / or nickel; the shape surface is of symmetry of revolution around an axis; and - the mold is a mold of a pipe.

  The invention further relates to a method of manufacturing an object by casting, of the type comprising the step of casting the liquid metal in a mold, characterized in that the mold is a mold as defined above.

  The process according to the invention may comprise the following characteristics: the metal is a cast iron, in particular gray cast iron or spheroidal graphite cast iron; and the method is further characterized by a step of annealing the object manufactured by casting.

  The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings, in which: FIG. 1 is a diagrammatic view of a manufacturing facility of pipes according to the invention; 2 is a detail view of the installation of FIG. 1, showing the mold on a larger scale, FIG. 3 is a partial sectional view of the mold according to the invention taken along line III-III. of Figure 2; and - Figure 4 shows a variant of the mold of Figure 3.

  In Figure 1 is shown a pipe-making plant of cast iron according to the invention, designated by the general reference 2.

  The installation 2 comprises a receptacle 4, a pouring device 6, a pouring channel 8, a rotating mold 10, a cooling device 12 and a device 14 for extracting a completed pipe. The installation 2 is used to manufacture pipes 16.

  The container 4 is a refractory crucible containing liquid metal, for example cast iron.

  The pouring device 6 is a rocker having a volume corresponding to the amount of liquid metal required for a pipe 16. The rocker can be inclined between a liquid metal receiving position of the container and a pouring position of the liquid metal in the flow channel. casting 8.

  The pouring channel 8 is a channel which leads from the pouring device 6 to the mold 10. It comprises an inlet 20 situated close to the device 6 and an outlet 22 extending into the mold 10. The channel 8 is inclined with respect to the horizontal so that the outlet 22 is located lower than the inlet 20, thus allowing the liquid iron to flow by gravity.

  The rotary mold 10, also called "shell", has a symmetrical shape of revolution, in the generally cylindrical example of axis XX, inclined relative to the horizontal so that it is parallel to the channel 8 In what follows, the terms "axially" and "radially" will be used with reference to this axis XX. The mold 10 has an inside surface 24 of shape which is the negative surface of the pipe 16, as well as a cylindrical outer surface 26 (see FIG. 2). The surface 26 does not come into contact with the liquid metal during casting. As shown in Figure 3, the inner surface 24 is provided with hollow 27 for driving the liquid metal.

  The mold 10 includes a plain end 28 facing the inlet 20, and a plug end 30, which is turned away from the inlet 20. The plain end 28 forms the spigot end of the pipe 16, while the end plug-in end 30 forms the interlocking end of the pipe 16.

  The mold 10 can be rotated about the X-X axis by a drive device not shown. Furthermore, the mold 10 can be driven in translation along the axis XX between a casting start position, in which the outlet 22 is in front of the end plug end 30, and a end of casting position , in which the outlet 22 is in front of the end end 28.

  The cooling device 12 is a watering head which is adapted to project cooling liquid, for example water, onto the outer surface 26 of the mold 10. In a variant, this device 12 can be constituted by a cooling system. circulation of water in the known manner.

  The extraction device 14 is adapted to extract the cast pipe 16 axially from the mold 10.

  The devices 6, 12 and 14, the container 4 and the pouring channel 8 are known per se and are not described in more detail.

  The mold 10 according to the invention is entirely CuCrZr which is a copper-based alloy. The alloy comprises more than 50% by weight of copper, preferably more than 60% by weight of copper, and in particular more than 75% by weight of copper.

  In the present case, it is made of an alloy of copper, chromium and zirconium, in particular according to DIN 17666. A particularly suitable alloy comprises 1% by weight of chromium and 0.15% by weight of zirconium, the remainder being copper and unavoidable impurities. This alloy has an average thermal conductivity at 220 C of 340 W / mK.

  The alloy may comprise between 0.25% and 2% by weight of chromium, and preferably between 0.5% and 1.5% by weight.

  The alloy may comprise between 0.05% and 0.25% by weight of zirconium, and preferably between 0.10% and 0.20% by weight.

  According to the invention, at least a part of the material of the mold 10 has a thermal conductivity greater than 150 W / mK, especially greater than 300 W / mK, and in particular greater than 325 W / mK.

  The manufacture of a pipe 16 by means of the installation 2 according to the invention is carried out as follows.

  Liquid cast iron, corresponding to the amount of cast iron required for the pipe 16, is introduced into the pouring device 6 via the crucible 4.

  Then, the mold 10 is rotated about the X-X axis and is brought into its start casting position.

  Then, the molten iron is poured from the device 6 into the channel 8, flows along it and is poured into the mold 10 at the end plug-in end 30.

  Successively, the mold 10 is brought to its end of casting position.

  The molten iron in the mold is pressed against the surface 24 by centrifugation, solidifies and forms the pipe 16.

  Due to the high thermal conductivity of the mold material 10, the temperature gradient between the inner surface 24 and the outer surface 26 is small. In other words, the heat provided by the molten iron is rapidly removed from the surface 24 to the surface 26. As a result, there is little thermal stress in the mold 10, which reduces or avoids an appearance of cracks. The mold 10 therefore has a long life.

  For example, for the CuCrZr mold 10 having a wall thickness of 20 mm and a cast iron of 1400 C, the inner surface 24 reaches a temperature of 150 C, while the outer surface 26 reaches a temperature of 100 C. The temperature gradient is therefore 50. Molds of the state of the art steel lead, under similar conditions, to a temperature gradient of about 580 C.

  Surprisingly, the heat is removed from the inner surface 24 fast enough that the latter does not reach the melting temperature (about 1083 C) of the mold material. It should be noted that generally the thermal conductivity of the mold material is chosen to be large enough to prevent melting of the mold at the surface 24.

  The melt solidifies rapidly in the mold, with a solidification rate of between 1.20 mm / s and 0.80 mm / s. Due to the rapid solidification, the carbon contained in the cast iron precipitates as Fe3C (iron carbide) in a zone in contact with the surface 24, while in a zone remote from the surface 24, the carbon is essentially in the form of graphite. The rapid solidification thus leads to a large gradient of iron-graphite carbide in the pipe 16. During annealing, which transforms the iron carbide into graphite, the outer surface of the pipe 16 is heated faster than the inner surface of the pipe. this. Accordingly, for a required conversion rate of graphite iron carbide, the pipe 16 may be annealed at a low temperature or for a low time. The mold 10 according to the invention is therefore particularly suitable for casting objects of gray cast iron or spheroidal graphite cast iron.

  In addition, after the extraction of a pipe 16, the mold 10 quickly reaches its initial temperature.

  Thus the pipes can be manufactured with a high rate.

  The mold 10 according to the invention also leads to small local deformations of the outer surface of the pipe thanks to rapid solidification. The pipe has a better appearance.

  During the casting of the first pipes, the plastic deformation of the mold 10 according to the invention is low. Even the first pipes produced respect the dimensions required and can be used. Thus, the installation has a significant yield.

  In Figure 4 is shown a variant of the mold of Figure 3. In the following, only the differences compared to the mold of Figure 3 will be described, the identical elements bearing the same references.

  The mold 10 is a composite mold. 11 includes an outer heat-removal layer 40 which forms the outer surface 26 and an inner protective layer 42 which forms the inner surface 24 of shape.

  The layer 40 is made of the same material as the mold 10 described above.

  The layer 42 is made of a material that has a hardness greater than the hardness of the layer 40. It is for example made of chromium or nickel, or an alloy based on these metals. The layer 42 protects the layer 40 against scratches generated during the extraction of the pipe 16.

  In addition, the coefficients of thermal expansion of the materials of the layers 40 and 42 are chosen such that the thermal expansion, due to the heating by melting, of the inner layer 42 is greater than the expansion of the outer layer 40. Thus, during casting, the inner layer 42 is pressed against the outer layer 40, and does not detach from it.

  In general, at least a portion of the mold which serves to evacuate the heat of the cast iron from the shaped surface is of a material having a high thermal conductivity such as the copper-based alloy.

  Alternatively, the alloy of the mold 10 or the heat removal portion is copper-based comprising small amounts of tellurium or small amounts of silver and phosphorus.

  In a variant that is not shown, the mold 10 comprises an inner thermal discharge layer of high thermal conductivity which forms at least a portion of the inner surface 24 and an outer layer of a different material, such as chromium or nickel or chromium. and nickel to increase hardness or chromium and zinc to prevent scale formation.

  Although the mold according to the invention is particularly suitable for cast iron pipes, it can be used for casting other objects of other metals.

2868346 11

Claims (17)

  A mold for casting liquid metal, of the type comprising a shaped surface (24) corresponding to the surface of the product (16) to be produced by casting and intended to come into contact with the liquid metal, characterized in that it comprises a heat removal portion (10, 40) made of a material having a thermal conductivity greater than 150 W / mK.   2. Mold according to claim 1, characterized in that the material has a thermal conductivity greater than 300 W / mK.   3. Mold according to claim 2, characterized in that the material has a thermal conductivity greater than 325 W / mK.   4. Mold according to any one of the preceding claims, characterized in that the heat removal portion is a copper-based alloy, especially comprising more than 50% by weight of copper.   5. Mold according to claim 4, characterized in that the copper-based alloy comprises chromium, especially between 0.25% and 2% by weight.   6. Mold according to any one of claims 4 or 5, characterized in that the alloy comprises zirconium, especially between 0.05% and 0.25% by weight.   7. Mold according to any one of the preceding claims, characterized in that the heat removal portion forms at least a portion of an outer surface (26) of the mold.   8. Mold according to any one of the claims   previous, characterized in that the heat dissipating portion forms at least a portion of the shaped surface (24).   Mold according to claims 7 and 8 taken together, characterized in that the heat discharge portion extends continuously between the shaped surface (24) and the outer surface (26).   Mold according to any one of claims 1 to 9, characterized in that it comprises a coating (42) forming at least a part of the shape surface (24) and extending from the shape surface (24). ) to the heat removal portion.   11. Mold according to claim 10, characterized in that the coating (42) and the heat removal portion (40) have thermal expansion coefficients such that the thermal expansion of the coating (42) is greater than the thermal expansion the heat removal portion (40) under the effect of the cast metal.   12. Mold according to claim 10 or il, characterized in that the coating (42) is a material having a hardness greater than the hardness of the heat removal portion (40), in particular chromium or nickel or an alloy based on chromium and / or nickel.   13. Mold according to any one of the preceding claims, characterized in that the shape surface (24) is of symmetry of revolution about an axis (X-X).   14. Mold according to claim 13, characterized in that it is a mold of a pipe.   15. A method of manufacturing an object by casting, of the type comprising the step of casting the liquid metal in a mold, characterized in that the mold is a mold (10) according to any one of the preceding claims.   16. The method of claim 15, characterized in that the metal is a cast iron, including gray cast iron or spheroidal graphite cast iron.   17. The method of claim 15 or 16, further characterized by a step of annealing the manufactured object by casting.