EP0065208B1 - Method and installation for producing thick-walled hollow castings - Google Patents

Abstract

1. Method for the feederless production of thick-walled container-type castings (1) of spheroidal graphite cast iron, characterised in that the entire mould (3, 6) is of nonyieldable construction comprising a mould core (6) and an outer mould (3), that the castings (1) are cooled both at their inner and at their outer surface, the cooling at the inner surface being effected by cooling of the mould core (6) by coolant, and that the ingates (2) are so dimensioned that the cast iron solidifies in them before the eutectic solidification of the casting (1) sets in.

Description

The invention relates to a method and a device for the feeder-free production of thick-walled, container-like castings made of spheroidal graphite cast iron. In the case of feeder-free casting, the mold is closed with the exception of the gates, and the cast iron does not flow after the mold has been filled from funnel-shaped feeders attached to various parts of the mold, but only for a short time through the gates. Castings of this type are required, for example, as transport containers for used fuel elements from nuclear power plants. This places particularly high demands on the quality of the casting. It must have a fine-grained and tough cast structure that is free from volume deficit errors, in particular free from micropores.

Thick-walled sand castings have long solidification times, since the considerable amounts of heat released can only be dissipated via the insulating molding material. In the case of spheroidal graphite cast iron, this can result in a rough globulitic cast structure. In addition, under these conditions, flat temperature gradients occur between the residual melt and the solidifying edge shell, which favor the occurrence of volume deficit errors, in particular micropores. In the case of a coarse cellular cast structure, the volume expansion, which predominates locally in graphitic eutectic crystallization and exerts a pressure that saturates to feed, cannot completely feed the micropores. The harmful consequences are micro-voids, which lead to indications in non-destructive testing methods and which limit micro segregation or, in extreme cases, even carbide deposits on the eutectic grain (cell), which impair the toughness of the material.

From DE-B-21 13267 it is known in the case of electroslag remelting in the production of thick-walled hollow bodies that a monolithic support body with cooling is used as the core, which can be pulled out of the melted block after the cooling has been switched off and the expansion associated therewith, and then the cooling has been switched on again . The document does not deal with the problem of producing cast iron from spheroidal graphite without micropores, but rather with a method for drawing the core. DE-B-1952009 solves the same problem with a water-cooled core during electro-slag remelting by pulling back wedge-shaped parts of the core by means of a spindle drive, the core diameter being reduced for pulling. From DE-A-28 27 091 it is known in conventional casting of steel into slabs or blocks to build a mold from individual walls from water-cooled cooling boxes. DE-C-665119 relates to the production of hollow bodies, in particular cast steel hollow bodies in a centrifugal casting mold which rotates about a vertical axis, the lower part of the inner wall of the casting body being additionally cooled with a coolant in order to counteract the formation of voids. These two documents do not deal with the problem of the invention of the pore-free casting of spheroidal graphite cast iron.

Based on this prior art, the object of the invention is seen in obtaining a fine-grained, low-segregation and non-porous casting structure in a thick-walled, container-like casting via a steeper temperature gradient which favors the shell-like solidification in conjunction with a shortened solidification time, as is otherwise only possible in thin-walled casting structures Castings can be reached.

This object is achieved in a method of the type mentioned at the outset in that the entire shape of the mandrel and the outer shape is built up relentlessly, that the castings are cooled both on their outer and on their inner surface, the cooling on the inner surface by cooling the Mold core is done by coolant and that the gates are dimensioned so that the cast iron starts in them before the eutectic solidification of the casting begins. The accelerated solidification of the cast iron, particularly in the core area, and the corresponding formation of a steep temperature gradient in the mold cavity, promote the formation of a fine cellular cast structure and shift the largely globulitic solidification in the direction of a solidifying edge shell. Both factors reduce the risk of micropores forming. The unrelenting structure of the entire mold causes the cooling casting to shrink onto the core so that no gap is formed and the good heat transfer is retained. The measure that the castings are dimensioned in such a way that the cast iron starts in them before the eutectic solidification of the casting begins, i.e. with the residual melt cooling to the liquidus temperature, together with the unyielding build-up of the overall shape, causes the expansion of the metal during graphitic eutectic solidification can fully act as a pressure increase in the mold cavity. As a result, the formation of micropores is avoided in the solidifying casting.

In particular, the invention can be advantageously configured as follows.

The good heat dissipation can be promoted by the fact that the cooling of the mandrel is metered and regulated by liquid, non-combustible coolants that evaporate in the system, such as liquid nitrogen.

In addition to the stable design of the mold, the cooling of the castings on the outer surface also contributes to the effective pressure increase in the mold cavity during the eutectic solidification. For castings, such as containers for Fuel elements in which the outer jacket surface has to be provided with cooling fins for later practical use can already achieve sufficient cooling on the outer surface by forming the castings on their outer surface with large cooling fins. Then the outer shape can be built up, for example, from dimensionally stable, cold resin-bonded quartz sand.

The cooling of the outer surface of the casting can e.g. with smaller cooling fins or a smooth outer surface, can be improved by a metallic outer shape. The metallic outer shape improves the dissipation of the heat to the outside due to its greater thermal conductivity compared to a ceramic shape and thus convection cooling by the ambient air due to the higher temperature. This can be improved by cooling fins on the outer shape. In addition, you can cool the metallic outer shape, mainly by. cool and evaporate liquid, non-flammable coolants evaporating in the system in a controlled manner

The measures to improve the external cooling of the castings promote shell-like solidification and thereby increase the pressure increase in the residual melt which improves the tightness of the casting during the eutectic solidification.

In the case of a mold which is particularly suitable for the process and consists of a mold core and an outer mold, the outer contour of the mold core is formed by a lost mold made of sheet steel, on the inner surface of which cooling elements through which coolant is arranged and the space between the lost mold, the cooling elements and the free space inside the mold core is filled with moldable, fine-grained substances. Sheet steel with a thickness of 10 to 20 mm is suitable for the lost shape. The fine-grained substances serve to stabilize the core and promote heat transfer between the lost mold shape and the cooling elements, in which cooling is effected by the coolant flowing through. The outer surface of the core is generally provided with a size customary in foundries in order to avoid welding.

The cooling elements can be designed as cooling boxes in which inflow and outflow pipes are arranged next to one another in order to enable uniform heat dissipation. The cooling boxes are advantageously held by metallic elements such as wedges and pressed against the lost shape. Instead of cooling boxes, cooling coils can also be used.

The fine-grained substances with which the spaces between the cooling elements, the lost formwork and the free space are filled can be ceramic molding materials, as are common in foundries. To increase the thermal conductivity, fine-grained metallic materials, preferably steel gravel, can also be used, or metallic materials can also be added to the molding material.

The outer shape is advantageously made of sheet steel and is provided with cooling elements. These can either be cooling boxes or cooling coils. Additional cooling fins improve heat dissipation.

An exemplary embodiment of the invention is explained below with reference to a drawing.

• Fig. 1 einen gegossenen Behälter für Brennelemente aus Kernkraftwerken in perspektivischer Darstellung.
• Fig. 2 eine schematische Darstellung einer Giessform für das Giessen des Behälters nach Fig. 1 im senkrechten Schnitt.

It show in detail

• Fig. 1 shows a cast container for fuel elements from nuclear power plants in a perspective view.
• Fig. 2 is a schematic representation of a mold for casting the container of FIG. 1 in a vertical section.

For the feeder-free production of a thick-walled, container-like cast piece 1 from spheroidal graphite cast iron, namely a container for fuel assemblies, the entire mold was made up of a mold core 6 and a metallic or non-metallic outer mold 3 and the cast piece 1 both on its outer and on its inner surface cooled, the cooling on the inner surface by cooling the mandrel 6 with liquid nitrogen. The gates 2 were dimensioned such that the cast iron solidified in them before the eutectic solidification of the casting 1 began. The outer shape 3 was made of dimensionally stable, cold resin-bonded quartz sand. To produce the outer mold 3, cores 4 were used for the cooling fins 5 to be produced on the outer surface of the casting 1. The lost mandrel 6 consisted on the outside of an approximately 6 m long cylindrical iron sheet jacket 7 with 15 mm wall thickness with a 30 mm thick welded-on cover 8. Before the cover 8 was welded on, the cooling boxes 9, 10 were introduced in two planes and countered with steel wedges 11 pressed the sheet metal jacket 7. A good cooling of the cooling boxes 9, 10 was achieved by a system of parallel, vertical cooling pipes, distributed over the entire circumference, a lower feed line and an upper discharge line laid alternately and each connected to a feed and discharge ring line. The lid 8 was also provided with a cooler 12. The core 6 was arranged upright. The casting was poured upwards. The casting temperature was 1320 ° C, the amount of magnesium-treated and inoculated iron was 115 t. The composition of the melt corresponded to a GGG-40.3, DIN 1693. The approximate dimensions of the casting 1 were 6 400 mm with respect to the length, the outer diameter with ribs 2 500 mm, the inner diameter 1 200 mm; the floor thickness was 400 mm.

After the casting, the core 6 was cooled with liquid nitrogen in such a way that evaporation occurred when it flowed into the cooling elements 9, 10, 12. The gates 2 were designed to freeze when the melt in the mold had a temperature of 1160 to 1200 ° C had achieved. Cooling was maintained throughout the solidification time. The coolant supply was only switched off shortly above the ya conversion in order not to disturb the ferrite formation. Overall, the setting time was shortened by 56% compared to pure sand casting.

After solidification and cooling in the mold, the casting was drawn and the fine-grained penetrations in the core were removed, the cooling elements arranged in several planes were removed and finally the formwork, that is, the sheet metal jacket 7 with the lid 8 was removed by cutting and pulling. The rest of the casting was cleaned in the usual way.

The ultrasound inspection of the cleaned and internally machined casting with various angle probes and with frequencies of 1 to 2 MHz did not show any indications with an identifiable substitute defect size of 3 mm.

Claims (14)

1. Method for the feederless production of thick-walled container-type castings (1) of spheroidal graphite cast iron, characterised in that the entire mould (3, 6) is of nonyieldable construction comprising a mould core (6) and an outer mould (3), that the castings (1) are cooled both at their inner and at their outer surface, the cooling at the inner surface being effected by cooling of the mould core (6) by coolant, and that the in- gates (2) are so dimensioned that the cast iron solidifies in them before the eutectic solidification of the casting (1) sets in.

2. Method according to claim 1, characterised in that the cooling of the mould core (6) is effected in a quantity-regulated and controlled manner by liquid non-combustible coolants which evaporate in the system, such as liquid nitrogen.

3. Method according to one of the above claims, characterised in that the cooling of the castings (1) at their outer surface is effected by constructing the outer surface with large-dimensioned cooling ribs (5).

4. Method according to one of the above claims, characterised in that the cooling of the castings (1) at the outer surface is effected by means of a metallic outer mould (3).

5. Method according to claim 4, characterised in that the metallic outer mould (3) is cooled by cooling ribs.

6. Method according to claim 4, characterised in that the metallic outer mould (3) is cooled in quantity-regulated and controlled manner by liquid coolants which are non-combustible and which evaporate in the system, such as liquid nitrogen or water.

7. Casting mould comprising a mould core (6) and an outer mould (3) for carrying out the method according to one of the above claims, characterised in that the outer contour of the mould core (6) is formed by a lost mould made of steel plate (7, 8) on whose inner surface there are arranged cooling elements (9, 10, 12) through which coolant flows, and the spaces between the lost mould (7, 8) and the cooling elements (9, 10, 12) and the free space in the interior of the mould core (6) are filled with mouldable, fine-particle substances.

8. Casting mould according to claim 7, characterised in that the cooling elements are constructed as cooling boxes (9, 10) in which inflow and outflow pipes are arranged side by side.

9. Casting mould according to claim 8, characterised in that the cooling boxes (9, 10) are held by metallic elements such as wedges (11) and pressed thereby against the lost mould (7).

10. Casting mould according to claim 7, characterised in that the cooling elements are cooling pipe coils.

11. Casting mould according to one of claims 7 to 10, characterised in that the fine-paritcle-size substances are metallic substances, preferably steel shot.

12. Casting mould according to one of claims 7 to 11, characterised in that the outer mould (3) is made of steel plate.

13. Casting mould according to claim 12, characterised in that the outer mould (3) is provided with cooling elements.

14. Casting mould according to claim 12 or 13, characterised in that the outer mould consists of cooling ribs.

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