A METHOD OF CASTING METALLIC ARTICLES

Abstract: For casting of iron alloys in permanent molds (10) thin-walled permanent copper molds are used which are subjected to forced cooling so that the surface layer of the articles (11) is quenched to rapidly form a solidified shell which permits the articles (11) to be transferred from the permanent molds (10) to a temperature regulation (21, 22) zone after a quite short dwell time in the permanent molds (10). This allows good utilization of the permanent molds (10) , and their thin walls and good heat conductivity cause the temperature drop over these walls to be so small that cracks and deformation are avoided and the permanent molds (10) thus get a long life. The melt is dosed most expediently by injection from below.

Description: A method of casting metallic articles The invention relates to a method of casting iron alloy articles in a permanent mold casting plant. In the past, casting in permanent molds was used only to a limited extent for making articles consisting of iron or iron alloys because permanent mold casting for this use has difficulty in competing with casting in expendable sand molds. One reason is that the permanent molds have a short life because of deformation and cracks with consequent high mold costs. To this should be added low production rate and reheating of castings requiring annealing. The object of the invention is to provide a method of the present type which enables an increased production rate and reduces the permanent mold costs so much that in many cases the method is able to compete with conven¬ tional casting in sand molds. This object is achieved by quenching the outer layer of the cast articles until their shape is stable in relatively thin-walled permanent molds consisting at least predominantly of copper, and immediately removing the articles and placing them in a temperature regulation zone. The quenching causes rapid formation of a shell of solidified material strong enough for the article to be removed from the permanent mold and be transferred to the temperature regulation zone. The dwell time in the permanent mold is thus very short and the produc¬ tion rate per permanent mold is correspondingly great. The use of thin-walled permanent molds with copper as the main material causes the temperature drop over the molds to be low so that cracks and deformation are « avoided and the molds therefore have a long life. y $\$.E The permanent molds may, as known in the art, have an internal coating, e.g. of ceramics, so that the temperature drop over the mold material proper can be regulated, and moreover this provides protection against mechanical and chemical influences. To obtain the required quenching, it is generally necessary that the permanent molds are subjected to forced cooling which can take place in several, general¬ ly known manners. When the articles are heat insulated in the temperature regulation zone, the required heat treatment can take place solely under the action of the heat provided by the articles themselves, so no external heat supply is necessary. In practice, the heat insulation is expediently provided for by placing the articles in and covering them by a heat insulating granular material. The rapid cooling resulting from the stated method makes it desirable that also the supply of melt to the permanent mold can take place rapidly. The invention also relates to an apparatus for rapid supply of melt to a permanent mold for use in the stated method. This apparatus contains a crucible mounted below the permanent mold and containing melt, as well as means for urging melt up through a riser pipe com¬ municating with a gate opening in the bottom of the permanent mold and is characterized in that the riser pipe is formed by a piston rod whose lower end carries a piston slidable in a cylinder whose interior com- municates with the crucible through a valve, and that the piston rod and the piston are formed with a channel extending from the underside of the piston to the upper end of the piston rod. As the piston can be activated through the permanent mold by a downwardly directed pressure against its top face, the activation pressure simultaneously serves to keep the permanent mold firmly engaged with the end of the piston rod, and when a horizontal dividing face is used between the permanent mold parts, the pressure also contri¬ butes to keeping the two permanent mold parts firmly engaged with each other. The use of a piston for press¬ ing melt up into the permanent mold also makes it easier to automatize and regulate the supply of melt than the conventional use of pressurized air. The cylinder may expediently be placed in a depression in the bottom of the crucible. The invention will be explained more fully below with reference to the drawing, in which figs. 1, 2 and 3 schematically illustrate their separate phases of an embodiment of the method of the invention, fig. 4 is a vertical section through an embodiment of the apparatus of the invention, and fig. 5 shows a modified embodiment of a part of the apparatus. In figs. 1-3 the symbol 10 designates a permanent copper mold consisting of two parts. In the example shown, the two parts are symmetrical and arranged to manufac¬ ture cylindrical articles 11, and the permanent mold has at one end a pouring gate 12 for the supply of the molten iron. This is fed in an ordinary manner from a ladle 13 pivotally mounted above the permanent mold, as shown in fig. 1. The two permanent mold parts form or are placed at one side of their respective box-shaped casings 14. It is attempted to keep the outer side of the permanent mold at a constant temperature level by (not shown) cooling means, and when the melt is then poured into the per¬ manent mold, a temperature difference will occur be¬ tween the inside and outside of the permanent mold. This difference in temperature is quite small owing to the good heat conductivity of the permanent mold material. However, the difference in temperature may be made even smaller, which in itself prolongs the life of the permanent mold, by providing the inside of the permanent mold with a coating, e.g. an approxi¬ mately 0.2 mm thick layer of a ceramic material which also protects the copper wall against wear. Owing to the great heat conductivity of the permanent mold and the consequent low difference in temperature, a solidi- fied shell will be formed within few seconds of the fil¬ ling of the permanent mold with liquid iron; this shell is strong enough for the article to be removed from the permanent mold, which is done immediately after the shell formation. As shown in fig. 2, the article may be removed by urging the two boxes 14 apart, e.g. by activation of a (not shown) cylinder whose piston rod carries one box, so that the article 11 with a sprue can fall down on an underlying conveyor belt 16, which, in the shown em¬ bodiment, is covered by a layer. of a granular material 17, e.g. kieselguhr, vermiculite or just dry sand fed from a pipe 18. The conveyor belt travels in the direc¬ tion shown by arrows 19 and conveys the articles J.1 be- low a second pipe 20 from which so much of the material 17 is fed as will cause the articles to be entirely covered and encased by protecting, heat insulating material. If the insulating material is kieselguhr, just the pipe 18 is necessary, which can then apply so thick a layer that the articles themselves will sink below the surface of the layer and be covered. In this state the articles move on the conveyor belt into a heat regulation tunnel 21 which is defined by heat insulating walls 22 and in which a controlled, slow cooling takes place. During the first phase of the cooling process the inherent heat of the articles will penetrate out into the surface of the castings and soften the regions which became too hard by the quenching in the permanent mold. A suction pipe 23 is fitted at a suitable point in the upper wall of the tunnel 21 where the temperature of the articles have dropped to such a value as permits an increase in the cooling rate. This suction pipe 23 draws out all the insulation material on the conveyor belt 16 as indicated by an arrow 24 so that the articles 11 will be disposed uncovered on the belt. Immediately after the suction pipe 23 the tunnel 21 communicates with a shaft 26 which is formed by insu- lating walls 25 and communicates with a suction chan¬ nel 27. This suction channel has mounted in it a fan 28 drawing cooling air countercurrently through the last portion of the temperature regulation tunnel 21 and out through the suction channel 27 as indicated by arrows 29. In the shaft 26 there is mounted a heat exchanger 30 through which a heat conveying medium, such as water, flows as indicated by arrows 31. This medium absorbs at any rate a great deal of the heat removed by the Cooling OMPI air from the articles 11 on the conveyor belt 16. The waste heat thus collected may be reused for any suit¬ able purpose. The same applies to the waste heat from the permanent molds. The cast articles 11 leave the conveyor belt 16 at the end of the cooling tunnel 21 and the only operation left is the removal of the sprue. Compared with conventional casting in expendable sand molds the described permanent mold casting method of- fers a plurality of advantages. Thus, no plant for preparation, transport and cleaning of moulding sand is needed, and this also obviates environmental draw¬ backs in the form of dust, smoke, noise and vibrations Moreover, it is easier to automatize the removal of castings and to clean them. The investment requirement is considerably reduced and the casting price can be lowered. The apparatus shown in fig. 4 for rapid supply of melt to a thin-walled, two-part permanent mold here desig-- nated by 40 comprises a crucible which is generally designated by 41 and has an inner wall or lining 42 of a refractory and electrically insulating material, e.g. stone or casting or stamping mass, an outer plate wall 43 of steel and an intermediate layer 44 of an insulating material, e.g. kieselguhr. The lining 42 has a relatively ' thick base whose centre is formed with a cylindrical depression 46 with a threaded wall, in which one end of a vertical cylinder is screwed. This cylinder has slidably mounted in it a piston 48 secured to the end of a piston rod 49 which extends up through the crucible and out through a central hole in a cover 50 consisting of a plate material and insulation material. The cover is formed with a filling hole '69 for melt. A nozzle 51 is screwed into the upper end of the piston rod 49, and a channel 52 extends through this nozzle, the piston rod and the piston 48 and thus establishes communication between the interior of the cylinder 47 and the upper end of the nozzle 51. The piston 48 is circu ferentially provided with a plurali¬ ty of axial, through bores 53, and a ring-shaped valve flap 54 is mounted on the underside of the piston in alignment with these bores. Below and in engagement with the base 45 of the lining 42 is placed an inductor member 55 of both heat insu¬ lating and electrically insulating material, e.g. casting mass. A U-shaped channel 56 extends through the base 45 and the member 55. An inductor core 58 extends through an opening 57 in the inductor member 55 disposed inside the channel 56, and this opening is disposed at right angles to the U formed by the channel. The inductor core carries a primary winding 59 outside the member, and the melt in the channel 56 and the crucible form the secondary winding of the inductor in which strong electric currents are produced which heat the material. As the iron in the branch extending through the core is heated more than the iron in the other branch because of the heavier magnetic field and the consequent stronger eddies, the melt constantly circulates in the direction indicated by arrows. It being assumed that the cylinder 47 in the position shown is filled with melt 60, a downward pressure on the permanent mold 40 will cause the piston 48 to be urged down so that melt will be pressed up through the channel 52 and the gate opening 61 of the permanent mold 40 and up into the mold cavity 62 of the permanent mold. The per¬ manent mold is here divided along a horizontal dividing OMPI face 63 so that the pressure will urge the two per¬ manent mold parts together. Both the piston rod and the nozzle are kept warm by heat conduction from the melt in the crucible. When the permanent mold 40 is again lifted, the material in its gate 61 has solidified, while the contents in the nozzle 51 is still so liquid that it now runs back into the crucible. While the permanent mold is imptied and made ready, the piston 48 goes upwards, and new melt penetrates into the cylinder 47. The upward movement of the melt on the piston and the valve flap is sufficient for moving these, but might optional¬ ly be supplemented with mechanical actuation of the piston. • In the modified embodiment of the piston as shown in fig. 5 here designated 65, the valve flap is replaced by channels 66 and 67 in the piston and the cylinder, respectively, here designated by 68. When the piston 65 is in the top position, it is rotated by (not shown) means to such an angle that the communication between the channels is interrupted. In the bottom position the communication is re-established by rotation in the opposite direction. In the experimental work, permanent copper moulds of a Wall thickness of 6 mm were used. Since copper has a conductivity of about 400 watts/degrees Celcius x m, such a mould has a heat resistance per surface of unit of about ^ 00 = 15 x 10 " degrees Celcius x m /watts. Somewhat higher values can be tolerated in practice, e.g. owing to the use of copper which is not quite pure, but contains small amounts of additives. A thin ceramic coating will increase the useful live of the mould at the expense of the rate at which heat can be conducted away through the wall of the mould. The details of the shown and described plant and appa¬ ratus can be modified in many ways. For example, in¬ stead of a horizontal cooling tunnel a vertical cooling shaft may be used in which the articles can move through a spiral-shaped path. The permanent molds may consist of more than two parts and may optionally be provided with cores. The heating may be effected in other ways than by induction, e.g. by means of electric arcs. In special cases, particularly in the casting of very thin- walled articles, it may be desirable to apply a vacuum in the mold cavity of the permanent mold.

Claims: 10 P a t e n t C l a i m s 1. A method of casting iron alloy articles in a per¬ manent mold casting plant, c h a r a c t e r i z e d by quenching the outer layer of the cast articles until their shape is stable in relatively thin-walled permanent molds consisting at least predominantly of copper,- and immediately removing the articles and placing them in a temperature regulation zone. 2. A method according to claim 1, c h a r a c t e r i z ¬ e d by heat insulating the articles in the temperature regulation zone. ' 3. A method according to claim 2, c h a r a c t e r i z ¬ e d in that the heat insulation is effected by placing the articles in and covering them by a heat insulating, granular material. 4. An apparatus for rapid supply of melt to a permanent mold for use in the method according to claim 1, 2 or 3, comprising a crucible mounted below the permanent mold and containing melt, as well as means for urging melt up through a riser pipe communicating with a gate opening in the bottom of the permanent mold, c h a r a c t e r ¬ i z e d in that the riser pipe is formed by a piston rod whose lower end carries a piston slidable in a cylinder whose interior communicates with the crucible through a valve, and that the piston rod and the piston are formed with a channel extending from the underside of the piston to the upper end of the piston rod. 5. An apparatus according to claim 4, c h a r a c - t e r i z e d in that the cylinder is placed in a depression in the bottom of the crucible. r _ OMP

Inventors: HANSEN VERNER DAHL, HANSEN, VERNER, DAHL
Applicants: HANSEN VERNER DAHL, HANSEN, VERNER, DAHL
Application year: 1983
Main classification: B22D15
IPC codes: B22D15, B22D23, B22D27, B22D29