Method And Means For Shaping A Stream Of Melt Flowing From A Tapping Hole

Abstract

A falling stream of a liquid tends to obtain a circular cross sectional shape, owing to the surface tension of the liquid. In the manufacture of metal powder by atomizing a falling stream of molten metal it is desired, however, that the stream shall have a flattened cross sectional shape. It has been found useful to make the molten metal flow through an orifice having a rectangular cross sectional area, and to make the molten metal follow the short sides of the orifice for a longer distance than it follows the long sides of the orifice.

Description

The present invention relates to processes for manufacturing or granulated stock from a melt, in which a stream of molten metal is atomized or disintegrated by one or more gas or liquid jets of some suitable atomizing medium which is directed under high pressure against the molten stream from specially shaped atomizing nozzles in such a way that the stream is separated into free drops which are in turn cooled to form the desired powder.

More specifically, the invention relates to a method and means of shaping the molten stream to be atomized in such an atomization process. The invention may also be defined as a method of delaying the contraction to circular cross section to which a stream of melt is subjected by its own surface tension as soon as it flows out through the orifice of the gap-like opening, the melt being spread in the longitudinal direction of the gap but otherwise in a coherent flow. Since it is possible in the manner characteristic of the invention to delay contraction of the stream of melt, it is, therefore, possible to allow the jets of atomizing medium which are to disintegrate the molten stream to intersect this stream while it still has the most advantageous shape for the disintegration process. This has greatly increased the production of powder per quantity of atomizing medium as well as offering other advantages which will be dealt with in the following.

The simplest way of performing the invention is to allow the melt which is to be atomized to flow out through a specially constructed outlet opening in nozzle stone so that the melt forms a coherent stream with, at least to start with, a specified cross section. By a nozzle stone is meant the actual lining around an outlet opening in a casting box, for example. Since the method according to the invention is fulfilled using outlet openings of varying construction, the invention itself covers several special embodiments for such outlet openings.

When manfacturing powder by atomizing such molten metals and metal alloys which easily form stable oxides which are difficult to reduce, it is necessary to use an inert atomizing medium and allow the atomization to take place in an inert atmosphere. If, furthermore, the particles must be spherical in shape it is usually necessary to use a gaseous atomizing medium. In order to completely avoid oxidation of the powder produced, the inert atomizing gas, which may be argon, for example, must be extremely pure, i.e. it must have a very low oxygen content. Such a pure inert gas, however, is relatively expensive and the costs of the actual powder production will therefore be greatly dependent on how effectively the quantity of atomizing gas can be used. It is therefore important that the quantity of gas used per quantity of atomized metal is kept as low as possible.

The atomizing gas used can of course be used again if a recirculation means is introduced in the system but such recirculated atomizing gas must be cooled down after it has been removed from the atomizing chamber and then cleaned from any particles which have accompanied it. It must then be brought up to the necessary pressure with the help of a compressor to enable the atomizing nozzles to function. In order to prevent the gas from becoming contaminated by oil and/or moisture when pressurizing in the compressor, a special type of compressor must be used, which is considerably more expensive than conventional compressors. Thus, even with a recirculation system, it is important for the atomizing nozzles from which the inert gas is directed towards the molten stream to be able to operate with as low a gas pressure as possible and for the quantity of gas necessary per atomized quantity of melt to be as low as possible so that the smallest possible compressor can be used without the splitting effect on the molten stream becoming unsatisfactory.

The amount of gas necessary to atomize or separate a certain quantity of melt into fine particles which can be cooled to a solid powder to a great extent, but not entirely, dependent on how the gas nozzles from which the jets of atomizing gas are directed towards the molten stream are shaped. A number of different designs are already known for such nozzles. However, the type which we have found to be the most suitable is described in our U.S. Patent application Ser. No. 94,148. In their most simple form the nozzles described in this application consist of two parallel gaps arranged one on each side of the molten stream and directed towards the stream so that it is intersected at an angle of 25.degree.-60.degree. first by a flat, extremely thin gas jet which causes the melt to change direction and follow the gas jet as a layer of molten metal spread over this and partly divided into free drops while a second flat, thin gas jet from a second nozzle intersects the first gas jet at an angle of 30.degree.-60.degree. and the layer of molten melt spread out over this gas jet and accelerates it in a new direction, whereupon the melt is finally divided into free drops which are cooled to form a solid powder. The separating effect of the first gas jet on the molten stream under otherwise identical conditions depends on the thickness of the molten stream in the direction of movement of the gas jet. Since the gas jet is a flat jet having certain extension laterally, the effect in other words will be the same for a tapping stream having circular cross section and a tapping stream having rectangular or elliptical cross section and with the thickness (or small axis) in the direction of movement of the gas jet, if this axis is equal to the diameter of the circular molten stream, obviously assuming that the width of the molten stream does not exceed that of the gas stream. It has also been suggested to atomize a stream of molten metal having an elliptical cross sectional area. With an elliptical or rectangular stream of melt it is thus possible to increase the quantity of atomized melt per time unit without having to increase the quantity of gas. Similarly, an unaltered quantity of melt can be atomized in a shorter time and using less gas.

An elliptical or rectangular cross section of the molten stream can be obtained if the melt is allowed to flow down into the atomizing chamber through an outlet opening having the appropriate shape in a nozzle stone which in turn is arranged in the lining of a casting ladle or box. However, the surface tension of the melt causes the molten stream to contract towards circular cross section if the melt is brought to follow the inner wall of the opening at the short sides of the opening of the nozzle stone.

In the simplest variant of the invention a nozzle stone is used, the outlet opening of which towards the orifice has two inner long walls which are parallel to the direction of flow of the melt or slightly converging, opposite to each other and extending in the spreading direction of the melt stream, while the inner short walls of the outlet opening at least nearest the orifice diverge from each other, i.e. the opening expands towards the orifice in the spreading direction of the molten stream. Due to surface tension conditions, the melt will then follow the diverging oblique short walls at the short sides of the opening and its speed components will therefore be directed towards the sides, which means that the time is extended before the tapping stream noticeably begins to contract towards the circular cross section.

So that the surface tension conditions will really make is possible for the melt to follow the diverging oblique short walls of the gap opening, the angle between these and the vertical should not be too great. The angle permitted will therefore vary with the surface tension conditions and viscosity of the melt and is most easily determined by means of practical experiments.

In another variant of the invention the short sides at the opening of the nozzle stone are made longer than the long sides so that these short sides extend outside the long sides. This is most easily achieved by applying a boss at the opposite short sides of the tapping opening. In this latter variant no speed component is achieved towards the side but the melt is guided by the bosses along the short sides for a longer period and the result will therefore be substantially the same as with the previous variant. These bosses may therefore be rounded towards the melt so that a somewhat more stable stream is obtained.

However, the contraction of the molten stream is most effectively delayed by a combination of the means described above, i.e. by applying a pair of bosses outside the orifice of the outlet gap, forming an extension of the short walls of the gap opening, the opposite sides, possibly rounded towards the melt, diverging from each other at an angle which permits the melt to really follow these diverging sides.

The invention will be described in more detail with reference to the accompanying drawings, and is defined in the following claims.

FIG. 1 shows a cross section through a means for atomizing a melt and cooling the drops of melt obtained to a solid powder, while FIGS. 2, 5, 8 and 11 shows nozzle stones according to the invention, seen from the outlet side of the melt. FIGS. 3, 4, 6, 7, 9, 10, 12 and 13 show the longitudinal and transverse section, designated by the same numbers in Roman numbers as in FIGS. 2, 5, 8 and 11.

The means shown in FIG. 1 consists of a vertical atomizing chamber 1, for example of stainless steel, at the upper end of which is a casting box 2. This is filled with the molten metal 3 to be atomized and cooled until it forms a solid powder. The casting box is provided at the bottom with a nozzle stone 4 having a tapping opening 5 inside it. The melt 3 flows gradually out of the tapping opening 5 in the form of a molten stream 6. On each side of the tapping opening 5 is an atomizing nozzle 7, 8 at the lower side of the casting box, these having the shape of narrow slit nozzles which run parallel to each other in a direction perpendicular to the plane of the figure. Sharp, flat gas jets are directed from these two nozzles on each side of the molten stream towards the stream in such a way that a first gas jet 9 from the nozzle 7 intersects the molten stream 6 at an angle of, for example, 45.degree., thus causing the melt to alter direction and follow the gas jet in the form of a layer spread out over this. A short distance from this first intersection point the stream is intersected by a second gas jet 10 from the nozzle 8 which intersects the molten layer over the gas jet 9 and accelerates the melt in a new direction so that this finally divided into free drops 11 which are spread out inside the atomizing chamber and fall freely through this, being cooled to form a solid powder 12 which is collected at the lower end of the chamber and fed through a rotating outlet valve 13. An inert gas, preferably argon, is used as atomizing gas, this being supplied to the nozzles in compressed state. The nozzle 7 is supplied with excess gas which is pumped from the lower part of the atomizing chamber out through a pipe 14 and, when it has passed through a cooler 15, a gas cleaner 16 and a compressor 17, is supplied to the nozzle at suitable pressure. The nozzle 8 is supplied with atomizing gas through a tube 18 which either comes from a similar circulation system as that for the tume 14 or from an external pressure source. The valve 19 on the wall of the atomizing chamber is used for the supply or removal of gas from the granulating chamber so that this is permanently filled witn an inert atmosphere of suitable pressure. If an inert atmosphere is permitted to flow continuously through the atomizing chamber, the molten drops obtained can be cooled considerably more easily. The granulation chamber is provided at the bottom with a watercooled sheath 20 to which cooling water is supplied through a supply pipe 21 and removed through an outlet pipe 22.

FIG. 2 shows a nozzle stone 23 provided with the theoretically simplest construction of a gap-like outlet opening for a melt. It may replace the nozzle stone 4 in FIG. 1 for example. The nozzle stone 23 is provided with a gap-like opening 24 limited by a pair of short walls 25 and a pair of long walls 26 arranged opposite to each other. FIG. 3 shows a section III--III of the nozzle stone 23 while FIG. 4 shows a longitudinal section of the same nozzle stone along the line IV--IV. As is clear from these two figures, the outlet opening 24 of the nozzle stone has an upper inlet part 27 which is funnel-shaped, becoming narrower downwardly towards an outlet part where the opposite pairs of inner walls run parallel to each other. If a melt is permitted to flow out through an outlet opening of the type shown in FIGS. 2-4 a flat stream of melt is formed which, because of the surface tension of the melt, starts to contract towards a circular cross section immediately outside the orifice of the outlet opening. The purpose of the inlet part 27 is to see that the lower part of the outlet opening is kept permanently filled with melt since it can otherwise only function as a funnel section which will not even give the tapping stream the desired shape from the start.

FIG. 5 shows from below a nozzle stone 28 provided with a gap-like outlet opening 29 which is constructed in accordance with the present invention. FIG. 6 shows a cross section through the nozzle stone according to section VI--VI while FIG. 7 shows a longitudinal section through the same stone along the section VII--VII. As can be seen from FIGS. 5-7, the outlet opening of the nozzle stone 28 also has a funnel-shaped inlet part 32. From the inlet part the two long sides 31 of the outlet opening 29 converge slightly towards each other right up to the orifice of the outlet gap, whereas the gap walls 30 limiting the short sides of the outlet opening, for having slightly converged from the funnel-shaped inlet part 32, start to diverge from each other towards the orifice. This means that the gap opening expands in its longitudinal direction towards the orifice and this in turn causes the melt when it flows through the outlet opening 29 to follow the gap walls along their oblique edges because of the surface tension conditions and the melt will therefore acquire a speed component which is directed towards the sides in the spreading direction of the molten stream which in turn causes the contraction of the molten stream outside the orifice of the gap to be considerably delayed.

The slight converging of the funnel-shaped inlet part 32 and the long sides 31 towards each other right up to the orifice ensures that the entire outlet opening is kept filled with melt. If the device according to the present invention is to function it is necessary for the orifice of the outlet opening, in spite of the short sides 30 diverging from each other, to have the smallest cross-sectional area of the outlet opening, or at least a cross-sectional area which does not exceed the cross-sectional area in any other section of the outlet opening. Should any other section than the actual orifice be the narrowest section of the outlet opening, the quantity of melt flowing out will not be sufficient to be able to follow the diverging short walls in the manner which is characteristic of the invention.

FIG. 8 shows a second variant 33 seen from below of a nozzle stone according to the invention. This nozzle stone is provided with a gap-like outlet opening 34 limited by long sides 35 and short sides 36. The nozzle stone is also provided on its lower side with two bosses 37 and 38, the opposite sides of which form extensions of the short walls 36 of the gap. FIGS. 9 and 10 show transverse and longitudinal sections IX--IX and X--X, respectively, of the nozzle stone 33. As can be seen from these figures, the nozzle stone 33 also has an upper funnel-shaped inlet part 39 to the outlet opening 34. The long edges 35 of the outlet opening 34 converge slightly from the funnel-shaped inlet part and, at least nearest to the inlet part, the short sides 36 may also converge. However, in this example they run parallel to each other along the bosses 38 and 38. Since the bosses 37 and 38 provide a direct continuation of the short sides 36, the melt will be guided along the short sides for a longer distance than along the long sides 35. This means that the contraction to which the melt flowing through the outlet opening 34 is subjected will be considerably delayed in comparison with melt flowing through a gap opening shaped as shown in FIGS. 2-4. If the edges of the bosses 7 and 8 facing each other are rounded in the manner shown in FIGS. 8-10, a more stable stream is obtained than if these edges had been flat.

FIG. 11 shows from below the nozzle stone according to the invention which has so far proved to be the most suitable. This nozzle stone 40 has a gap-like outlet opening 41 limited by inner longitudinal walls 42 opposite to each other and inner short walls 43 opposite to each other. FIGS. 12 and 13 represent transverse and longitudinal sections, respectively, through the nozzle stone 40 along the dotted section lines XII--XII and XIII--XIII. As can be seen from FIGS. 12 and 13, the outlet opening 41 is also provided with a funnel-shaped inlet part 44. At the short sides 43 of the funnel-shaped outlet opening 41 on the lower side of the nozzle stone two bosses 45 and 46 are arranged extending below the bottom of the nozzle stone in such a way that they provide a continuation of the short walls 43. As can be seen from FIG. 13, the short walls 43 also diverge from each other along the bosses 45 and 46 at the orifice of the outlet opening 41. Since the short walls 43 diverge from each other towards the orifice of the gap and since these short walls in the bosses 45 and 46 extend below the long walls 42, the melt will be guided along the short sides of the gap-like outlet opening for a long way and it will also acquire speed components in the spreading direction of the molten stream. Both these factors contribute to a further delay in the contraction of the molten stream towards circular cross section. As previously stated, the outlet opening may not be narrower at any section than it is at the orifice in this case as well.

The nozzle stones shown in FIGS. 5-13 thus cause streams of a melt spread in a certain direction to retain their spread shape for a sufficient period of time for them to be disintegrated into fine drops with the help of flat jets of some suitable atomizing medium, for example an inert gas. The flat jets are directed in such a way that their direction of movement intersects the spread stream of melt substantially straight across its spreading direction. The nozzle stones shown in FIGS. 2-13 should therefore be fitted in the device according to FIG. 1 in such a way that the gap-like outlet openings 24, 29, 34 and 41 extend perpendicular to the plane of FIG. 1.

The invention is not limited to the examples shown above but can be varied within the scope of the basic inventive idea.

Claims

What is claimed is:

1. A nozzle stone for a casting ladle or a casting box, having a substantially rectangular outlet opening of a pair of long and short edges for a molten stream of metal spread in a plane extending in the longitudinal direction of the rectangular outlet, said rectangular opening defined by, inwardly from the opening, projecting walls of the rectangular outlet, and walls arranged in pairs opposite each other, said walls forming a pair of short sided walls and a pair of long sided walls, said long sided walls defining narrow edges of said rectangle and wherein a projecting boss at each of the short edges of the rectangular outlet opening protrude outside the outer edges of the long walls whereby the bosses provide an extension of the narrower edges of the rectangular opening.

2. The nozzle stone according to claim 1 and wherein the long sided walls of the rectangular opening diverge outwardly at the rectangular opening.

3. The nozzle stone according to claim 1 and wherein the bosses are convexly round towards each other.

4. The nozzle stone according to claim 1 and wherein the bosses are convexly round towards each other and their faces divergent in respect to each other.