WO1995032819A1

WO1995032819A1 - Manufacturing of high alloy wire

Info

Publication number: WO1995032819A1
Application number: PCT/SE1995/000612
Authority: WO
Inventors: Christer ÅSLUND
Original assignee: Anval Nyby Powder Ab
Priority date: 1994-05-30
Filing date: 1995-05-30
Publication date: 1995-12-07

Abstract

The invention relates to a method of manufacturing wire from a high alloy material in which thin-walled tubes of a ductile carbon steel filled with a powder of the high alloy material are placed in a can and powdered filler material is introduced between the tubes and the can, and then the can is sealed, heated and extruded to a bar comprising extruded wires in a matrix material. In order to release the extruded wire from the matrix in an easy way it is necessary that the transition temperature of the carbon steel of the tubes is substantially below the transition temperature of the carbon steel of the filler material which is at or above room temperature, and that the tubes in the can are placed at a distance from each other which must not be less than 2 times the maximum particle size of the filler material.

Description

Manufacturing of high alloy wire

The present invention refers to a method of manufacturing wire, especially welding wire, from a powdered high alloy mate¬ rial by extrusion of a sealed can containing rods of the high alloy material enclosed by a powdered filler material.

High alloy materials having a good corrosion resistance and/ or wear resistance are used within areas having wear problems, for instance for coating of rolls, conveyors, guiders in rolling mills etc. They are also used in parts which are subjected to high temperatures, for instance within the aircraft and the space industry. A large usage is within the nuclear industry where critical components such as valves in pipe systems are coated with such alloys. The coating can be applied according to several methods, some of the most usual ones are TIG- or MIG- welding, and the coating is then often treated by grinding or polishing. In order to perform such a welding, a welding wire of the alloy in question is required, of high quality and having a diameter under- 3 mm.

Conventionally metal wire is produced by rolling or drawing, which, however, is not possible with these high alloy material, which are difficult to process owing to a low ductility and an extremely high yield strength also at an increased temperature. Nor can a thin wire, having a diameter under 3 mm, be produced by casting. In order to produce welding wire from a high alloy material it has thus been necessary to use other techniques. One possibility has been to grind down a thicker wire to the required thickness, which of course is costly owing to the big material loss. Another way has been to produce a tube electrode from carbon steel which is filled with a metal powder of such an analysis that the melted electrode gets the correct final ana¬ lysis. A disadvantage with this method is the insufficient homogeneity of the final wire, which has led to that wires of this type are not approved of for a number of applications.

US patent 4,209,122 describes a method of manufacturing wire from a high alloy material for coating, a method which requires a thin and homogenous welding wire. With this method a plurality of rods casted from the high alloy material are positioned in a can, the spaces between the rods and the can are filled with the powdered filler material, and after vibration the can is closed forming a billet. After heating to the forging temperature the billet is extruded under pressure and area reduction, is allowed to air' cool and then the extruded rods are released from the can and the filler material. In this way metal wire can be obtained having a diameter down to 0.25 mm. A disadvantage with this method is, however, that the separation of the extruded wire from the matrix material is time consuming and expensive. In order to release the wires the ends of the extruded billet have to be cropped off, the can peeled and the matrix material pickled away in a pickling acid attacking this but leaving the alloy wire intact. This technique is very costly as the pickling time is long and the pickling for instance for environmental reasons is an expensive process.

In order to overcome the disadvantages being associated with the manufacturing of wire in accordance with the patent above it is in US patent 4,777,710 suggested to manufacture wire accord¬ ing to an improved method in which the extruded wires more easily can be separated from the surrounding material. The described method is characterized in that the rods, before being positioned in the can, are coated with a parting agent which facilitates the separation, that the filler material is brittle at room temperature, and that the rods are placed in the can with a maximum adjoining contact forming a densely packed geo¬ metric pattern. In order to make the separation of the extruded rod from the surrounding material easier a special rod separat¬ ing apparatus has been constructed to be used for said purpose. Although the method here described will give a wire of the required dimension this wire, as well as wire produced by the method mentioned above, can have an insufficient ductility or flexibility owing to the fact that the manufacturing has started from a casted material. The described method is not suitable for manufacturing of welding wire in a powder metallurgical way.

SE patent 7502944-7 describes a method of producing tubes, rods or similar shaped, elongated metallic objects having a very homogenous structure and uniform physical and chemical charac¬ teristics. In this method a sealed can filled with powder of a metal alloy, which powder has spherical particles and a grain size smaller than 1 mm obtained by argon atomization, is extruded. Before the extrusion the density of the powder in the can is increased, for instance by vibration, and then the can is sealed and subjected to a cold isostatic pressing to reach at least 80% of the theoretical density. This method can, however, not be used for producing wire but only for producing material of larger dimensions.

There is consequently still a need for an improved method for manufacturing and separation of a thin wire of a high alloy material, especially a welding wire, showing high ductility and homogeneity.

It has now turned out that by combining different steps it is possible in an economically and technically favourable way to manufacture and separate a wire having a surprisingly high duc¬ tility and tolerance, that is having a high degree of uniformity both in the longitudinal and the cross direction, by extruding a sealed can containing tubes filled with powder of the alloy, which rods are separated fro each other by means of the filler material, and subsequent mechanical processing of the extruded bar and finally refining pickling of the separated wires. The ductility and the homogeneity of the manufactured wire is higher than for a wire produced by extruding a cast rod and allows an easy spooling of the wires, after butt welding to a continuous wire, on a coiler.

The invention refers to a method of manufacturing wire of high alloy material comprising the following steps: positioning of a plurality of rods of a high alloy material in parallel relation with each other in a can, closing one end of said can, introducing a powdered filler material between the rods and the can, compacting the filler material by agitation and closing the other end of said can, heating said can to the forging temperature of said rods, extruding the can to a bar and, after allowing the bar to cool, removing the extruded wires of high alloy material from the can and the filler material, which method is characterized in that the rods of the high alloy material are thin-walled carbon steel tubes filled with a powder of the high alloy material, the carbon steel of the.tubes having a transition temperature sub¬ stantially below room temperature, the filler material is a powder of a carbon steel having a transition temperature at or above room temperature, which when heated forms a coherent matrix, and the carbon steel tubes are positioned in the can at a dis¬ tance from each other and from the wall of the can which must not be below 2 times the maximum particle size of the filler material.

The high alloy material according to the invention refers to a high alloy material not having a pronounced transition tem¬ perature alternatively a very low transition temperature and being very difficult to process also at higher temperature owing to high hardness and low ductility. Of special interest are cobalt-base alloys, such as stellites, and nickle-base alloys, comprising super alloys, which are especially used for so called hard facing. Within this definition can also be comprised cer¬ tain high alloyed stainless steels.

The impact strength is a measure of the ability of a material to resist an impact without breaking. In particular it indicates the sensitivity of the material to brittle fracture, that is a fracture without any elongation. At a low temperature the impact strength is low (brittle fracture) , but is increased when the temperature is increased, first slowly and then turns into a high value (ductile fracture) when the temperature has passed a transitional area, the so called transition temperature. At the transition temperature a material passes from a brittle into a ductile behaviour, or in other words the material is 50% brittle and 50 % ductile. The higher the transition temperature the greater is the risk of a brittle fracture at room temperature.

It is essential that the high alloy material according to the invention has a transition temperature being well below the transition temperature of the material which is used as a filler material, alternatively does not have a defined transition temperature. This implies, in other words, that the impact strength at room temperature must not be lower than the impact strength of the matrix/filler material. In the same way the impact strength of the carbon steel tubes must be high at room temperature in order to prevent cracks to propagate into the high alloy material, preferably 4-5 times higher than the impact strength of the matrix material, in order to obtain satisfactory separation of the extruded wires without risking a fracture. For the carbon steel in the carbon steel tubes a typical value of a Charpy V test (10 x 10 mm, ASTM E 23) can be an impact strength of 150-200 J, preferably not below 100 J. The same preferably applies for the can material. The impact strength of the matrix material should at room temperature be below about 20 J, which implies that the material can be regarded as brittle.

Ductility refers to the ability of the material to be elon¬ gated without rupture. At higher temperatures, for instance at the so called hot working temperature or the forging tempera¬ ture, recrystallization takes place in most materials, that is a continuous recovering of the structure, which makes it possible to reach an elongation of several hundred per cent. Recrystallization only occurs to a very restricted extent in the high alloy materials which are processed according to the inven¬ tion, which makes rolling and forging thereof very difficult.

According to a preferred embodiment of the method of the invention, a powder of an alloy is used wherein each particle has the required analysis. The powder preferably consists of substantially spherical particles having a grain size < 500 μ , preferably < 250 μm, obtained by gas atomization, especially in argon.

The carbon steel tubes are made from a carbon steel having a low transition temperature and a high ductility. The carbon content of the carbon steel should be < 0.20%, preferably 0.12 - 0.15%, for the tubes to have a sufficient ductility to be able to be extruded and mechanically worked without cracking. It is also important that the tubes are thin, that is have a wall- thickness preferably not exceeding 1-2 mm, as they are to be pickled away after the extrusion. The diameter of the tube is in general 5-15 mm depending on the length of the tube and the required final dimension. The length of the tubes has to be adapted to the press equipment used and consequently to the length of the can.

The can into which the carbon steel tubes are to be posi¬ tioned should be made by a carbon steel material having a suffi- cient ductility to be extruded, for instance of the same material as the above mentioned carbon steel tubes. In order to avoid inhomogeneities it is convenient that the inside of the can or the screen has notches, for instance in the shape of longitudinal wings, which prevent the tubes filled with powder to get too close to the inside wall of the can.

As a filler material can be used any carbon steel in powder form having a transition temperature at or above room tempera¬ ture and clearly above the transition temperature of the carbon steel which is used in the tubes containing the alloy powder and which is inert in relation thereto. It is essential that the matrix formed by the filler material after extrusion has a very low ductility, which allows for the removing thereof by mechan¬ ical means, preferably at room temperature. The carbon steel can also be mixed with a ceramic material in order to increase the brittleness. The carbon steel tubes on the other hand shall have a high ductility to be able to resist this mechanical working without cracking. Appropriate matrix carbon steels have a carbon content > 0.5%, preferably about 0.7-1.1%. The filler material should in addition have a grain size < 500 μm. A filler material of a carbon steel having a carbon content of 0.8% has a transi¬ tion temperature above 100°C and a very low impact strength at room temperature, normally 5-15 J.

In order to keep the tubes at an adequate distance from each other in the can the tubes are placed in a screen. Said screen can be made from wire nettings of an adequate mesh size or in any other conventional way. Optionally it can in part or totally be provided with longitudinal intermediate partitions in order to further facilitate the separation of the extruded wires. The screen can be made from the same material as the filler material, but it can also consist of any other material which does not have an effect on the tubes and which is brittle at room temperature.

In order to attain a free flow at the filling and by that homogenous filling with the filler material it is necessary that the distance between the carbon steel tubes in the can is larger than 2 times the maximum particle size of the filler material. In order to guarantee a free flow at the filling and a uniform distribution of filler material around the tubes the smallest distance between the carbon steel tubes should therefore be about 2, preferably 3 times the maximum particle size. It is essential that no carbon steel tube being filled with alloy powder gets into contact with an adjacent tube or with the wall of the can as this would make a homogenous deformation of the tubes impossible, which tubes would rather get an oval or in any other way irregular form.

In order to make the filling easier, of the carbon steel tubes as well as of the can, they can be vibrated at any suit¬ able step of the process. After that the sealed can be subjected to a cold isostatic pressing in order to prevent segregation of the alloy powder in the carbon steel tubes as well as the filler material in the can during the continued handling. When segre¬ gated, coarser and finer particles will land up in different places in the can which leads to an inhomogeneous deformation of the steel tubes during the following extrusion. By said press¬ ing, which is performed at a pressure of 200-500 MPa, preferably 350-450 MPa, approximately the same density is reached in the filler material and in the tubes, that is of the size 65-75% of the theoretical density. This means that an increase of the den¬ sity of about 5% is obtained compared with the filling density, and implies that the powder particles are pressed against each other and will not segregate during the subsequent handling. That the density will not be higher depends on the hardness of the filler material and the high alloy material.

The extrusion of the sealed can which has been heated to forging temperature, that is the temperature suitable for pro¬ cessing which is about 1100-1400°C depending on the sintering temperature of the alloy to be worked, can be performed in any conventional way. In particular the so called Ugine-Sejournet method is used in which glass is used as lubricant.

According to a preferred method of the invention the carbon steel tubes filled with alloy powder, the screen and optionally also the inside of the can is coated with a parting agent faci¬ litating the separation of the extruded wire. The parting agent can be a high melting fine-grained material, which is not reac¬ tive with the coated parts or the matrix, such as a powdered ceramic material. Suitable substances are alumina, silica, manganese oxide and glass powder. The parting agent can be applied by dipping of the actual parts in a mixture comprising an aqueous suspension of the parting agent or by spraying or in any other way.

According to another preferred method of the invention the extruded bar is immediately after the extrusion quenched in water. Thus the brittleness of the matrix material is additio¬ nally increased, which creates micro-cracks, which in turn allows for a simplified release of the extruded wire.

The invention is further illustrated by the accompanying drawings in which

Figure 1 is a schematic view in longitudinal section showing a filled can which can be used in the method of the invention;

Figure 2 is a schematic top view showing a screen which is used in the can of Figure 1;

Figure 3 is a schematic view in cross-section showing an alternative design of a can, which can be used according to the invention;

Figure 4 is a micro photograph showing a section through an extruded stellite wire produced according to Example 2 in a magnification of 50 x.

A method of the invention for manufacturing a thin metal wire from a powder of the required composition can in short be described as follows with reference to Figures 1 and 2.

Thin-walled highly ductile carbon steel tubes 3 are posi¬ tioned in a screen 4 to be kept separated from each other, are closed in one end, filled with a spherical gas atomized alloy powder of the required analysis having a particle size < 500 μm and are then closed in the other end. The bundle of tubes is then positioned in a can 1 of carbon steel, having one end 5 closed, and the spaces between the thin-walled tubes, the screen and the can are filled with a powdered filler material 2, being a carbon steel having a low ductility at room temperature. The can is vibrated, closed in the other end 6 by welding and cold isostatically pressed at room temperature in order to compact the matrix and alloy powders in order to prevent segregation. After heating the can to working temperature it is extruded to a bar comprising separate wires of high alloy material in a carbon steel matrix. The bar is then mechanically worked at room tem¬ perature in order to uncover the wires, and the uncovered metal is then clean pickled, the wires are connected to each other by welding, ground to the exact dimension and coiled.

According to a preferred method of the invention a stellite welding wire is manufactured as follows.

Thin-walled high ductile carbon steel tubes 3 are placed in a screen 4 in order to be kept separated from each other, are closed in one end and filled with a spherical gas atomized stellite powder of the required analysis having a particle size

< 250 μm. After closing the other end of the tubes and coating the tubes with a parting agent, the bundle of tubes is posi¬ tioned in the can 1 of carbon steel, one end of which is closed, and then the spaces between the thin-walled tubes 3, the screen 4 and the can 1 are filled with a powder 2 of a carbon steel having a low ductility at room temperature and a particle size

< about 500 μm. The can is vibrated, sealed by welding and cold isostatically pressed at room temperature under a pressure of 350-450 MPa in order to compact the matrix and alloy powders to a theoretical density of 65-75% in order to prevent segregation. After heating the can to 1100-1250°C it is extruded to a bar comprising separate wires of high alloy material in a carbon steel matrix. The bar is quenched in water in order to increase the formation of martensite in the matrix material and is then mechanically worked at room temperature in order to uncover the wires. The uncovered wires are then cleaned from the parting agent, pickled to remove the thin layer from the carbon steel tubes, welded together and ground to the exact dimension.

The can 1 illustrated in Figure 1 is in the upper end closed by a welded front plate 5. After the introduction of the carbon steel tubes 3, filled with a powder of the high alloy material, positioned in the screen discs 4 and filling of filler material 2 the can is vibrated and then sealed by welding of the back plate 6.

The screen disc 4 illustrated in Figure 2 is made from a perforated plate having square openings 8 of 8 x 8 mm separated by metallic strands of a width of 2 mm. Along the circumference the screen disc is provided with spacer means 7 giving an appro¬ priate distance to the wall of the can 1.

Figure 3 illustrates in cross-section an alternative design of a can 1 which has been constructed with longitudinal wings 9 in order to facilitate the breakage of the can after extrusion. In such a can the carbon steel tubes 3 are positioned in a screen centred between the wings 9.

The invention is illustrated in detail by means of the following not limiting example on manufacturing of stellite welding wire having a composition corresponding to Stellite®6 (registered trade mark for Stoody Deloro Stellite, USA) .

Example 1

In a can of carbon steel having a diameter of 144 mm and a length of 625 mm a screen in the form of two discs of a perfor¬ ated plate having apertures of 8 x 8 mm and a plate width between the apertures of 2 mm is positioned. The screen discs were provided with spacer wings in part to prevent direct con¬ tact with the wall of the can and in part to keep the discs in place. The discs were positioned one in each end of the can, about 50 mm from the plates by means of which the can is sealed. The front plate of the can was welded. Carbon steel tubes having a length of 525 mm had previously been placed in this screen and after being closed in one end filled with a gas atomized stelli¬ te powder, an alloy of cobalt-base type which is used in weld¬ ing. The stellite powder consisted of spherical particles having a maximum grain size of 250 μm and had been obtained from Anval Nyby Powder AB as Alloy 6. A filling density of about 65% was obtained. After the filling with the alloy powder the tubes were closed also in the other end by pressing and then the screen was positioned in the can in such a way that the carbon steel tubes were in contact with the front plate. The carbon steel tubes had a diameter of 8 mm and a wall thickness of 1 mm and the carbon steel of the tubes had a carbon content of 0.12% (corresponding to Swedish standard SIS 1311) . The can was made from the same steel. Then the spaces between the tubes and between the can and the tubes were filled under vibration with a carbon steel powder of a high carbon content, 0.85% C. The powder had a maximum particle size of 500 μm and consisted mainly of spherical part¬ icles giving a filling density of 65%. The can was then sealed by welding the other end plate and cold isostatically pressed at 350 MPa (3500 bar) to prevent segregation of the powder during the continued handling. This caused the density to increase about 5% to approximately 70%. The sealed can was then heated to 1150°C and extruded, by the so called Ugine-Sejournet method, having glass as a lubricant, to a round bar having a diameter of 42 mm, corresponding to an area reduction of about 12 times. The carbon steel tubes had been extruded to wires with a length of about 5 m and a diameter of about 1.8 mm, embedded in a matrix of filler material. The powder in the matrix and the carbon steel tubes had been condensed to full density.

After air cooling of the bar an examination of the micro- structure showed that the matrix was heavily cracked with a large number of micro cracks. There were, however, no cracks in the carbon steel tubes surrounding the stellite wire.

In order to remove the matrix surrounding the wire longitudi¬ nal notches were cut in the can and then the bar was worked at room temperature in an hydraulic press and subjected to a stepwise threepoint bending. In this way the matrix cracked and . the stellite wires could be uncovered comparatively easily. The wires then were pickled in 4% hydrofluoric acid and 15% nitric acid to remove the carbon steel tubes and small residual amounts of the adhering matrix material. The pickling was finished after about 2 hours. After the clean pickling the wires were welded together and tolerance ground to a diameter of 1.65 mm, a stan¬ dard dimension for TIG-welding.

The wire obtained had a sufficient ductility and homogeneity to enable spooling.

Comparative Example 1

If in the method above a carbon steel having a carbon content of 0.5% was used instead as a filler material fewer cracks were formed in the matrix during the mechanical working of the extruded bar and remainders of the matrix were left on the extruded wire. This made the pickling take a long time, almost 12 hours, which is not reasonable in industrial connections.

Comparative Example 2

The method of Example l was repeated, but with a screen of such a design that some of the carbon steel tubes came into contact with each other or with the wall of the can.

Analysis of the extruded bar showed that the extruded wires did not have a uniform cross-section but rather an oval form falling outside the tolerances for the final wire.

Example 2

Welding wire was prepared according to the same process as in Example 1 with the difference that the screen with the filled carbon steel tubes was dipped into a solution of suspended alumina in water with a small amount of glue as a binder. The alumina was obtained from Alcoa, Canada, and had a particle size < about 2 μm. In order to further increase the brittleness of the matrix the extruded bar was quenched in water immediately after the extrusion instead of being air cooled.

Analysis of the extruded bar after extrusion and water cooling showed that there were a large number of micro cracks in the matrix material and that the alumina formed a layer between the extruded wire and the matrix. Figure 4 shows a photograph of the micro structure of a section of an extruded stellite wire 10 having an approximate diameter of 1.2 mm in the matrix material 2 in a magnification of 50 x. 11 shows the surrounding carbon steel tubes having a thickness of about 0.2 mm. From the figure can be seen that a heavy crack had been formed in the matrix material and that the tube at 12 had been released from the matrix which can be explained by the presence of parting agent.

After mechanical working in the way stated above the wires were uncovered very easily and optional matrix residues on the wires could easily be drawn off. The layer of alumina was removed by blasting or brushing. The thin carbon steel tubes which had been drawn out into a thin film on the stellite wires could easily be removed by pickling in nitric acid. This pick¬ ling lasted no longer than about 1 hour.

Claims

1. A method of manufacturing wire of a high alloy material comprising the following steps positioning a plurality of rods of a high alloy material in parallel relation with each other in a can, closing one end of said can, introducing a powdered filler material between the rods and the can, compacting the filler material by agitation and closing the other end of said can, heating said can to the forging temperature of said rods, extruding the can to a bar and, after allowing the bar to cool, removing the extruded wires of high alloy material from the can and the filler material, which method is characterized in that the rods of the high alloy material are thin-walled carbon steel tubes filled with a powder of the high alloy material, the carbon steel of the tubes having a transition temperature sub¬ stantially below room temperature, the filler material is a powder of a carbon steel having a transition temperature at or above room temperature, which when heated forms a coherent matrix, and the carbon steel tubes are positioned in the can at a distan¬ ce from each other and from the wall of the can which must not be below 2 times the maximum particle size of the filler materi¬ al.

2. Method according to claim 1, characterized in that the powder of the high alloy material is composed of spherical particles having a particle size < 500 μm, preferably < 250 μm, obtained by gas atomization.

3. Method according to claim 1 or 2, characterized in that the filler material has a particle size < 1000 μm, preferably < 500 μm.

4. Method according to any of claims 1-3, characterized in that the carbon steel in the tubes has a carbon content < 0.20 %, preferably 0.12-0.15 %, and in that the carbon steel in the filler material has a carbon content > 0.5 %, preferably 0.7-

1.1 * _ .

5. Method according to any of claims 1-4, characterized in that the carbon steel tubes are kept at a distance from each other by being placed in a screen in the can.

6. Method according to any of claims 1-5, characterized in that the impact strength of the carbon steel tubes at room temperature is at least 4-5 times higher than the impact strength of the matrix material.

7. Method according to any of claims 1-6, characterized in ■ that the can after being sealed and before the heating is com¬ pacted by means of cold isostatic pressing to a theoretical density of 65-75 %.

8. Method according to any of claims 1-7, characterized in that the carbon steel tubes filled with high alloy powder, the screen and optionally the inside of the can are coated with a parting agent in order to facilitate the removal of the extruded wire from the matrix material.

9. Method according to any of claims 1-8, characterized in that the extruded bar is quenched in water in order to increase the brittleness of the matrix material.

10. Method according to any of claims 1-9, characterized in that a welding wire is manufactured from a cobalt-base alloy.