EP0377390A1 - Method of making bimetallic tubes, and tubes made by this method - Google Patents

Abstract

The method according to the invention relates to the production of a bimetallic tube, the cross-section of which comprises an outer annular zone and an inner annular zone, these zones being of different compositions. The method resides in preparing a blank (1) comprising two tubular components (2, 3) housed one inside the other and of different compositions and separated by an annular space (4) which is filled with divided metal. The whole assembly is made rigid by two end pieces (7, 8) and coextrusion is carried out at a suitable temperature to ensure a metallurgical bond between the inner component (2) and the outer component (3). The bimetallic tube obtained is particularly suitable for uses in which one of its two outer or inner walls is exposed to a medium which is more aggressive than the other.

Description

The invention relates to a method of manufacturing bimetallic tubes, the section of which comprises an external annular zone and an internal annular zone of different compositions. It is particularly applicable to steel tubes.

The invention also relates to the tubes obtained by this method, in particular the steel tubes, as well as the tubular blank enabling the manufacturing method of the invention to be implemented.

Such tubes can in particular be used whenever only their outer or inner wall is in contact with a fluid whose composition, temperature or other characteristics require the use of a metal or alloy of particular composition and cost relatively high. We can then limit the thickness of the annular zone made of such a metal or alloy and use for the rest of the section of the tube a metal or alloy of much lower cost, whose essential function is then to ensure the holding tube mechanics.

A method of manufacturing such bimetallic tubes is known. It consists in producing a blank comprising two tubular components of different compositions which one fits into the other. One of the components is made of stainless or refractory steel or of a refractory alloy. The other component is, for example, unalloyed or alloyed steel.

These two components must have a cylindrical shape of revolution and be machined with the precision necessary to be able to be fitted into each other with the minimum of play. After heating to the desired temperature, is carried out, in a manner also known, coextrusion of this blank, with a determined reduction rate of section, in order to obtain a bimetallic tube. It is then found that, if the composition of the metals or alloys involved, the surface condition of the walls in contact, as well as the extrusion conditions are suitable, a good bond of metallurgical nature is obtained between the two components.

In practice, this process is relatively expensive to implement, in particular because it is necessary to carry out precise machining of the two components of the blank. Each of the two components must be machined so as to have a constant thickness. In addition, for the two components, the machining of the inner wall over a relatively large length presents difficulties which make it expensive. Finally, special precautions may have to be taken to limit the oxidation of the walls facing the two components of the blank, during the heating thereof, before coextrusion. Additional difficulties are due to the difference in coefficient of expansion which most often exists between the two components of the blank. Indeed one of the two components is often made of an austenitic steel or other alloy with a coefficient of expansion much higher than that of the other component which is made of an unalloyed or low alloyed steel.

When it is the external component which has the highest coefficient of expansion, one observes, during the heating of the blank, an increase in the clearance between the two components. This increase can be a cause of oxidation and can cause irregularities in the flow of one of the components with respect to the other when passing through the die.

When, on the contrary, the internal component has the highest coefficient of expansion, the two components are clamped against each other when heated.

We looked for the possibility of simplifying the process for producing bimetallic tubes by coextrusion, in order to make it at the same time safer, more reproducible and more economical.

In particular, research has been carried out on the possibility of eliminating the need for a precise fitting of the two components of the blank into one another with a clearance reduced to a minimum.

The possibility has also been sought of using internal and external components with different expansion coefficients without risk of notable oxidation on heating or of flow anomalies during coextrusion.

Finally, we looked for the possibility of effectively protecting the walls facing the two components from oxidation during the heating of the blank, before coextrusion.

The process which is the subject of the invention makes it possible to achieve these results and to obtain a bimetallic tube which does not have the defects of those produced by the known process. This bimetallic tube is also the subject of the invention.

The method of manufacturing a bimetallic tube by hot coextrusion according to the invention consists in producing a blank comprising two tubular components of coaxial revolution. These two components are made up of metals or alloys, of different compositions, housed coaxially one inside the other.

The sections of each of these tubular components are determined, in a plane perpendicular to the common axis, so as to provide between their facing walls an annular space, of radial width not less than 3 mm; the radial width of this annular space is preferably at least equal to 2% of the external diameter of the internal component and is not greater than the radial width of the tubular component of smaller thickness. This annular space is filled with a metal or divided alloy, the composition of which is compatible with the compositions of the two tubular components, then it is closed in a sealed manner at each of the two ends by means of closure. The blank is then heated to the extrusion temperature which is determined according to the characteristics of the metals or alloys which constitute it, then the coextrusion of this blank is carried out, by means of a press, through a die, so as to obtain a bimetallic tube, the reduction ratio between the solid section of the blank and that of the bimetallic tube obtained being at least equal to 4.

Preferably, the radial width of the annular space is not substantially greater than 10 mm.

Advantageously, the blank comprises a first tubular component made of unalloyed, or alloyed or even stainless steel, the second tubular component being made of a different material such as stainless or refractory steel or of a stainless alloy or refractory containing in total at least 50% by mass of elements of the group comprising Co, Cr, Mo, Ni, or in a nickel-based alloy.

Preferably, when the first tubular component is made of stainless steel and the second tubular component is made of stainless or refractory steel, the content of additives in the steel of the second component is higher than that of the steel in the first component .

Preferably, the radial width of the wall of the first component is greater than that of the wall of the second component. Preferably also, the mechanical characteristics of resistance to deformation of the steel of the first component are superior to those of the steel or alloy of the second component.

Depending on the intended applications of the bimetallic tube obtained by the process according to the invention, the first tubular component of the blank is the external component or the internal component, the second tubular component of the blank being, respectively, the internal component or the external component.

Preferably, the metal or divided alloy with which the annular space is filled consists for the most part of granules advantageously of generally substantially spherical shape, the mean diameter being less than 1 mm. This metal or divided alloy can be of any material compatible with the composition of the first and second tubular components. It may for example be a non-alloy, or alloy, or stainless steel or a stainless or refractory alloy containing in total at least 50% by mass of elements of the group comprising Co, Cr, Mo, Ni, or an alloy based on nickel. Preferably, the divided metal or alloy is packed in the annular space so as to reach an apparent density of at least 50% of the real density of this metal or alloy.

The means for closing the annular space of the blank are preferably two metal end pieces arranged at the two ends of the blank. These end pieces are advantageously made of unalloyed or alloyed steel.

Preferably also, each end piece is connected to the two corresponding ends of each component of the blank by sealed annular weld beads. Optionally, a vacuum can be created in the annular space before heating the blank to extrusion temperature.

The preform is extruded by means of a press, comprising a piston provided with a needle which engages in the preform, previously housed in a container, then in the die integral with the latter. This causes the flow of the blank and therefore of its components to flow through the annular space between the needle and the die, the lubrication being provided by a layer of glass.

The invention also relates to the tubular blank comprising the two coaxial tubular components whose structure has been described above and which makes it possible to implement the method according to the invention.

The invention also relates to a bimetallic tube of revolution, seamless, produced by coextrusion; this tube has an outer layer and an inner layer, made of different metals or alloys, bonded together metallurgically by a bonding layer from a divided metal; this, during the coextrusion process, was welded to itself as well as to the internal component and the external component.

• La figure 1 est une vue en coupe d'une ébauche, permettant de fabri­quer par le procédé suivant l'invention un tube bimétallique, le premier composant tubulaire de cette ébauche étant le composant ex­terne.
• La figure 2 est une vue en coupe d'une ébauche, permettant de fabri­quer par le procédé suivant l'invention un tube métallique, le premier composant tubulaire de cette ébauche étant le composant interne.

The figures and examples below describe, without limitation, two particular embodiments of the process for manufacturing bimetallic tubes according to the invention.

• Figure 1 is a sectional view of a blank, for manufacturing by the method according to the invention a bimetallic tube, the first tubular component of this blank being the external component.
• Figure 2 is a sectional view of a blank, for manufacturing by the method according to the invention a metal tube, the first tubular component of this blank being the internal component.

We see in Figure 1 a blank 1 for manufacturing, according to a first embodiment of the method according to the invention, a tube bimetallic which, by itself, is part of the invention. This blank 1, seen in section along a plane passing through the axis X1-X1, comprises two tubular components 2, 3 with cylindrical walls of revolution, arranged one inside the other, coaxially with respect to X1 - X1. The first tubular component 2, with a greater radial thickness "e1", is an external component made of low-alloy steel whose total content of additives is less than 5%. This thickness "e1" is more than twice that of "e2" of the second tubular component 3, which constitutes the internal component of the blank. An annular space 4 is formed between the outer wall 5 of the second tubular component 3 and the inner wall 6 of the first tubular component 2. The radial width "e3" of this annular space 4 is, in the case of this FIG. 1, much less to the radial thickness "e2" of the second tubular component 3. This radial width "e3" may be closer to the radial thickness "e2" of the second tubular component 3, the thinner of the blank, without however exceeding it . The second tubular component 3 can be produced, depending on the uses, in stainless or refractory steel or else in an alloy containing, in total, at least 50% by mass of elements of the group comprising Co, Cr, Mo, Ni or still in a nickel-based alloy. Two annular end pieces 7, 8 are each arranged at one end of the blank 1. These two pieces 7, 8 can be made of unalloyed or low-alloyed steel; they may have a composition close to that of the tubular component of the blank whose wall is the thickest. This composition is in particular determined to allow a tight junction by welding with the two tubular components 2, 3 of the blank 1. These 2 end pieces 7, 8 ensure the centering of the two tubular components 2 and 3 relative to the 'common axis X1 - X1 through annular ribs 9, 10 which engage with each other.

Before the final positioning of at least the last of these two end pieces, the annular space 4 is filled with a metal or divided alloy whose composition is compatible with the compositions of the two tubular components. This metal or divided alloy can be chosen, for example, from non-alloyed, alloyed, stainless or refractory steels or also from alloys containing, in total, at least 50% by mass of elements, from the group comprising Co, Cr, Mo, Or.

This divided metal is preferably in the form of granules, for the most part substantially spherical and with an average diameter of less than 1 mm.

Packing of this metal or alloy divided in the annular space 4 is carried out, by any suitable method, in order to obtain an apparent density at least equal to 50% of the actual density. This packing can in particular be carried out by vibration or compression. After the last of the two end pieces 7, 8 have been put in place, a sealed connection is made between each of them and the corresponding ends of the tubular components 2, 3 by sealed annular weld beads 11, 12, 13, 14. To avoid excess thickness and allow good penetration, chamfers are formed, inclined at approximately 45 °, on the end edges of the tubular components and of the end pieces in the areas where these weld beads are to be made.

The blank 1, thus prepared, is then heated by known means such as a gas oven, or electric radiation or induction oven or oven with a salt bath or the like. The heating temperature depends, on the one hand, on the characteristics of the metals or alloys which constitute the blank and, on the other hand, on the coextrusion conditions: strength of the press, dimensions of the blank, reduction rate of the section , nature of the lubricant used. This heating temperature is greater than 1000 ° C. The lubricants giving the best results are glasses. The reduction ratio between initial section of the blank and section of the tube obtained must be at least 4 and preferably at least 6 in order to obtain a good metallurgical bond, by means of the layer of divided metal. , between the external and internal layers of the tube obtained from components 2, 3 of the blank. The compositions and the thicknesses of the two tubular components 2, 3 of the blank 1 are determined according to the conditions of use of the bimetallic tube obtained. As a general rule, the first component 2, which is less alloyed, is in contact with the least corrosive fluid and its thickness is determined essentially to give the tube the necessary mechanical strength. This explains why this first component is, more often than not, thicker than the second. The composition of the second component 3 is chosen for its resistance to corrosion by the most corrosive fluid. In the case of the present figure 1 this fluid is that which circulates inside the tube. Experience shows that a wise choice of the metal or alloy constituting this second component makes it possible to provide very little wear and therefore a relatively small thickness of this component 3 compared to the thickness of the first component 2 necessary to ensure the mechanical strength of the tube.

As a practical example of using this first mode of implementing the method according to the invention, a blank 1 of structure similar to that of FIG. 1 is prepared. It comprises: a first component 2, external of 223 mm outside diameter, 140 mm inside diameter and 870 mm long in carbon steel with low additions of Mn and V of type 20 MV6 (AFNOR standard), a second internal component 3, of 126 mm outside diameter, 100 mm internal diameter and 870 mm long, in stainless steel type AISI 316 (AISI STANDARD).

The annular space 4 between the two components 2, 3 of 7 mm in radial width, is filled with divided stainless steel type AISI 316 L mainly in the form of substantially spherical granules of diameter between 0.1 and 1 mm. Tamping by vibration achieves an apparent density of approximately 60% of the actual density. This annular space is closed by two end plates 7, 8 also made of carbon steel type 20MV6. Each of these plates is provided with an annular rib 9, 10 of a few mm in height which engages in the annular space 4 filled with divided stainless steel. These two end plates 7, 8 are each connected to the two components 2, 3 by sealed weld beads 11, 12, 13, 14, produced by arc welding under argon.

This blank is then heated to a temperature between 1150 and 1200 ° C. in a gas oven, then, after coating conventionally carried out with a layer of lubricating glass, both on the exterior surface and on the interior surface, the following is introduced. the blank in the container of a press and coextrusion is carried out through a die of 117 mm in diameter. The press piston is provided with a needle of 94 mm in diameter which makes it possible to obtain, after coextrusion then release, a bimetallic tube of 114.3 mm of outside diameter and 92.6 mm of inside diameter. The reduction ratio between the section of the blank 1 and that of the tube obtained is therefore approximately 9.3.

Micrographic examinations carried out on samples taken in several points of the bimetallic tube show an excellent metallurgical bond produced by means of the layer of metal divided between the external layer and the internal layer at the time of the passage through the die. This layer of divided metal also makes it possible, before coextrusion, during the heating phase of the blank, to absorb the phenomena of differential radial expansion of one component with respect to the other; this bonding layer also facilitates, during coextrusion, the sliding of one of the two components of the blank relative to the other without risk of tearing, cracking or creasing.

FIG. 2 shows a blank 21, making it possible to manufacture, according to a second embodiment of the method according to the invention, a metal tube which, in itself, is also part of the invention. This blank, seen in section along a plane passing through its axis X2-X2 comprises two tubular components 22, 23 with cylindrical walls of revolution, arranged one inside the other, coaxially with respect to X2-X2. The first tubular component 22 is an internal component made of carbon steel. Its radial thickness, e11, is greater than that, e12 of the second tubular component 23 which is external. Between these two components is formed an annular space 24 between the outer wall 25 of the first tubular component 22 and the inner wall 26 of the second tubular component 23. The radial width e13 of this annular space is in the case of this blank much less than the radial thickness e12 of the thinnest external component 23 while being greater than 2% of the external diameter of the internal component 22, not less than 3 mm and not more than 10 mm. The second external component 23 is made of stainless or refractory steel or else of an alloy containing, in total, at least 50% by mass of elements of the group comprising Co, Cr, Mo, Ni.

Two annular end pieces 27, 28 made of carbon steel ensure the centering of the two components 22, 23 by means of the annular ribs 29, 30. Before placing the last end piece, the annular space 24 is filled of a metal or divided alloy whose composition is compatible with the compositions of the two tubular components and which is preferably in the form of granules for the most part substantially spherical and of average diameter preferably less than 1 mm. This divided metal can be an alloyed or unalloyed carbon steel, a stainless or refractory steel or even a alloy containing, in total, at least 50% by mass of elements of the group comprising Co, Cr, Mo, Ni. This divided metal is vibrated to obtain an apparent density at least equal to 50% of the actual density. The end pieces 27, 28 are connected to the corresponding ends of the components 22, 23 by annular and sealed weld beads 31, 32, 33, 34.

The blank 21 thus prepared is heated, by known means, to a suitable temperature above 1000 ° C. This temperature is determined, in known manner, taking into account the characteristics of the metals or alloys which constitute the blank and the conditions extrusion. The extrusion is then carried out in a known manner, by placing the blank, after coating, in a known manner both on its outer surface and on its inner surface, by a lubricating glass, in the container, provided with a die, an extrusion press. The thrust of the blank is provided by a piston provided with a needle which passes through the blank and engages in the die. Lubrication is preferably provided in a known manner by glasses. The sections of the needle, of the die, and of the blank are determined so as to obtain a reduction ratio of at least 4 and, preferably, at least 6.

By way of example, a blank of structure similar to that of FIG. 2 is prepared, comprising a first internal component 22 of carbon steel of type 20 MV 6 (AFNOR STANDARD) 189 mm in outside diameter, 60 mm in inside diameter and 870 mm long. The second external component 23, made of 316 stainless steel (AISI STANDARD) has an outside diameter of 223 mm, an inside diameter of 200 mm and a length of 870 mm. The surface conditions of the facing walls forming the annular space are prepared so as to avoid the presence of oxide. One can for example practice before mounting the blank brushing or grinding of these walls. The annular space 24 of 5.5 mm in radial width is filled with divided stainless steel, type 316 (AISI STANDARD), mainly in the form of substantially spherical granules with a diameter between 0.1 and 1 mm. After tamping by vibration, the apparent density of this divided steel is approximately 60% of its actual density. After the end pieces 27, 28 of 20MV6 steel have been put in place, they are connected to the two components 22, 23 by sealed weld beads 31, 32, 33, 34 produced by arc welding under argon.

The blank thus produced is heated between 1050 and 1200 ° C. in a gas oven and then, after coating with a layer of lubricating glass, in known manner both on the outer surface and on the inner surface, coextrusion using a press. The piston of this press comprises a needle of 52.1 mm in diameter which engages in the blank 21 then in the die of 66 mm in diameter.

After coextrusion then dewaxing, a bimetallic tube of approximately 63.5 mm outside diameter and 51.3 mm inside diameter is obtained. The reduction ratio compared to the initial section of the blank of 223 mm outside diameter and 60 mm inside diameter is approximately 31.

Micrographic examinations, carried out on samples taken at several points of the bimetallic tube, show an excellent metallurgical bond produced by means of the layer of divided metal between the internal layer and the external layer of the tube. Furthermore, taking into account the characteristics of the process, tubular products which do not require tight tolerances can be used for the two components, in particular for the facing surfaces forming the annular space, which makes it possible to lower the manufacturing costs.

For certain applications, it may be necessary to eliminate, before preheating, the rest of the air contained in the annular space 24 filled with metal or divided alloy. This can be achieved by creating a vacuum in this annular space, through a passage formed in an end piece. A closing means then makes it possible to close this passage before heating the blank or, at the latest, before coextrusion.

Numerous modifications can be made to the process according to the invention which are not outside the scope of this invention.

It is the same for the bimetallic tubes which are the subject of the invention.

Claims (17)

1) Process for manufacturing a bimetallic tube by hot coextrusion of a blank comprising two tubular components, of revolution, made of metals or alloys of different compositions, housed coaxially one inside the other, characterized in that determines the sections of each of these tubular components 2, 3, 22, 23, in a plane perpendicular to the common axis, so as to provide between their facing walls 5, 6, 25, 26 an annular space 4, 24 of radial width e3, e13 not less than 3 mm, at least equal to 2% of the external diameter of the internal component and not greater than that of the tubular component 3, 23 of thinner thickness, then in that it is filled with a metal or divided alloy, the composition of which is compatible with the compositions of the two tubular components, this annular space 4, 24 which is then closed, in a leaktight manner by closing means 7, 8, 27, 28 arranged at the two ends, then in that we heats the blank 1, 11 to the determined extrusion temperature, according to the characteristics of the metals or alloys which constitute it, then in that the coextrusion of this blank is carried out by means of a press through a die of so as to obtain a bimetallic tube, the reduction ratio between the solid section of the blank and that of the bimetallic tube obtained being at least equal to 4. 2) Method according to claim 1 characterized in that the annular space 4, 24 has a radial width not substantially greater than 10 mm. 3) Method according to claim 1 or 2 characterized in that the blank comprises a first tubular component 2, 22 in unalloyed or alloyed or stainless steel and a second tubular component 3.23 made of a different material such as stainless steel or refractory or a stainless or refractory alloy containing, in total, at least 50% by mass of elements of the group comprising Co, Cr, Mo, Ni, or in a nickel-based alloy. 4) Method according to claim 3 characterized in that when the first component is made of stainless steel and the second component made of stainless steel or refractory, the content of additives in the steel of the second component is greater than that of the steel of first component. 5) Method according to one of claims 1 to 4 characterized in that the radial width of the wall of the first component 2, 22 is greater than that of the wall of the second component 3, 23. 6) Method according to one of claims 3 to 5 characterized in that the mechanical characteristics of resistance to deformation of the steel of the first component 2, 22 are greater than those of the steel or alloy of the second component 3, 23 . 7) Method according to one of claims 1 to 6 characterized in that the metal or divided alloy which is filled with the annular space 4, 24 consists mainly of substantially spherical granules with an average diameter less than 1 mm. 8) Method according to one of claims 1 to 7 characterized in that the metal or divided alloy is an unalloyed, or alloyed, or stainless steel or a stainless or refractory alloy containing in total at least 50% by mass of elements from the group comprising Co, Cr, Mo, Ni. 9) Method according to one of claims 1 to 8 characterized in that the metal or alloy divided in the annular space 4, 24 is packed so as to reach an apparent density of at least 50% of the actual density of this metal or alloy. 10) Method according to one of claims 3 to 9 characterized in that the first component of the blank is the external component 2. 11) Method according to one of claims 3 to 9 characterized in that the first component of the blank is the internal component 22. 12) Method according to one of claims 1 to 11 characterized in that the means for closing the annular space 4, 24 of the blank 1, 21 are two annular metal end pieces 7, 8, 27, 28 arranged at both ends of the blank. 13) A method according to claim 12 characterized in that each end piece 7, 8, 27, 28 is connected to the two corresponding ends of each component of the blank by welding seams waterproof annulars 11, 12, 13, 14, 31, 32, 33, 34. 14) Method according to one of claims 1 to 13 characterized in that there is a vacuum in the annular space 4, 24 before heating the blank 1, 21 to extrusion temperature. 15) Method according to one of claims 1 to 14 characterized in that the coextrusion of the blank is carried out by means of a press, comprising a piston provided with a needle which engages in the blank, previously housed in a container, then in the die integral with this container, said piston thus causing the components of the blank to flow through the annular space between needle and die, lubrication being ensured by a layer of glass. 16) Bimetallic tube, of seamless revolution, produced by coextrusion of a blank comprising two components, this tube comprising an outer layer and an inner layer made of different metals or alloys characterized in that a metallurgical bond between these two layers is provided by a bonding layer which comes from a divided metal which has welded to itself and to both the internal component and the external component during coextrusion. 17) Tubular blank for implementing the method according to one of claims 1 to 15 characterized in that it comprises two tubular components of revolution, made up of metals or alloys of different compositions housed coaxially one inside the other, the sections of each of these tubular components in a plane perpendicular to the common axis being determined so as to provide between their facing walls an annular space of radial width not less than 3 mm, at least equal to 2% of the outside diameter of the internal component and not greater than that of the tubular component of smaller thickness, this annular space being filled with a metal or divided alloy whose composition is compatible with the compositions of the two tubular components, this annular space being closed in leaktight manner by closing means arranged at both ends.

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