ES2719701T3 - Stainless steel tubes and method for their production - Google Patents

Abstract

A method (100, 110, 120, 130) for producing a stainless steel alloy tube, the method comprising: (a) hot working (101) a stainless steel cast piece into a workpiece of form pre-tubular or in a cylindrical bar, the workpiece having a pre-tubular shape having a tubular shape; (b) climbing (102) the cylindrical bar or machining (103) the internal diameter of the workpiece pre-tubularly to obtain a tubular workpiece (201, 301); and (c) cold work (104) the tubular workpiece (201, 301).

Description

DESCRIPTION

Stainless steel tubes and method for their production

Technical field

The present invention relates to the production of tubes made of stainless steel alloys, particularly austenitic-ferritic stainless steel alloys such as duplex and super duplex stainless steel, and the method for their production.

State of the art

There are several techniques and processes in the field of metallurgy to produce components made of various metals and alloys. The selection of the metal or alloy used in these components depends on the application and the conditions to which the components will be exposed.

A product especially prone to exposure to severe conditions is a tube that, together with other tubes, forms a tube for the transfer of substances such as oil and gas extracted from the well. As such, these tubes must cope with high levels of pressures and tensions. These levels may be even higher when the pipes are underwater, which may be the case in, for example, underwater equipment. In addition to the high pressure, there are many corrosive and erosive agents in the sea that corrode the pipes, which affects their structural integrity and that can lead to the failure of the pipes. Another example of an environment with similar extreme conditions is the wells.

The pipes and pipes that are used or found in these adverse environments should have, among other things, great strength and high resistance to corrosion by cracking and pitting. A set of metals that can have these characteristics are stainless steel alloys, and more specifically, austenitic-ferritic stainless steel alloys.

But not only the tubes are prone to failures, also the joints of the tubes are susceptible to failures. In this sense, welding the tubes together in order to form a tube is a critical process: if the joints between the tubes are not welded properly, defects such as phase precipitation can occur and cause additional stresses in the tubes which, in turn, adversely affect corrosion resistance. In addition, the tube welding process is particularly expensive.

A common method for tube production is hot extrusion. The length of the tubes produced, however, is largely limited by the maximum power of the piston that presses the workpiece into the die and the size of the extrusion press is large. Therefore the productivity of the machinery is low and its efficiency is reduced.

United States Patent No. 8,479,549 B1 refers to a method of producing cold-worked centrifugal tubular products in which the tubular workpiece cast from a corrosion resistant alloy has the material of its internal diameter removed, and then A metal shaping process reduces the walls of the tubular workpiece. When the metal forming process is stretch forming, the walls of the workpiece can be reduced with several passes because the workpiece is not able to process large reductions in a single pass, hence the progressive reduction of walls may be provided with subsequent passes for stretch forming. WO2015 / 200325 discloses a method of forming seamless stainless steel tubes by the U-O method.

Therefore, it would be convenient to produce pipes that can be used in environments characterized by demanding conditions, and that have, as far as possible, the purpose of reducing the number of joints and, in addition, the costs of welding the tubes. It would also be convenient to make the method for the production of these tubes as effective as possible in terms of productivity.

Description of the invention

The stainless steel tubes and the method for the production thereof disclosed in the present invention are intended to solve the drawbacks of the tubes and the prior art methods.

A first aspect of the invention relates to a method of producing a tube from a stainless steel alloy. The method comprises the stages:

(a) hot work a stainless steel cast piece until it becomes a pre-tubular work piece or a cylindrical bar;

(b) climbing the cylindrical bar or machining the workpiece pre-tubularly to obtain a tubular workpiece; Y

(c) cold work the tubular workpiece.

A pre-tubular workpiece is a tube or a workpiece with a tubular shape that is machined or shaped to obtain the final dimensions of the tube, while a cylindrical rod is a rod with a rounded cross-section that is, for example , circular or oval.

A hot work process plastically deforms a stainless steel cast by turning it into a pre-tubular workpiece or a cylindrical bar while changing its microstructure and, therefore, the properties of the cast.

The shape of the cast may resemble, for example, but not limited to a bullion or a bar. The shape may have regular or irregular geometries, such as, for example, rectangular prisms, hexagonal prisms, circular prisms, cylinders, etc.

In order for the process to be effectively applied to the casting, the stainless steel casting is heated to a temperature preferably higher than its recrystallization temperature. The cast is then deformed plastically so that its mechanical properties are improved improved for the production of tubes that are characterized by an elongated shape and reduced (i.e. thin) walls.

The internal structure of the castings normally offers cavities, grain sizes and variable segregations in stainless steel that appear during casting. Therefore, while melting, the different temperatures present throughout the material, together with the effect of gravity, generate a heterogeneous internal structure in the form of said cavities, grains with different size and shape, and segregation at macro scale and / or micro scale of the alloy elements.

The hot work process homogenizes the microstructure of the resulting workpiece or bar. Therefore, with hot work, the cast is compacted internally causing changes in the resulting microstructure. In particular, the workpiece or bar can be recrystallized, that is, a new internal structure of crystals can be formed, generating fine grains that improve the mechanical properties since internal stresses disappear due to deformation. A consequence of hot work is that the workpiece or bar has greater ductility and, in the end, higher cold reductions can be applied in a single stage, which leads to the production of longer tubes.

The effect of the hot work process on the microstructure can be estimated by a deformation ratio. The relationship is defined as the original cross section of the cast or work piece divided between its cross section after hot work. Reaching a deformation ratio of approximately 3 or more can be advantageous in that an increase in the toughness and tensile strength of the workpiece or bar, in the longitudinal direction, is achieved.

A drilling or trembling process removes a part of the bar with a hole that generally passes through the entire bar. The removed part may substantially correspond to a central part of at least one side or side of the bar. In the case of the pre-tubular workpiece, its internal diameter is machined. After climbing the bar or machining the internal diameter of the workpiece pre-tubularly, a tubular workpiece is obtained. A cold work process reduces the section or area of the tubular workpiece in order to lengthen the tube to be produced. The process, therefore, redistributes stainless steel: the part of the steel that is removed from the workpiece in the radial direction, which generally corresponds to the walls of the produced tube, is added to the workpiece in the axial direction. The cross section is reduced thereby extending the pipe or tube.

Since the workpiece or bar has been hot worked, its rather fine internal structure provides better conditions - compared to the conditions of the cast before hot work - for cold work. Consequently, the degree of reduction may be greater than if no hot work is performed. The reduction is directly related to the attainable length of the tube.

In preferred embodiments of the invention, the method further comprises (d) tuning the workpiece or bar, and stage (d) is performed after stage (a).

Tempering the workpiece pre-tubularly or the cylindrical rod with a liquid minimizes phase transformations, particularly on its surface. The liquid can reduce, for example, the formation of a vapor phase on the surface of the workpiece or bar that prevents it from cooling rapidly. With a tempering process, the workpiece or bar can maintain the mechanical properties that it has after a hot work or solubilization annealing process, for example. Tempering is done after hot work. In some embodiments, the workpiece or bar is tempered after having been subjected to hot work and heat treatment such as, for example, annealing by solubilization.

In these preferred embodiments, tempering is performed with water at a temperature not exceeding 50 ° C, and preferably not exceeding 35 ° C.

The liquid used in the cooling stage may be water at a temperature equal to or below 50 ° C such that the workpiece or bar can be cooled rapidly. Preferably, the liquid is at an even lower temperature than this value, such as equal to or below 35 ° C, and therefore the cooling of the workpiece or bar takes less time, and thus its mechanical properties suffer Less changes In preferred embodiments of the invention, the method further comprises (e) melting the stainless steel cast. In addition, in these embodiments, melting the stainless steel cast-step (e) - is performed before hot-working the stainless steel cast on a workpiece or bar-Stage (a).

The hot melt in some embodiments is melted by melting the stainless steel alloy and pouring it into a mold. The dimensions of the cast piece produced, both in terms of its length and its section -or diameter-, determine the maximum dimensions of the tube that can be produced since the stainless steel in the cast piece will be redistributed in order to form the tube, although a part of said alloy may be lost during the production of the tube, for example, during the trembling, machining or cold work of the workpiece. Therefore, the amount of stainless steel alloy needed for the cast varies according to the dimensions of the tube to be produced.

In preferred embodiments of the invention, the stainless steel alloy is an austenitic-ferritic stainless steel alloy.

Austenitic-ferritic stainless steel, including duplex stainless steel and super-duplex stainless steel, has greater strength than austenitic stainless steel and ferritic stainless steel. In addition, austenitic-ferritic stainless steel is more resistant to pitting and cracking corrosion and induced stress corrosion than austenitic or ferritic stainless steels. This makes austenitic-ferritic stainless steel suitable for products to be placed in environments with adverse conditions, particularly in wells and underwater (for example, at the bottom of the sea), where the level of pressure and quantity of corrosive substances or agents is high.

However, austenitic-ferritic stainless steel has low ductility and, therefore, the conformation of products with this alloy requires greater forces than the formation of austenitic or ferritic stainless steel products.

It is important that, in step (c), the microstructure of austeniticoferritic stainless steel alloys is controlled, so that at the end of the process the appropriate percentages of austenite and ferrite phases are present in the tubular workpiece or tube, because the mechanical properties and corrosion resistance is largely determined by these phases. The alloy elements are selected so that the elements from Algeria (which promote the formation of ferrite) and elements from Gamagens (which promote the formation of austenite) are balanced. In addition, a correct heat treatment temperature and rapid cooling stabilize both phases and prevent the formation of unwanted third phases.

In this sense, the distance between the phases is also important to avoid hydrogen stress-induced cracking (HISC). In particular, it is desirable that the separation of austenite be a maximum of 30 gm (i.e. microns or micrometers) with a presence of the ferrite phase between 40% and 60% (including the endpoints in the range of possible values).

In some of these preferred embodiments, the austenitic-ferritic stainless steel alloy is a duplex stainless steel. In some other of these preferred embodiments, the austeniticoferritic stainless steel alloy is super duplex stainless steel.

In preferred embodiments of the invention, hot work comprises one of: lamination, forging, and a combination thereof.

The lamination of the stainless steel cast piece homogenizes its internal structure in terms of grain size, porosity, cavities, among others. Rolling mills plastically deform the cast, which usually offers grains that are larger inside than on its surface - the part in contact with the cast iron. The laminated workpiece can have many different shapes, such as, for example, cylindrical, rectangular, sheet-shaped, among others. Continuous or reversible rolling mills known in the art can be used, for example, to plastically deform a cast as, for example, a bar or ingot.

The stainless steel cast can also be forged during the hot work stage, in which case the cast can be retained - although not necessarily - with pliers, bars, or the like, and a hammer or a die applies blows to deform it. The forging can be done by a user (for example, a blacksmith) or by a machine (for example, free forging). It is also possible to use a rotary forging press to deform the cast.

It is convenient to carry out the process of forging progressively (i.e. sequential strokes, each causing a small deformation) so that the deformations can crystallize without cracking.

In some cases, lamination and forging can both be performed sequentially from the cast. In some embodiments of the invention, the method further comprises (f) annealing the workpiece or bar by solubilization, at a temperature between 1030 ° C and 1120 ° C (including the endpoints in the range of possible values).

In order to reduce the hardness of the bar or workpiece and increase its ductility, the bar or workpiece may be annealed by solubilization. On the other hand, annealing by solubilization can also reduce the internal stresses of the workpiece or bar. The bar or workpiece is heated, therefore, above its recrystallization temperature, maintained for some time at a temperature greater than said recrystallization temperature, and then rapidly cooled (for example, water cooling).

In some embodiments of the invention, step (f) is performed on the pre-tubular shaped workpiece or the cylindrical bar, that is, the solubilization annealing step can be performed after the molten part is hot worked and before climbing the bar or machining the workpiece pre-tubularly so that the increase in ductility achieved with plastic deformation is further improved. In embodiments in which the method comprises tempering the bar or workpiece after hot work, solubilization annealing can take advantage of tempering - Stage (d).

In some embodiments, step (f) is performed on the tubular workpiece, that is, after trembling and before cold work since with the increase in ductility, the reduction of walls and elongation of the wall can be improved. tubular product during the cold work process and, therefore, it is possible to apply a greater reduction in a single pass, and / or produce a longer work piece or tube with the same applied forces, or a work piece or tube with the same length as that or that has not undergone annealing by solubilization performed but applying less force.

Since cold work can generate tensions within the tubular workpiece, the solubilization annealing step can also be performed after cold work so that these internal tensions are eliminated, at least partially.

In some cases, after a solubilization annealing process, the workpiece can be tempered with a liquid such as water.

In preferred embodiments of the invention, the method further comprises (g) heating the stainless steel cast to a temperature greater than 1000 ° C, and preferably greater than 1200 ° C. In addition, in these embodiments, step (g) is performed before step (a).

The stainless steel casting is heated to a temperature higher than its recrystallization temperature, generally above 1000 ° C, so that it can be deformed, at the hot working stage, at a high temperature. It is convenient to reach a temperature of at least 1000 ° C because intermetallic phases (delta sigma or delta ferrite) can be formed in some stainless steel alloys (for example, duplex stainless steel), in the temperature range between 600 ° C and 1000 ° C These intermetallic phases can cause or generate cracks in the workpiece. Preferably, the temperature in the oven is greater than or equal to 1200 ° C in order to perform a hot work process more efficiently and without the risk that the temperature of the molten part - due to the heat being transferred to the support (for example, an anvil), the environment, etc.- passes below the recrystallization temperature and / or below 1000 ° C.

In the preferred embodiments of the invention, the cold work comprises one of: stretch forming and step rolling cold pilgrim step rolling.

In embodiments in which the cold work comprises stretching forming, a stretching forming machine, which includes, among other things, a mandrel and a plurality of rollers with, normally, three or four rollers, reduces the thickness of the walls of the workpiece and makes the workpiece longer. The tubular workpiece can be subjected to either the forward stretch conformation or the reverse stretch conformation.

The tubular workpiece is fixed to the mandrel by means of the hole, for example formed with the trepanning or machining of the stage (b). When the workpiece is fixed, the mandrel can move the workpiece in a direction of movement of the rollers. The rollers apply forces to the workpiece in the axial, longitudinal and tangential directions. The compression force in a radial direction reduces the thickness of the wall, which combined with the forces in the other two directions results in an elongation of the workpiece or tube.

Stretching can improve the grain structure of the tubular workpiece or tube by making the internal structure more homogeneous throughout the entire workpiece, and can improve its mechanical properties.

In embodiments in which the cold work comprises pilgrim step rolling, a pilgrim passing mill can reshape the workpiece into an elongated tube with thinner walls. The train ring dies, which can be ring-shaped, compress the workpiece in a radial direction and thus reduce its external diameter. The mandrel, which can secure the workpiece using a hole of the workpiece - for example formed with the tremming or machining of stage (b) - moves and rotates the workpiece, and can also reshape the diameter internal workpiece or tube.

The mandrel feeds and rotates the workpiece, successively, while two ring dies deform the workpiece, which causes a reduction in both the external diameter and the thickness of the walls. The workpiece is first rotated to a large extent (i.e. large angle variations, for example, approximately 60 °) to deform the section currently processed by the dies, and then rotated on a small scale (i.e., small angle variations, for example, approximately 20 °) to adjust the shape of the section such that it has a polished circular section, that is, a substantially rounded outer diameter. Pilgrim's lamination is a semi-continuous process that is particularly effective in long-term productions. The tubular workpiece can be fed, in a forward motion, at a speed between 2 and 50 mm / s (the endpoints are included in the range of possible values), while the feed rate or the movement speed towards in front of the stretch forming machine it can be between 0.5 mm / s and 10 mm / s (including endpoints in the range of possible values). Although the feed rate in the stretch forming machine may be less than in the pilgrim's lamination, a smaller number of passes may be necessary to produce a stretch forming tube.

In some embodiments of the invention, stretch forming or pilgrim lamination reduces, at least, the wall thickness of the workpiece between 25% and 35% (including endpoints in the range of possible values).

In some embodiments of the invention, stretch forming or pilgrim lamination reduces, at least, the thickness of the walls of the workpiece between 35% and 50% (including the endpoints in the range of possible values).

In some embodiments of the invention, stretch forming or pilgrimage lamination reduces, at least, the thickness of the walls of the tubular workpiece between 50% and 75% (including endpoints in the range of possible values).

In some embodiments, cold work comprises stretching forming, and stretching forming reduces the thickness of the walls of the tubular workpiece by at least 70% in one pass.

Due to the mechanical properties obtained after some processes or stages of some embodiments of the invention, the workpiece can withstand a wall reduction between 65% and 70% (including endpoints in the range of possible values) in One pass With such reductions, the stretch forming machine takes less time to process the workpiece and reduce the number of passes needed to reach the desired thickness. This is even more significant if it is considered that cold work progressively reduces the ductility of the workpiece after each pass or deformation produced and, therefore, the forces necessary to further deform the workpiece increase.

Another aspect relates to stainless steel tubes produced with the method described above with respect to the first aspect of the invention.

The tube comprises

- an external diameter greater than or equal to 152 mm, preferably greater than or equal to 200 mm and preferably greater than or equal to 250 mm;

- an average wall thickness greater than or equal to 2.8 mm, and less than or equal to 70 mm, preferably greater than or equal to 12 mm and preferably greater than 39 mm; Y

- a length greater than 5 m, preferably greater than or equal to 10 m, and preferably greater than 12 m. The tube preferably comprises an austenitic-ferritic stainless steel alloy and, more preferably, one of a duplex stainless steel and a super-duplex stainless steel;

In some preferred embodiments, the tubes comprise an austenite separation of less than 30 microns (micrometers).

Austenitic-ferritic stainless steel tubes offer austenite separations that are below 30 gm (micrometers) are particularly resistant to HISC.

In some preferred embodiments, the tubes comprise a ferrite content greater than or equal to 40%, and less than or equal to 60%.

Brief description of the drawings

To complete the description and in order to provide a better understanding of the invention, a set of drawings is provided. These drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be construed as limiting the scope of the invention, but simply as an example of how the invention can be carried out. The drawings comprise the following figures:

Figures 1A to 1D are flow charts of methods according to some embodiments of the invention. Figure 2 is a representation of a stretch forming machine, which can be used for cold work in some embodiments of the invention.

Figure 3 is another representation of a stretch forming machine.

Figure 4 is a photograph of the microstructure of a tube produced with a method according to an embodiment of the invention.

Description of an embodiment of the invention

Figure 1A is a flow chart 100 depicting the steps performed in a method of an embodiment of the invention.

In step 101 of the method, a stainless steel cast piece is hot worked until it becomes a pre-tubular work piece or a cylindrical bar, specifically, the cast piece deforms plastically in an environment that has a higher temperature at the recrystallization temperature of the casting so that its internal structure is altered. In general, the cast has a microstructure that includes grains of different sizes, segregations of material, and cavities that appear during casting. Hot work, that is, the plastic deformation of the casting, reduces the aforementioned defects within the resulting workpiece or bar since a new crystalline structure can be formed. This structure can be characterized by a more homogeneous distribution of the grains, and a lower presence of cavities and / or alloy segregations. Consequently, the amount of internal stresses is smaller, which improves some mechanical properties of the workpiece or bar; Ductility, for example, may increase due to the hot work of step 101.

Some non-limiting examples of hot work are forging, rolling and wire drawing.

When the casting is hot worked on a cylindrical bar, the bar is climbed in step 102. A drilling or cutting machine makes a hole in the cylindrical bar, preferably a through hole with circular cross-section. In the embodiments in which the hot work - Stage 101 - produces a workpiece pre-tubularly, the workpiece is subjected to a machining process of its internal diameter in step 103. After step 102 or step 103, a tubular workpiece is obtained.

In step 104, the tubular workpiece is subjected to cold work: the workpiece is plastically deformed at a temperature below its recrystallization temperature. In particular, in step 104 the walls of the workpiece are reduced and the length of the tube produced is increased.

Some non-limiting examples of cold work are pilgrim's lamination and stretch forming. In these cases, the mandrel of the stretch forming machine or pilgrim lamination holds the workpiece by means of the hole formed in step 102 or machined in step 103 so that the tubular workpiece can be subject to deformations produced by the machine.

Figure 1B is a flow chart 110 depicting the steps of a method of producing a tube according to another embodiment.

Flowchart 110 comprises steps 101, 102, 103 and 104 that correspond to hot work, creeping, machining and cold work, respectively, as described above with respect to flow chart 100.

The method of Figure 1B further comprises step 105: foundry, in which a stainless steel alloy is melted and poured into a mold. The stainless steel is allowed to dry forming the cast, which can take the form of, for example, an ingot or a bar. The volume of stainless steel in the casting can determine the maximum amount of steel that can be used for the production of the tube, since, generally, no steel is added afterwards, rather, a little of the steel is removed during a or more successive steps 101-104 of the method.

Then, the castings are subjected to at least hot work (step 101), trepanning (step 102) or machining of the internal diameter (step 103), and cold work (step 104).

The cast and / or workpiece subjected to the methods described with respect to flowcharts 100, 110 comprise a stainless steel alloy, the stainless steel alloy being an austenitic-ferritic stainless steel alloy which is preferably , duplex stainless steel or super duplex stainless steel.

Figure 1C shows the flowchart 120 corresponding to a method according to another embodiment of the invention.

The embodiment comprises stages 101, 102, 103 and 104 corresponding to hot work, trembling, machining and cold work, respectively, and further comprising the tempering-stage 106-which takes place after stage 101, and before stage 102 or stage 103.

In step 106, the pre-tubular workpiece or the cylindrical bar is rapidly cooled, so that the internal structure obtained in step 101 is largely maintained. Therefore, rapid cooling reduces the amount of phase transformations that can occur in the workpiece or bar and, in particular, on its surface after hot work.

Figure 1D is a flow chart 130 according to another embodiment of the invention.

First, a stainless steel cast piece melts-stage 105-. With hot work - stage 101 - the cast is deformed in such a way that its microstructure changes and, consequently, its mechanical properties are also altered. The resulting workpiece is tempered - stage 105 - in order to maintain the altered mechanical properties, and then subjected to trepanning - stage 102 - in order to form a hole inside it or machined - stage 103 - in order to re-form the inner hole. The tubular workpiece obtained is then deformed, in a cold work process reducing the walls and increasing the length of the tube-stage 104.

The tubes produced in some of these embodiments have a length greater than 5 m. In some of these embodiments, the length of the tubes produced is more than 10 m. And in some of these embodiments, the length of the tubes produced is more than 12 m. These tubes may have an external diameter greater than or equal to 252 mm, preferably greater than or equal to 200 mm, and preferably greater than or equal to 250 mm; It can also have a wall of medium thickness greater than or equal to 2.8 mm, and less than or equal to 70 mm, and preferably greater than or equal to 12 mm and less than or equal to 39 mm.

Figure 2 shows a stretch forming machine 200. The workpiece 201 having a tubular geometry is placed on the mandrel 202 of the machine, and held in place with a jaw chuck 203. The jaw chuck 203 causes the workpiece 201 to rotate in accordance with the motor's turning movement - a mandrel 202 (not shown) provides said turning movement. The machine 200 further comprises a carriage 204 in which a plurality of rollers 205a-205d are arranged in an equidistant configuration with a progressive phase difference of 90 ° between the rollers 205a-205d.

Both the mandrel 202 and the plurality of rollers 205a-205d include rotational movements during the operation of the machine 200 such that the workpiece 201, as it passes through the roller assembly 205a-205d, has its external diameter reduced, which in turn causes a reduction in the thickness of its walls, and its length increases along the axis is illustrated in the Figure.

In the stretch forming machine 200, there are up to 10 degrees of freedom that are adjusted and controlled during the production of tubes: the rotation of the mandrel 202, the rotation of each of the four rollers 205a-205d, the position of each of the four rollers 205a-205d in relation to the workpiece 201 or mandrel 202 - horizontal position adjustments of the rollers 205b and 205d, and vertical position adjustments of the rollers 205a and 205c-, and the distance of the portion of the mandrel between jaw chuck 203 and carriage 204.

In some embodiments, the stretch forming machine comprises two, three, six or more rollers and, consequently, the machine may have more or less degrees of freedom. In these other embodiments, the rollers can also be arranged following constant phase differences with respect to an imaginary circumference along which the rollers are distributed; Constant phase differences correspond to 360 ° divided by the number of rollers in the car.

The carriage 204 moves towards the jaw chuck 203, and the rollers 205a-205d, which rotate in a direction opposite to the turning movement of the mandrel 202 and the workpiece 201, provide forces in the axial, radial and tangential directions. Although the rollers apply a compressive force on the workpiece 201, the carriage 204 must face and resist the forces applied by the rollers 205a-205d. Therefore, these forces - mainly those in the axial and radial directions, since the tangential component is much smaller than the other two - determine the structural requirements of the carriage 204.

The rollers can be moved axially with each other allowing three different roller configurations, depending on the process requirements. An axial displacement to the zero line allows faster shaping feed rates. An axial displacement that is four times different, one for each roller, allows greater precision and perfect surface qualities combined with height reduction rates. The midpoint, an axial displacement evenly allows stronger stretching operations, which means greater reductions, because each torque forming roller functions as a counter bearing and takes on the strength of the opposite roller. The result is a perfect run at high feed speeds.

Figure 3 shows a stretch forming machine 300 in a 2D view. Similar to the machine 200 of Figure 2, the mandrel 302 holds the workpiece 301, and the jaw chuck 303 which also holds the workpiece 301 causes the workpiece to rotate according to the turning movement of mandrel 302.

As the carriage 304 moves toward the jaw chuck 303, the rollers 305a, 305b apply a compressive force to the workpiece 301 and incrementally produce a longer tube with thinner walls.

The existence of so many degrees of freedom in the forming machine by stretching - and machine, by extension, the corresponding process - makes its operation a complex task. To this end, a computer numerical control manages the entire process and operation in such a way that the tubes produced have, throughout their volume, the mechanical and microstructural properties sought in the least possible number of passes. In this sense, the computer numerical control can adjust the parameters related to the aforementioned degrees of freedom so that the axial and radial forces of the rollers 305a, 305b plastically deform the inner part of the workpiece 201 in order to generate compression forces within its structure.

It is of particular relevance to determine an appropriate relationship between the speed 311 at which the carriage 304 moves towards the jaw chuck 303 and the rotation speed 312 of the mandrel 302. If this ratio is too high, the rollers 305a, 305b cannot properly deform workpiece 301. On the contrary, if the ratio is too small, the time it takes to process workpiece 301 may be unnecessarily long. It is also convenient to adjust the angle of attack 310 of the rollers 305a, 305b, that is, the relative angle between the rollers, 305a 305b and the workpiece 301, as it is being shaped by stretching. The angle of attack 310 can range between 6 ° and 45 ° (including endpoints in the range of possible values). Overly pronounced angles of attack can also lead to irregular deformations of workpiece 301.

Preferably, the end of the workpiece 301 that will first be in contact with the rollers 305a, 305b has the edges of its opening chamfered so that the rollers do not deform the workpiece irregularly, which could do that the tube becomes unusable since the mechanical properties of that part of the tube may be different from the rest of the tube.

Stretch shaping not only changes the shape of the workpiece, it also changes its microstructure: the resulting grains can be oriented and have a homogeneous fine size, both of which provide improved mechanical properties.

Figure 4 shows the microstructure of a tube produced with a method according to an embodiment of the invention. The tube comprises an austenitic-ferritic stainless steel alloy with an austenite phase 401 and a ferrite phase 402. In some embodiments, the austenitic-ferritic stainless steel alloy is a duplex stainless steel. In some other embodiments, the austenitic-ferritic stainless steel alloy is a super-duplex stainless steel.

On average, the separation of austenite phases 401 is approximately 30 micrometers or less, which is convenient for resisting HISC phenomena. Such separation can be observed using the illustrated segment 403, which is equivalent to 30 gm.

In this text, the term "comprises" and its derivations (such as "understanding", etc.) should not be understood in an exclusive sense, that is, these terms should not be construed as excluding the possibility that what is described and define may include elements, stages, etc.

The invention is obviously not limited to the one or more specific embodiments described herein, but also encompasses any variation that may be considered by any person skilled in the art (for example, in terms of the choice of materials, dimensions , components, configuration, etc.), within the scope of the invention as defined in the claims.

Claims (12)

1. A method (100, 110, 120, 130) for producing a tube of a stainless steel alloy, the method comprising: (a) hot working (101) a stainless steel cast piece until it becomes a pre-tubular work piece or a cylindrical bar, the pre-tubular work piece having a tubular shape; (b) climbing (102) the cylindrical bar or machining (103) the internal diameter of the workpiece pre-tubularly to obtain a tubular workpiece (201, 301); Y (c) cold work (104) the tubular workpiece (201, 301). 2. The method (120, 130) of claim 1, wherein: The method further comprises (d) tempering (106) the pre-tubular workpiece or the cylindrical rod; and (d) is performed after (a). 3. The method (120, 130) of claim 2, wherein (d) tempering (106) of the pre-tubular workpiece or of the cylindrical bar is performed with water at a temperature not exceeding 50 ° C, and preferably not higher than 35 ° C. 4. The method (110, 130) of any of the preceding claims, wherein: the method further comprises (e) melting (105) the stainless steel cast; Y (e) is done before (a). 5. The method (100, 110, 120, 130) of any of the preceding claims, wherein the stainless steel alloy is an austenitic-ferritic stainless steel alloy. 6. The method (100, 110, 120, 130) of claim 5, wherein the stainless steel alloy is duplex stainless steel or super-duplex stainless steel. 7. The method (100, 110, 120, 130) of any of the preceding claims, wherein the hot work comprises one of: rolling, forging and a combination thereof. 8. The method (100, 110, 120, 130) of any of the preceding claims, further comprising (f) annealing the workpiece pre-tubularly or the cylindrical bar, at a temperature between 1030 ° C and 1120 ° C. 9. The method (100, 110, 120, 130) of any of claims 1-7, further comprising (f) annealing the tubular workpiece (201, 301) by solubilization, and wherein (f) is perform at least one of the following: after (b) and before (c), and after (c). 10. The method (100, 110, 120, 130) of any of the preceding claims, wherein: The method further comprises (g) heating the stainless steel cast to a temperature greater than 1000 ° C, and preferably greater than 1200 ° C; Y (g) is done before (a). 11. The method (100, 110, 120, 130) of any of the preceding claims, wherein the cold work (104) comprises one of: stretch forming and pilgrimage lamination. 12. The method (100, 110, 120, 130) of claim 11, wherein the cold work (104) comprises stretching forming, and stretching forming reduces, at least, the thickness of the walls of the work piece (201, 301) by 70% in one pass.