STAINLESS STEEL TUBES AND METHOD FOR PRODUCTION THEREOF

Abstract

A method for producing a tube of a stainless steel alloy tube which comprises the steps of hot working a stainless steel casting into a pretubular shaped workpiece or into a cylindrical bar, trepanning the cylindrical bar or machining an inner diameter of the pretubular shaped workpiece to obtain a tubular workpiece, and cold working the workpiece. The hot working comprises one of: rolling, forging, and a combination thereof. The cold working comprises flow forming or pilgering. The stainless steel tube produced with the method comprises an outer diameter greater than or equal to 152 mm, an average wall thickness greater than or equal to 2.8 mm and less than or equal to 70 mm, and a length greater than 5 m.

Claims

1. A method for producing a tube of a stainless steel alloy, the method comprising: (a) hot working a stainless steel casting into a pretubular shaped workpiece or into a cylindrical bar; (b) trepanning the cylindrical bar or machining the inner diameter of the pretubular shaped workpiece to obtain a tubular workpiece; and (c) cold working the tubular workpiece.

2. The method of claim 1, wherein: the method further comprises (d) quenching the pretubular shaped workpiece or cylindrical bar; and (d) is performed after (a).

3. The method of claim 1, wherein: the method further comprises (e) casting the stainless steel casting; and (e) is performed prior to (a).

4. The method of claim 1, wherein the stainless steel alloy is an austenitic-ferritic stainless steel alloy.

5. The method of claim 4, wherein the stainless steel alloy is duplex stainless steel or super-duplex stainless steel.

6. The method of claim 1, wherein the hot working comprises one of: rolling, forging, and a combination thereof.

7. The method of claim 1, further comprising (f) solution annealing the pretubular shaped workpiece or cylindrical bar, at a temperature between 1030 C. and 1120 C.

8. The method of claim 1, further comprising (f) solution annealing the tubular workpiece, and wherein (f) is performed in at least one of the following: after (b) and prior to (c), and after (c).

9. The method of claim 1, wherein: the method further comprises (g) heating the stainless steel casting to a temperature higher than 1000 C., and preferably higher than 1200 C.; and (g) is performed prior to (a).

10. The method of claim 2, wherein quenching the pretubular shaped workpiece or cylindrical bar is performed with water at a temperature not higher than 50 C., and preferably not higher than 35 C.

11. The method of claim 1, wherein the cold working comprises one of: flow forming and pilgering.

12. The method of claim 11, wherein the cold working comprises flow forming, and the flow forming at least reduces thickness of walls of the workpiece by 70% in one pass.

13. A stainless steel tube produced with the method of claim 1, characterized by: an outer diameter greater than or equal to 152 mm; an average wall thickness greater than or equal to 2.8 mm, and less than or equal to 70 mm; and a length greater than 5 m.

14. The stainless steel tube of claim 13, wherein: the outer diameter is greater than or equal to 200 mm; the average wall thickness is greater than 12 mm; and the length is greater than 10 m.

15. The stainless steel tube of claim 13, wherein the tube comprises an austenitic-ferritic stainless steel alloy with: an average austenite spacing less than or equal to 30 microns; and a ferrite content greater than or equal to 40%, and less than or equal to 60%.

16. The stainless steel tube of claim 13, wherein the stainless steel tube is seamless.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

[0072] FIGS. 1A to 1D are flowcharts of methods in accordance with some embodiments of the invention.

[0073] FIG. 2 is a representation of a flow forming machine, which may be used for cold working in some embodiments of the invention.

[0074] FIG. 3 is another representation of a flow forming machine.

[0075] FIG. 4 is a photograph of the microstructure of a tube produced with a method in accordance with an embodiment of the invention.

DESCRIPTION OF A WAY OF CARRYING OUT THE INVENTION

[0076] FIG. 1A is a flowchart 100 depicting the steps carried on a method of an embodiment of the invention.

[0077] In step 101 of the method, a stainless steel casting is hot worked into a pretubular-shaped workpiece or cylindrical bar, namely, the casting is plastically deformed in an environment that has a temperature higher than the casting's recrystallization temperature so that its internal structure is altered. Generally, the casting has a microstructure including differently-sized grains, material segregations, and cavities that appear during its casting. Hot working, that is, plastically deforming the casting, reduces the aforementioned defects within the resulting workpiece or bar since a new crystalline structure may be formed. This structure may be characterized by a more homogeneous distribution of grains, and a lower presence of cavities and/or alloy segregations. Consequently, the amount of internal stresses is lower, which improves some mechanical properties of the workpiece or bar; the ductility, for instance, may increase due to the hot working of step 101.

[0078] Some non-limiting examples of hot working are forging, rolling and drawing.

[0079] When the casting is hot-worked into a cylindrical bar, the bar is trepanned in step 102. A drilling or cutting machine drills a hole into the cylindrical bar, preferably a through hole with circular cross section. In the embodiments in which hot workingstep 101produces a pretubular shaped workpiece, the workpiece is subject to a machining process of its inner diameter in step 103. After step 102 or step 103, a tubular workpiece is obtained.

[0080] In step 104, the tubular workpiece is subject to cold working: the workpiece is plastically deformed at a temperature below its recrystallization temperature. Particularly, in step 104 the walls of the workpiece are reduced and the length of the tube produced is increased.

[0081] Some non-limiting examples of cold working are pilgering and flow forming. In these cases, the mandrel of the flow forming or pilgering machine holds the workpiece by means of the hole formed in step 102 or machined in step 103 so that the tubular workpiece may be subject to the deformations produced by the machine.

[0082] FIG. 1B is a flowchart 110 that depicts the steps of a method for producing a tube in accordance with another embodiment.

[0083] The flowchart 110 comprises steps 101, 102, 103 and 104 corresponding to hot working, trepanning, machining and cold working, respectively, as described above with respect to flowchart 100.

[0084] The method of FIG. 1B further comprises step 105: casting, by which a stainless steel alloy is melt and poured in a mold. The stainless steel is left to dry forming the casting, which may take the shape of, for example, an ingot or a bar. The volume of stainless steel in the casting may determine the maximum amount of steel which may be used for producing the tube since, generally, no steel is added afterwards, rather, some steel is removed during one or more of the successive steps 101-104 of the method.

[0085] Then, the casting is at least subject to hot working (step 101), trepanning (step 102) or machining of the inner diameter (step 103), and cold working (step 104).

[0086] The casting and/or workpiece subject 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 that is, preferably, duplex stainless steel or super-duplex stainless steel.

[0087] FIG. 1C shows flowchart 120 corresponding to a method in accordance with another embodiment of the invention.

[0088] The embodiment comprises steps 101, 102, 103 and 104 corresponding to hot working, trepanning, machining and cold working, respectively, and further comprises quenchingstep 106which takes place after step 101, and before step 102 or step 103.

[0089] In step 106, the pretubular shaped workpiece or cylindrical bar is rapidly cooled so that the internal structure obtained in step 101 is largely maintained. Therefore, quenching reduces the amount of phase transformations that may occur throughout the workpiece or bar and, particularly, on its surface after hot working.

[0090] FIG. 1D is a flowchart 130 in accordance with another embodiment of the invention.

[0091] First a stainless steel casting is castedstep 105. With hot workingstep 101, the casting is deformed such that its microstructure changes and, consequently, its mechanical properties are altered as well. The resulting workpiece is quenchedstep 105so as to maintain the altered mechanical properties, and then trepannedstep 102so as to form a hole inside or machinedstep 103so as to reshape the hole inside. The tubular workpiece obtained is then deformed in a cold working process by reducing the walls and increasing the length of the tubestep 104.

[0092] The tubes produced in some of these embodiments feature a length longer than 5 m. In some of these embodiments, the length of the tubes produced is longer than 10 m. And in some of these embodiments, the length of the tubes produced is longer than 12 m. These tubes may feature an outer 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; they may also feature an average wall 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.

[0093] FIG. 2 shows a flow forming machine 200. A 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 makes the workpiece 201 rotate in accordance with the rotary motion of the mandrel 202an engine (not illustrated) provides said rotary motion. 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 90 phase difference between the rollers 205a-205d.

[0094] Both the mandrel 202 and the plurality of rollers 205a-205d feature rotary movements during the operation of the machine 200 such that the workpiece 201, as it goes through the set of rollers 205a-205d, has its outer diameter reduced, which in turn causes a reduction of the thickness of its walls, and its length increasedalong the Y axis illustrated in the figure.

[0095] In the flow forming machine 200, there are up to 10 degrees of freedom which 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 relative to the workpiece 201 or mandrel 202horizontal position adjustments of rollers 205b and 205d, and vertical position adjustments of rollers 205a and 205c, and the distance of the portion of the mandrel between the jaw chuck 203 and the carriage 204.

[0096] In some embodiments, the flow forming machine comprises two, three, six or more rollers and, consequently, the machine may feature more or less degrees of freedom. In these other embodiments, the rollers may also arranged following constant phase differences with respect to an imaginary circumference along which the rollers are distributed; the constant phase differences correspond to 360 divided by the number of rollers in the carriage.

[0097] The carriage 204 moves towards the jaw chuck 203, and the rollers 205a-205d, which rotate in a direction contrary to the rotary 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 cope with and resist the forces applied by the rollers 205a-205d. Thus these forcesmainly those in the axial and radial directions, since the tangential component is much smaller than the other twodetermine the structural requirements of the carriage 204.

[0098] The rollers can be offset axially to each other which allows three different roll configurations, depending on the requirements of the process. An axial offset to zero-line allows faster forming feed rates. An axial offset that is four times different, one for each roller, allows higher accuracy and perfect surface qualities combined with high reduction rates. The middle way, a pair wise axial offset allows stronger flow forming operations which means higher reductions, because each forming roller of the pair works as a counter-bearing and takes the force of the opposite roller. The result is a perfect run-out at high feed rates.

[0099] FIG. 3 shows a flow forming machine 300 in a 2D view. Similarly to the machine 200 of FIG. 2, the mandrel 302 holds the workpiece 301, and the jaw chuck 303 also holding the workpiece 301 makes the workpiece rotate in accordance with the rotating motion of the mandrel 302.

[0100] As the carriage 304 moves towards the jaw chuck 303, the rollers 305a, 305b apply a compressive force to the workpiece 301 and incrementally produce a tube longer and with thinner walls.

[0101] The existence of so many degrees of freedom in the flow forming machineand, by extension, the corresponding processmakes its operation a complex task. To this end, a computer numerical control manages the whole process and operation such that the produced tubes feature, throughout their whole volume, the mechanical and microstructural properties sought in the lower number of passes possible. In this sense, the computer numerical control may 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 so as to generate compressive forces within its structure.

[0102] It is of particular relevance to determine an appropriate ratio between the rate 311 at which the carriage 304 moves towards the jaw chuck 303 and the rotary speed 312 of the mandrel 302. If this ratio is too high, the rollers 305a, 305b may not properly deform the workpiece 301. Conversely, if the ratio is too small, the time it takes to process the workpiece 301 may be unnecessarily long.

[0103] 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 flow formed. The angle of attack 310 may range between 6 and 45 (the endpoints being included in the range of possible values). Too pronounced angles of attack may also result in irregular deformations of the workpiece 301.

[0104] Preferably, the end of the workpiece 301 that will be first 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 render the tube unusable since the mechanical properties of that part of the tube may differ from the rest of the tube.

[0105] The flow forming not only reshapes the workpiece, it also changes its microstructure: the resulting grains may be oriented and have a homogeneous fine size, both of which provide improved mechanical properties.

[0106] FIG. 4 shows the microstructure of a tube produced with a method in accordance with 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.

[0107] In average, the spacing of the austenite phases 401 is about 30 microns or less, which is convenient for resisting HISC phenomena. Such spacing may be observed using the illustrated segment 403, which is equivalent to 30 m.

[0108] In this text, the term comprises and its derivations (such as comprising, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

[0109] The invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.