A method for preparing a cobalt-chromium-based alloy seamless pipe with high strength and high plasticity

Abstract

A method for preparing a cobalt-chromium-based alloy seamless tube with both high strength and high plasticity is provided, belonging to the technical field of cobalt-chromium-based alloy tube processing. The present invention employs a two-step process: first, coarse grains in the tube billet are effectively crushed through large-deformation three-roll planetary warm rolling, followed by a partial recrystallization heat treatment to form a bimodal structure with large and small grains. Based on the mechanism by which large grains deform preferentially under stress, the large grains are further deformed through small-deformation cold rolling or cold drawing, followed by a high-temperature, short-time heat treatment to form smaller grains. By rationally controlling the deformation amount per pass and matching the heat treatment temperature, a cobalt-chromium-based alloy tube with both high strength and high elongation is obtained, effectively solving the problem of high strength and low plasticity in conventionally processed cobalt-based alloy tubes.

Description

Technical Field

[0001] The present invention belongs to the technical field of cobalt-chromium-based alloy pipe processing, and in particular relates to a method for preparing a cobalt-chromium-based alloy seamless pipe with both high strength and high plasticity. Background Art

[0002] Cobalt-chromium alloys, due to their excellent mechanical properties, corrosion resistance, and biocompatibility, have been widely used in gas turbines, dentistry, orthopedics, and cardiovascular applications. In the cardiovascular field in particular, high-strength cobalt-based alloy stents can minimize strut width (currently the thinnest connecting rib width is 50μm) while maintaining radial support strength and radiographic visibility. This reduces irritation to the vessel wall, further reducing the potential for restenosis and improving the success rate of stent implantation. Furthermore, during stent expansion, plastic deformation occurs, leading to stress concentrations at the stent joints and potentially causing fracture. According to ISO and ASTM standards, L605 alloy has Rp > 1 GPa, Rm > 1.2 GPa, and A > 5% in the as-processed state; and Rp > 500 MPa, Rm > 900 MPa, and A > 30% in the annealed state. Therefore, the raw material for stents—the seamless tubing—must possess both high strength and high ductility.

[0003] However, the strength-elongation relationship of metal materials often exhibits an inverted relationship. According to the Hall-Petch relationship, fine-grained structures (tens of nanometers to tens of microns) exhibit higher strength but lower ductility, while coarse-grained structures exhibit higher elongation but lower strength. Properly controlling the grain size and distribution of metal materials is an effective approach to achieving a high-strength-high-elongation balance.

[0004] Currently, cobalt-chromium alloy tubes are typically produced using centrifugal casting or hot extrusion, followed by swaging / rolling / cold drawing. For example, Chinese invention patent CN113680985B discloses a low-cost, short-process method for producing seamless high-temperature alloy tubes. This method utilizes centrifugal casting of tube billets followed by rolling, with vacuum centrifugal casting as the core process. Chinese invention patent CN105331913B discloses a short-process method for producing Inconel 625 tubes using hot extrusion followed by cold drawing, with hot extrusion as the core process. Both patents focus on the short-process method for producing tube billets. The subsequent conventional rolling / drawing process involves a relatively small deformation per pass, making it difficult to effectively break up the coarse microstructures produced by casting or hot extrusion. Furthermore, cobalt-chromium alloys experience rapid work hardening, and frequent heat treatments result in larger grain sizes, making it difficult to achieve both high strength and high ductility. In summary, in the field of cobalt-chromium alloy seamless tube production, no method currently exists to simultaneously enhance both high strength and elongation. Therefore, improvements are necessary. Summary of the Invention

[0005] The technical problem solved by the present invention is: to provide a method for preparing a cobalt-chromium-based alloy seamless tube with both high strength and high plasticity. In response to the above-mentioned deficiencies in the prior art, the present invention adopts a two-step method. First, the coarse grains of the tube billet are effectively crushed by three-roll planetary warm rolling with large deformation, and then a bimodal structure of large grains + small grains is formed by partial recrystallization heat treatment; according to the mechanism that large grains deform preferentially when subjected to stress, the large grains are further deformed by cold rolling or cold drawing with small deformation, and then smaller grains are formed by high-temperature short-time heat treatment. By reasonably controlling the deformation amount per pass and matching the heat treatment temperature, a cobalt-chromium-based alloy tube with both high strength and high elongation is obtained, which effectively solves the problem of high strength and low plasticity of conventionally processed cobalt-based alloy tubes.

[0006] The technical solution adopted by the present invention is a method for preparing a cobalt-chromium-based alloy seamless pipe with both high strength and high plasticity, the method comprising the following steps:

[0007] Step 1: Preparation of tube blanks: preparing a cobalt-chromium alloy tube blank by hot extrusion or vacuum centrifugal casting, and then homogenizing heat treatment in a vacuum tube furnace; the average grain size after the homogenizing heat treatment is 200 to 300 μm;

[0008] Step 2, three-roll planetary hot rolling: The cobalt-chromium alloy tube is preheated by an induction heating device, and then three-roll planetary hot rolling is performed. The cross-sectional shrinkage rate of the tube during the three-roll planetary hot rolling is 85% to 90%. After rolling, the tube is naturally air-cooled.

[0009] Step 3, partial recrystallization annealing: The cobalt-chromium alloy tube rolled by three-roll planetary hot rolling in step 2 is cleaned, alkali-washed, pickled and partially recrystallized to form a bimodal structure of large grains and small grains;

[0010] Step 4, cold rolling or cold drawing: The cobalt-chromium alloy tube annealed in step 3 is cold rolled or cold drawn at room temperature, and then straightened and cleaned in sequence to obtain the desired tube; the deformation of each cold rolling or cold drawing pass is 8% to 12%, and a high-temperature short-time heat treatment is performed after every 1 to 2 passes;

[0011] Step 5: High-temperature short-time heat treatment: The cobalt-chromium alloy tube prepared in step 4 is subjected to high-temperature short-time heat treatment in an environment where the tube is sealed in a vacuum quartz tube and filled with argon gas at 0.1 MPa.

[0012] In the above step 1, the outer diameter of the cobalt-chromium alloy tube blank is 28 mm to 32 mm, and the wall thickness is 4 mm to 5 mm; the temperature of the homogenization heat treatment is 1250° C. to 1300° C., and the holding time is 2 to 3 hours.

[0013] In the above step 2, the cobalt-chromium alloy tube blank is preheated to a temperature of 300-400° C. and kept warm for 30 minutes.

[0014] In the above step 3, the partial recrystallization annealing temperature is 850-950° C., kept at this temperature for 30-60 minutes, and then furnace cooled.

[0015] In the above step 5, the high temperature short time heat treatment temperature is 1150-1200° C., the heat preservation time is 1-5 minutes, and the water cooling is carried out.

[0016] In the above step 1, the nominal composition of the cobalt-chromium-based alloy tube blank is L605 or MP35N.

[0017] The advantages of the present invention compared with the prior art are:

[0018] 1. This solution leverages the antioxidant and high-temperature resistance of cobalt-based alloys and adopts a two-step process. First, the coarse grains in the tube billet are effectively crushed through large-deformation three-roll planetary hot rolling. Then, a partial recrystallization heat treatment is performed to form a bimodal structure with large and small grains. Based on the mechanism that large grains deform preferentially under stress, the large grains are further deformed through small-deformation cold rolling or cold drawing. Then, a high-temperature, short-time heat treatment is performed to form smaller grains. By properly controlling the deformation amount per pass and matching the heat treatment temperature, a cobalt-chromium-based alloy tube with both high strength and high elongation is obtained, effectively solving the problem of high strength and low plasticity of conventionally processed cobalt-based alloy tubes.

[0019] 2. Although the recrystallization heat treatment in this solution is a means of refining grains, the deformation of the pipe is locally non-uniform, and the deformation degrees of the inner and outer surfaces and the core are different. During the recrystallization heat treatment, the outer surface deforms the most, the inner surface is second, and the core is the smallest. The distortion energy of each part of the pipe is different. During the heat treatment, the driving forces for grain growth on the outer surface, inner surface, and core are different, and the grain size of each part is uneven. Therefore, the present invention prepares the pipe by a two-step method, which can effectively refine the grain size and control the size distribution.

[0020] 3. In this solution, the three-roll planetary warm rolling process for cobalt-based alloy tube billets primarily utilizes radial compression, while cold rolling or cold drawing primarily utilizes axial drawing. The combined effect of these two processes effectively ensures uniform deformation of the grains in the microstructure in all directions, further refines the grain size, and improves the mechanical properties of the cobalt-based alloy seamless tube. Furthermore, the three-roll planetary warm rolling process achieves a large deformation per pass, reducing the number of processes compared to conventional cyclic rolling and improving efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the as-cast microstructure diagram of the cobalt-chromium alloy L605 tube in Example 1 of the present invention;

[0022] Figure 2 This is the homogenized microstructure diagram of the cobalt-chromium alloy L605 tube in Example 1 of the present invention;

[0023] Figure 3 This is a cross-sectional view of a Φ12×1.0 mm cobalt-chromium alloy L605 tube prepared in Example 1 of the present invention;

[0024] Figure 4 This is a longitudinal cross-sectional view of a Φ12×1.0 mm cobalt-chromium alloy L605 tube prepared in Example 1 of the present invention;

[0025] Figure 5 This is a graph showing the roughness of the inner and outer surfaces of the cobalt-chromium alloy L605 pipe prepared in Example 1 of the present invention;

[0026] Figure 6 This is a diagram showing the mechanical properties of the cobalt-chromium-based alloy L605 pipe prepared in Example 1 of the present invention;

[0027] Figure 7 This is a longitudinal microstructure diagram of a cobalt-chromium alloy MP35N pipe Φ11.95×1.0 mm prepared in Example 2 of the present invention;

[0028] Figure 8 This is a transverse microstructure diagram of a cobalt-chromium alloy MP35N tube Φ11.95×1.0 mm prepared in Example 2 of the present invention;

[0029] Figure 9 This is a diagram of the mechanical properties of the cobalt-chromium-based alloy MP35N pipe prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0031] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, the elements defined by the phrase "comprises..." do not exclude the presence of additional identical elements in the process, method, article, or apparatus comprising the elements.

[0032] See also Figure 1-6 , details the embodiments of the present invention.

[0033] Example 1:

[0034] This embodiment includes the following steps:

[0035] Step 1: Prepare a tube blank: Prepare a cobalt-chromium alloy L605 tube blank by hot extrusion or vacuum centrifugal casting, and then homogenize heat treat in a vacuum tube furnace; the cobalt-chromium alloy L605 tube blank has an outer diameter of 28 mm and a wall thickness of 4 mm; the homogenize heat treat temperature is 1250° C. and the holding time is 3 h; the average grain size after the heat treatment is 300 μm.

[0036] Step 2: Three-roll planetary rolling: The cobalt-chromium alloy L605 tube billet is preheated to 300°C using an induction heating device and held at this temperature for 30 minutes. Three-roll planetary rolling is then performed, achieving an 87% reduction of area. After rolling, the tube is naturally air-cooled. The specific process involves processing an L605 cobalt-chromium alloy tube billet measuring 28 mm x 4.0 mm (outer diameter x wall thickness) to 13 mm x 1.0 mm (outer diameter x wall thickness).

[0037] Step 3, partial recrystallization annealing: The cobalt-chromium alloy L605 pipe rolled by three-roll planetary hot rolling in step 2 is cleaned, alkali-washed, pickled and partially recrystallized annealed; the annealing temperature is 950° C., kept warm for 30 minutes, and furnace cooled to form a large grain + small grain bimodal structure.

[0038] Step 4: Cold Rolling or Cold Drawing: The cobalt-chromium alloy L605 tube, annealed in Step 3, is cold-drawn at room temperature, followed by straightening and cleaning to obtain the desired tube. Each cold-drawing pass achieves an 8.3% deformation, and short-term heat treatment at high temperature is performed after every one to two passes. The specific process involves processing an L605 cobalt-based alloy tube billet measuring 13 mm x 1.0 mm (outer diameter x wall thickness) to 12 mm x 1.0 mm (outer diameter x wall thickness).

[0039] Step 5, high temperature short time heat treatment: the cobalt-chromium alloy L605 pipe in step 4 is subjected to high temperature short time heat treatment in a vacuum quartz tube sealed and filled with argon gas at 0.1 MPa. The high temperature short time heat treatment temperature is 1200° C., the temperature is kept for 1 minute, and the pipe is water-cooled.

[0040] The cobalt-chromium-based alloy L605 micro-tubes prepared in this embodiment have a size of 12.0 mm × 1.0 mm (outer diameter × wall thickness) and a length of 1700 mm. The inner and outer surface roughness of the tubes are 0.255 μm and 0.225 μm, respectively. The average grain size is level 11. The room temperature tensile properties are: Rm = 1185 MPa, Rp0.2 = 640 MPa, and A = 49.5%.

[0041] Example 2:

[0042] This embodiment includes the following steps:

[0043] Step 1, tube blank preparation: hot extrusion or vacuum centrifugal casting is used to prepare a cobalt-chromium alloy MP35N tube blank, and then homogenization heat treatment is performed in a vacuum tube furnace; the outer diameter of the cobalt-chromium alloy MP35N tube blank is 32 mm and the wall thickness is 5 mm; the temperature of the homogenization heat treatment is 1300°C and the holding time is 2 hours; the average grain size after the heat treatment is 200 μm.

[0044] Step 2: Three-roll planetary rolling: The cobalt-chromium alloy MP35N tube billet is preheated to 400°C using an induction heater and held at this temperature for 30 minutes. Three-roll planetary rolling is then performed. This three-roll planetary rolling achieves a 90% reduction of area, and the tube is then naturally air-cooled. The specific process involves processing a cobalt-chromium alloy MP35N tube billet measuring 32 mm x 5.0 mm (outer diameter x wall thickness) to 14.5 mm x 1.0 mm (outer diameter x wall thickness).

[0045] Step 3: Partial Recrystallization Annealing: The cobalt-chromium alloy MP35N tubes rolled by three-roll planetary hot rolling in Step 2 are cleaned, alkali-washed, pickled, and partially recrystallized. The annealing temperature is 850°C, held for 30 minutes, and then furnace-cooled to form a bimodal microstructure with large and small grains.

[0046] Step 4: Cold Rolling or Cold Drawing: The cobalt-chromium alloy MP35N tubing, annealed in Step 3, is cold rolled at room temperature, followed by straightening and cleaning to obtain the desired tubing. The cold rolling process achieves deformations of 11.9% and 8.0% per pass, respectively, and two passes are followed by a short high-temperature heat treatment. The specific process involves cold rolling the tubing in varying dimensions from 14.5mm x 1.0mm (outer diameter x wall thickness) to 12.9mm x 1.0mm (outer diameter x wall thickness) to 11.95mm x 1.0mm (outer diameter x wall thickness).

[0047] Step 5, high temperature short time heat treatment: the cobalt-chromium alloy MP35N pipe in step 4 is subjected to high temperature short time heat treatment in a vacuum quartz tube sealed and filled with argon gas at 0.1 MPa. The heat treatment temperature is 1150° C., the heat is kept for 5 minutes, and the pipe is water-cooled.

[0048] The MP35N cobalt-based alloy micro-tubes prepared in this embodiment have a size of 11.95 mm × 1.0 mm (outer diameter × wall thickness) and a length of 1500 mm. The inner and outer surface roughness of the tubes is 0.255 μm, the average grain size is 9.5, and the room temperature tensile properties are: Rm = 1227 MPa, Rp0.2 = 632 MPa, and A = 42.6%.

[0049] In summary, the present invention adopts a two-step method. First, the coarse grains of the tube billet are effectively crushed by three-roll planetary warm rolling with large deformation, and then a bimodal structure of large grains + small grains is formed by partial recrystallization heat treatment. According to the mechanism of preferential deformation of large grains under stress, the large grains are further deformed by cold rolling or cold drawing with small deformation, and then small grains are formed by high-temperature short-time heat treatment. By reasonably controlling the deformation amount of each pass and matching the heat treatment temperature, a cobalt-chromium-based alloy pipe with both high strength and high elongation is obtained, which effectively solves the problem of high strength and low plasticity of conventionally processed cobalt-based alloy pipes.

[0050] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-limiting. The scope of the invention is defined by the appended claims, not the foregoing description, and it is intended that all variations within the meaning and range of equivalents of the claims be encompassed within the present invention, and any reference signs in the claims should not be construed as limiting the claim to which they relate.

[0051] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for preparing a cobalt-chromium-based alloy seamless pipe with both high strength and high plasticity, characterized by: The method comprises the following steps: Step 1: Preparation of tube blanks: preparing a cobalt-chromium alloy tube blank by hot extrusion or vacuum centrifugal casting, and then homogenizing heat treatment in a vacuum tube furnace; the average grain size after the homogenizing heat treatment is 200 to 300 μm; Step 2: Three-roll planetary hot rolling: The cobalt-chromium alloy tube is preheated by an induction heating device, and then three-roll planetary hot rolling is performed. The cross-sectional shrinkage rate of the tube during the three-roll planetary hot rolling is 85% to 90%. After rolling, the tube is naturally air-cooled. Step 3, partial recrystallization annealing: The cobalt-chromium alloy tube rolled by three-roll planetary hot rolling in step 2 is cleaned, alkali-washed, pickled and partially recrystallized to form a bimodal structure of large grains and small grains; Step 4, cold rolling or cold drawing: The cobalt-chromium alloy tube annealed in step 3 is cold rolled or cold drawn at room temperature, and then straightened and cleaned in sequence to obtain the desired tube; the deformation of each cold rolling or cold drawing pass is 8% to 12%, and a high-temperature short-time heat treatment is performed after every 1 to 2 passes; Step 5: High-temperature short-time heat treatment: The cobalt-chromium alloy tube prepared in step 4 is subjected to high-temperature short-time heat treatment in a vacuum quartz tube sealed and filled with argon gas at 0.1 MPa; The high temperature short time heat treatment in step 5 is performed at a temperature of 1150-1200° C., kept at that temperature for 1-5 minutes, and then cooled with water.

2. The method for preparing a cobalt-chromium-based alloy seamless pipe with high strength and high plasticity according to claim 1, characterized in that: In the above step 1, the outer diameter of the cobalt-chromium alloy tube blank is 28 mm to 32 mm, and the wall thickness is 4 mm to 5 mm; the temperature of the homogenization heat treatment is 1250° C. to 1300° C., and the holding time is 2 to 3 hours.

3. The method for preparing a cobalt-chromium-based alloy seamless pipe with high strength and high plasticity according to claim 1, characterized in that: In the above step 2, the cobalt-chromium alloy tube blank is preheated to a temperature of 300-400° C. and kept warm for 30 minutes.

4. The method for preparing a cobalt-chromium-based alloy seamless pipe with high strength and high plasticity according to claim 1, characterized in that: In the above step 3, the partial recrystallization annealing temperature is 850-950° C., kept at this temperature for 30-60 minutes, and then furnace cooled.

5. A method for preparing a cobalt-chromium-based alloy seamless pipe with high strength and high plasticity according to any one of claims 1 to 4, characterized in that: In the above step 1, the nominal composition of the cobalt-chromium-based alloy tube blank is L605 or MP35N.

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