Titanium alloy pipe and method of making the same

Abstract

The application discloses a titanium alloy pipe and a preparation method thereof. The method comprises the following steps: (1) performing multi-pass cold rolling on a titanium alloy pipe blank, performing inter-pass vacuum annealing after each pass of cold rolling, and obtaining a finished titanium alloy pipe; and (2) performing sectional stress relief annealing treatment on the finished titanium alloy pipe: heating at 5-7 ℃ / min to 440-500 ℃, keeping for 5-20 min, then heating at 4-6 ℃ / min to 460-520 ℃, keeping for 60-120 min, then cooling at 1-4 ℃ / min to 200-350 ℃, and cooling to room temperature in the furnace. Through the synergistic effect of the gradually decreasing inter-pass annealing temperature and the sectional stress relief annealing of the finished pipe, the pipe internal stress is effectively eliminated, and the 6-point / 12-point direction cracking problem in the flaring and flattening test is solved.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy processing technology, specifically relating to a titanium alloy pipe and its preparation method, and particularly to a method for improving the flaring and flattening qualification rate of finished titanium alloy pipes through segmented temperature and speed controlled vacuum annealing treatment, as well as the titanium alloy pipes prepared therefrom. Background Technology

[0002] Titanium alloys possess unique and excellent properties such as high strength, good corrosion resistance, and low specific gravity, making them widely used in aerospace, petroleum, and military industries. Currently, titanium alloy pipe production accounts for 20% of titanium product output, holding a significant position in the titanium and titanium alloy application market. However, titanium alloy materials have poor machinability, especially in the production of titanium alloy pipes, which faces difficulties in processing and high costs due to their high strength, high hardness, and poor thermal stability. In recent years, with the rapid development of science and technology, my country's demand for titanium alloy pipes has been increasing year by year, requiring continuous innovation and optimization in controlling key process parameters such as annealing of finished titanium alloy pipes.

[0003] In the 1960s, near-alpha titanium alloys were highly favored by researchers due to their excellent room-temperature and high-temperature mechanical properties, superior corrosion resistance, and outstanding processing plasticity, formability, and weldability. They were commonly used to manufacture high-pressure, lightweight hydraulic and fuel pipelines to meet the urgent needs of the aerospace industry for high-performance, lightweight piping systems. Among developed countries, the United States has long been a leader in pipe manufacturing technology and applications, possessing mature production equipment, a comprehensive standard system, and solid theoretical research. Research on titanium alloy pipes in my country started relatively late. When tackling the challenges of titanium alloy pipe manufacturing, flaring and flattening tests are necessary to assess the plastic forming ability of finished pipes and reveal potential defects. These tests determine whether the pipe meets the requirements of the application scenario and prevent internal cracks or fissures during service.

[0004] The most rigorous method for evaluating the stability and quality of titanium alloy pipe connections and forming processes is the flaring and flattening test. Through multiaxial complex stress (tension, compression, and bending), various latent defects in titanium alloy pipes can be exposed. Studies have shown that in the flattening test, cracks preferentially appear on the inner surfaces of the flattening ring at the 6 o'clock and 12 o'clock positions due to the large tensile stress. The main challenge encountered when conducting the flaring and flattening test on titanium alloy pipes is that the material must withstand extremely large local deformation forces while simultaneously remaining unbroken under pressure.

[0005] Currently, titanium alloy tube blanks are mostly produced using multi-pass cold rolling processes. In traditional processes, each cold rolling pass requires annealing heat treatment to eliminate internal stress and reduce subsequent rolling cracking. Oxidation annealing or vacuum annealing is typically used between cold rolling passes, with a single heating to the annealing temperature. The stress-relief annealing temperature for finished titanium alloy tubes is generally selected 50-200℃ below the recrystallization temperature. For cold-rolled TA18 titanium alloy thin-walled seamless tubes, existing research suggests that a vacuum heat treatment process of holding at 450℃ for 2 hours followed by furnace cooling can achieve satisfactory results in eliminating residual stress. Furthermore, existing research has conducted systematic experiments on the stress-relief annealing regime for TA18 titanium alloy tubes, studying the effects of different temperature points and holding times on mechanical properties.

[0006] However, existing technologies still have shortcomings. For example, in traditional multi-pass cold rolling processes, the annealing regime between each pass lacks systematic optimization; existing stress-relief annealing of finished pipes mostly adopts a one-stage heating process, which easily leads to excessive temperature differences between the inside and outside of the pipe, resulting in thermal stress concentration; existing technologies mostly use furnace cooling or air cooling, and the cooling rate is uncontrollable, which easily generates new thermal stress during the cooling process; existing technologies have not effectively solved the cracking problem in the 6 o'clock and 12 o'clock directions in the flaring and flattening test, resulting in a low flaring and flattening pass rate, which restricts the mass application of titanium alloy pipes in aerospace and other fields.

[0007] Therefore, there is an urgent need for a method to optimize the process performance and surface quality of finished titanium alloy pipes, improve the pass rate of flaring and flattening, and enable titanium alloy pipes to have both excellent plasticity and surface condition, so as to ensure the reliability and stability of the pipes in subsequent processing and actual service, and realize the independent and controllable engineering mass production and application of titanium alloy pipes. Summary of the Invention

[0008] This invention provides a method for preparing titanium alloy tubing, aiming to overcome the aforementioned deficiencies of existing technologies. This invention eliminates the influence of internal stress generated during cold deformation and other processing steps on the microstructure and properties of the tubing by performing multi-pass cold rolling on titanium alloy tubing in different states, combined with inter-pass vacuum annealing, and by performing segmented temperature- and speed-controlled vacuum annealing on the finished titanium alloy tubing. After annealing, the internal microstructure recovers, returning from the metastable state of cold deformation to a stable state, avoiding rapid grain growth and coarsening. This effectively improves the tubing's ultimate deformation capacity at room temperature and its uniform deformation capacity under radial load, significantly increasing the pass rate of flaring and flattening tests, providing dual protection for the application of titanium alloy tubing in aerospace and other military industries.

[0009] To achieve the above objectives, in one aspect, the present invention provides a method for preparing titanium alloy tubing, comprising the following steps:

[0010] (1) The titanium alloy tube blank is subjected to multiple cold rolling passes, and vacuum annealing is performed between each pass after cold rolling to obtain the finished titanium alloy tube.

[0011] (2) The finished titanium alloy tube is subjected to segmented stress-relief annealing, wherein the segmented stress-relief annealing includes:

[0012] Heating stage: First, heat to the first temperature T1 at the first heating rate and hold for the first holding time t1, then heat to the second temperature T2 at the second heating rate and hold for the second holding time t2;

[0013] Cooling stage: Cool to the third temperature at the first cooling rate, and then cool to room temperature with the furnace;

[0014] Wherein, the first temperature T1 is 440~500℃, the second temperature T2 is 460~520℃, and the first temperature is 20~60℃ lower than the second temperature; the first heating rate is 5~7℃ / min, the second heating rate is 4~6℃ / min, and the first heating rate is greater than the second heating rate; the third temperature is 200~350℃, and the first cooling rate is 1~4℃ / min.

[0015] In a preferred embodiment, the first temperature T1 can be 440℃, 450℃, 460℃, 470℃, 480℃, 490℃, or 500℃; the second temperature T2 can be 460℃, 470℃, 480℃, 490℃, 500℃, 510℃, or 520℃; and the third temperature can be 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, or 350℃.

[0016] In a preferred embodiment, the first heat preservation time is 5 to 20 minutes, for example, it can be 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 minutes, 11 minutes, 12 minutes, 13 minutes, 14 minutes, 15 minutes, 16 minutes, 17 minutes, 18 minutes, 19 minutes, or 20 minutes; the second heat preservation time is 60 to 120 minutes, for example, it can be 60 minutes, 70 minutes, 80 minutes, 90 minutes, 100 minutes, 110 minutes, or 120 minutes.

[0017] In another aspect, the present invention provides a titanium alloy tube prepared by the above method. When the finished titanium alloy tube is flared to 1.2 times the measured outer diameter in a flaring test, no cracks appear; when flattened to less than 10 times the measured wall thickness in a flattening test, no cracks appear.

[0018] Beneficial effects

[0019] The present invention has the following beneficial effects:

[0020] 1. The annealing temperature decreases gradually between passes to precisely match the microstructure evolution.

[0021] This invention, based on the characteristics of gradually thinning tube wall thickness and accumulating work hardening during multi-pass cold rolling, sets the vacuum annealing temperature between passes to decrease progressively (680~900℃, with a temperature difference of 20~80℃ between adjacent passes). In the early passes, where deformation is large, a higher annealing temperature is used to ensure sufficient recrystallization and eliminate work hardening; in the later passes, where the tube is close to the finished size, a lower annealing temperature is used to avoid excessive grain growth, ensuring the final tube has a fine-grained structure and good plasticity reserves. This "gradient annealing" strategy overcomes the defect of a constant annealing temperature in traditional processes.

[0022] 2. Segmented heating stress-relieving annealing to eliminate thermal stress concentration.

[0023] This invention employs a two-stage heating strategy for stress-relief annealing of finished pipes: First, the pipe is heated to a first temperature T1 (440-500℃) at a relatively fast first heating rate (5-7℃ / min) and held for a short period (5-20 min) to initially synchronize the internal and external temperatures. Then, it is heated to a second temperature T2 (460-520℃) at a slower second heating rate (4-6℃ / min) and held for a longer period (60-120 min). The first temperature is 20-60℃ lower than the second temperature, and the first heating rate is greater than the second heating rate. This heating regime effectively reduces the temperature difference across the pipe cross-section, minimizes thermal stress, protects the metal material from uniform heating, and ensures consistent PV and SV temperatures in the furnace, effectively controlling the overall temperature uniformity of the pipe.

[0024] 3. Precisely control the cooling rate to avoid creating new stress.

[0025] In this invention, the cooling process is carried out at a slow rate of 1~4℃ / min (preferably 2℃ / min) to a third temperature (200~350℃, preferably 300℃), and then cooled to room temperature in the furnace. The slow cooling rate avoids the formation of new stresses within the finished pipe due to excessively rapid cooling. The cooling rate, lower than the heating rate, does not generate large thermal stresses, thus stabilizing the internal structure of the alloy and preventing the formation of large new thermal stresses during the cooling process, allowing the structure to soften naturally.

[0026] 4. Significantly improves the pass rate of flaring and flattening.

[0027] This invention, through the synergistic effect of the aforementioned technical means, effectively eliminates the internal stress of cold-rolled pipes, improves the stability of the internal structure of the pipes, and solves the problem of cracking at the 6 o'clock or 12 o'clock direction in the flaring and flattening test. Titanium alloy finished pipes prepared by the method of this invention showed no cracks when flared to 1.2 times the measured outer diameter and no cracks when flattened to less than 10 times the measured wall thickness. The flaring and flattening results both meet the requirements of GB / T 242 and GB / T 244. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in this invention or related technologies, the accompanying drawings used in describing this invention or related technologies will be briefly introduced below. Obviously, the drawings in the following description only relate to some embodiments of the invention. For those skilled in the art, other drawings can be obtained based on these drawings without any inventive effort.

[0029] For each embodiment and comparative example, two flaring tests and three flattening tests were performed. To more clearly and conveniently observe the morphology of the pipe after flaring and flattening, the pipe was not placed in exactly the same position during the photography.

[0030] Figure 1 Photographs of the flared and flattened Φ22×2.0 mm finished titanium alloy tubes of Examples 1 and 2, wherein the first row is the flared and flattened photo of Example 1, and the second row is the flared and flattened photo of Example 2.

[0031] Figure 2 Photos of the flared and flattened Φ16×1.5 mm finished titanium alloy tubes of Examples 3 and 4, wherein the first row is the flared and flattened photos of Example 3, and the second row is the flared and flattened photos of Example 4.

[0032] Figure 3 Photographs of the flared and flattened finished titanium alloy tubes Φ22×2.0 mm of Comparative Example 1 and Comparative Example 2. The first and second rows of the left image are flared photos of Comparative Example 1 and Comparative Example 2, respectively. The right image is a flattened photo of Comparative Example 1 and Comparative Example 2 distributed from left to right.

[0033] Figure 4 The images show the flared and flattened Φ16×1.5 mm finished titanium alloy tubes of Comparative Examples 3 and 4. The first and second rows of the left image are the flared images of Comparative Examples 3 and 4, respectively, while the right image shows the flattened images of Comparative Examples 3 and 4, which are arranged from left to right. Detailed Implementation

[0034] To better illustrate the technical means and effects of the present invention, the present invention is further described below in conjunction with non-limiting embodiments. These embodiments (including descriptions mentioned in the embodiments) are intended to illustrate implementation methods of the present invention and are not intended to limit the scope of any claims. According to the present invention, those skilled in the art will understand that many changes can be made to the specific embodiments disclosed without departing from the spirit and scope of the present invention, and the same or similar results can still be obtained.

[0035] In the preparation method of the titanium alloy tube of the present invention, the temperature is slowly increased to 20°C below the second temperature T2 at a first heating rate of 5~7°C / min (preferably 6°C / min). At this time, no phase transformation occurs inside the alloy structure (atoms gain kinetic energy and undergo slight displacement, releasing internal stress and eliminating residual stress while maintaining the original structure). Then, the temperature is heated to the target temperature at a heating rate of 4~6°C / min (preferably 5°C / min) to further reduce the influence of thermal stress on the finished tube and prevent the tube from transitioning too quickly from the uniform heating state in the early stage to the target temperature. At the same time, it ensures that the furnace PV (process value) and SV (set value) temperatures are consistent, and more effectively controls the heating uniformity of the overall temperature range of the tube. During the cooling process, the temperature is reduced to 300°C at a first cooling rate of 1~4°C / min (preferably 2°C / min) to avoid excessively rapid cooling that could cause new stress in the finished tube. The cooling rate, which is lower than the heating rate, will not generate large thermal stress, thus stabilizing the internal structure of the alloy and preventing the generation of large new thermal stress during the cooling process, which would cause the structure to soften naturally. When the furnace temperature is below 100℃, the tubes can be removed from the furnace. After 300℃, there is no need to require a cooling rate, and the tubes can be cooled directly with the furnace. At this point, the microstructure of the finished tubes will not undergo a phase change due to the temperature difference. Furthermore, for vacuum furnaces, the lower the temperature, the slower the cooling rate. Below 300℃, the furnace itself can achieve a cooling rate of 1℃ / 5min, which is sufficient to meet the cooling environment requirements of the finished tubes.

[0036] I. Basic Preparation of Titanium Alloy Pipes

[0037] The methods for preparing extruded titanium alloy tubing used in the embodiments and comparative examples of this invention are the same and will be described uniformly here.

[0038] (1) Ingredient design

[0039] In the specific implementation of the titanium alloy pipe test materials involved in the various embodiments and comparative examples of the present invention, near-α titanium alloy was selected, and the element types and composition (wt%) ratio were as follows: 2.60%~3.40% Al, 1.50%~2.50% V, 0.08%~0.13% Fe, 0.05%~0.15% O, ≤0.04% C, ≤0.015% N, ≤0.004% H, and the remainder being Ti.

[0040] (2) Smelting and casting ingots

[0041] Based on the alloy composition design, the raw materials selected are sponge titanium, aluminum-vanadium alloy, titanium-iron alloy, and aluminum briquettes. After mixing the above raw materials, the electrode blocks with a height of 170~220 mm are pressed in a Φ550 mm mold and welded in a vacuum plasma welding box. A three-stage vacuum self-consumable melting process is adopted, and the ingot feeding end and bottom are sawn by 30~50 mm for forging and extruding tube blanks.

[0042] (3) Forging

[0043] The polished billet is heated to 1100℃ and held for 2 hours, then subjected to three upsetting and three drawing processes in two heats. The resulting forged bar is cooled and polished, then heated to 950℃ and held for 1.5 hours, followed by two upsetting and two drawing processes in the third heat. After cooling and polishing, a fourth heat is carried out using the same method. The previously cooled forged bar is polished and polished, then heated to 920℃ and held for 1.5 hours, followed by two upsetting and two drawing processes in the fifth heat. The previously forged bar undergoes the same cooling and polishing treatment, then heated to 890℃ and held for 1 hour, followed by two upsetting and two drawing processes in the sixth and seventh heats. The eighth heat is heated to 860℃ and held for 1 hour, followed by two upsetting and two drawing processes, then cooled and polished. The ninth heat is heated to 830℃ and held for 1 hour, then drawn and rounded to Φ145 mm.

[0044] (4) Hot extrusion

[0045] Remove surface cracks, slag, holes, and oxide scale from titanium alloy forged bars, process the bars to Φ125 mm, and use hot extrusion at a speed of 40~80 mm / s to obtain Φ50 mm pipes. Although hot extrusion is used, stress concentration still exists inside the pipes, so they need to be heated in the furnace to 700~800℃, held for 1~2 hours, and then air-cooled after being removed from the furnace. Subsequently, they are straightened under residual heat at 40~60℃.

[0046] II. Implementation Examples

[0047] Example 1

[0048] The titanium alloy finished tube prepared in this embodiment has a specification of Φ22×2.0 mm. The specific implementation steps are as follows:

[0049] Step 1 (vacuum annealing, tube rolling, surface treatment): Trim and straighten the Φ50 mm extruded tube, with both ends flush and free of burrs. The tube is held at 880℃ for 2 hours in a vacuum furnace (inter-pass annealing), with a feed rate not exceeding 4 mm and a rolling speed of 75 strokes / min, and rolled to Φ35×7.0 mm on a two-roll mill. Based on the surface condition of the preceding tube, it undergoes degreasing, polishing to remove micro-scratches, and internal and external pickling. After finishing, it is held at 830℃ for 2 hours in a vacuum furnace (inter-pass annealing, 50℃ lower than the first pass), cooled in the furnace to below 70℃, and then rolled to Φ27×5.5 mm on a two-roll mill, maintaining the same feed rate and rolling speed. The preceding tube undergoes surface treatment, is held at 780℃ for 2 hours in a vacuum furnace (inter-pass annealing, 50℃ lower than the second pass), cooled in the furnace to below 80℃, and then rolled to Φ25×4.0 mm on a two-roll mill, with a feed rate not exceeding 4 mm and a rolling speed of 70 strokes / min (5 strokes / min lower than the previous pass). mm; the preceding tube is surface treated and held at 730℃ in a vacuum furnace for 1 h (fourth inter-pass annealing, 50℃ lower than the third pass), cooled in the furnace to below 70℃ and taken out of the furnace. The feed rate and rolling speed remain unchanged, and it is rolled to Φ22×2.0 mm on a two-roll mill.

[0050] Step 2 (Segmented Stress-Relief Vacuum Annealing): Several 230 mm samples were cut from Φ22×2.0 mm titanium alloy tubes, with both ends flush. After removing burrs from the ends and oil stains from the inner and outer surfaces of the tubes, they were placed in a vacuum tube annealing furnace. The entire process involved furnace-based heating and cooling. First, the temperature was raised to 500℃ at 6℃ / min and held for 10 min; then, it was raised to 520℃ at 5℃ / min and held for 90 min; finally, it was cooled to 300℃ at 2℃ / min and then cooled in the furnace to below 70℃ before being removed for inspection.

[0051] Step 3 (Flanging and Flattening Test): Two 33 mm × 2 flared samples and one 53 mm × 3 flattened sample were cut from the annealed finished pipe. The flaring mandrel was made of 304 stainless steel with a taper of 70°~80°. The flattening plate was a 15 mm thick pure titanium plate. The flaring was done to 1.2 times the measured outer diameter, and the flattening to 10 times the measured outer diameter. The flattening rate was the same for both tests, 3~5 mm / min. Testing showed that the titanium alloy finished pipe prepared in this embodiment had qualified flaring and flattening performance, and the inner and outer surfaces of the samples were smooth, without cracks or other defects. Figure 1 As shown.

[0052] Example 2

[0053] The titanium alloy finished tube prepared in this embodiment has a specification of Φ22×2.0 mm. The specific implementation steps are as follows:

[0054] Step 1 (vacuum annealing, tube rolling, surface treatment): Trim and straighten the Φ50 mm extruded tube, with both ends flush and free of burrs. The tube is held at 900℃ for 2 hours in a vacuum furnace (inter-pass annealing), with a feed rate not exceeding 4 mm and a rolling speed of 80 strokes / min, and rolled to Φ34×6.5 mm on a two-roll mill. Based on the surface condition of the preceding tube, it undergoes degreasing, external polishing to remove micro-scratches, and internal and external pickling. After finishing, it is held at 860℃ for 2 hours in a vacuum furnace (inter-pass annealing, 40℃ lower than the first pass), then cooled in the furnace to below 70℃ before being removed from the furnace. The feed rate is not higher than 4 mm and the rolling speed is 75 strokes / min (5 strokes / min lower than the previous pass), and rolled to Φ30×5.0 mm on a two-roll mill. The preceding tube undergoes surface treatment, held at 820℃ for 2 hours in a vacuum furnace (inter-pass annealing, 40℃ lower than the second pass), and cooled in the furnace to below 80℃ before being removed from the furnace. The feed rate during rolling is not higher than 4 mm. The tube is rolled to Φ25×4.0 mm at a rolling speed of 70 times / min (5 times / min less than the previous pass) on a two-roll mill. The preceding tube is then surface-treated and held at 760℃ in a vacuum furnace for 1 h (intercalation between the fourth and third passes, 60℃ lower than the third pass). It is then cooled in the furnace to below 70℃ and removed from the furnace. The feed rate and rolling speed remain unchanged, and the tube is rolled to Φ22×2.0 mm on a two-roll mill.

[0055] Step 2 (Segmented Stress-Relief Vacuum Annealing): Several 230 mm samples were cut from Φ22×2.0 mm titanium alloy tubes, with both ends flush. After removing burrs from the ends and oil stains from the inner and outer surfaces of the tubes, they were placed in a vacuum tube annealing furnace. The entire process involved furnace-based heating and cooling. First, the temperature was raised to 460°C at 6°C / min and held for 10 min; then, it was raised to 480°C at 5°C / min and held for 90 min; finally, it was cooled to 300°C at 2°C / min and then cooled in the furnace to below 70°C before being removed for inspection.

[0056] Step 3 (Flanging and Flattening Test): Conduct the same flaring and flattening test as in Step 3 of Example 1. The titanium alloy finished pipe prepared in this example passed the flaring and flattening test, and the inner and outer surfaces of the sample were smooth, without cracks or other defects. Figure 1 As shown.

[0057] Example 3

[0058] The titanium alloy finished tube prepared in this embodiment has a specification of Φ16×1.5 mm. The specific implementation steps are as follows:

[0059] Step 1 (vacuum annealing, tube rolling, surface treatment): Trim and straighten the Φ50 mm extruded tube, with both ends flush and free of burrs. The tube is held at 880℃ for 2 hours in a vacuum furnace (inter-pass annealing), with a feed rate not exceeding 4 mm and a rolling speed of 75 strokes / min, and rolled to Φ35×7.0 mm on a two-roll mill. Based on the surface condition of the preceding tube, it undergoes degreasing, polishing to remove micro-scratches, and internal and external pickling. After finishing, it is held at 830℃ for 2 hours in a vacuum furnace (inter-pass annealing, 50℃ lower than the first pass), cooled in the furnace to below 80℃, and then rolled to Φ29×5.5 mm on a two-roll mill, maintaining the same feed rate and rolling speed. The preceding tube undergoes surface treatment, is held at 780℃ for 2 hours in a vacuum furnace (inter-pass annealing, 50℃ lower than the second pass), cooled in the furnace to below 70℃, and then rolled to Φ24×4.5 mm on a two-roll mill, with a feed rate not exceeding 4 mm and a rolling speed of 75 strokes / min. The preceding tube is surface treated and held at 740℃ in a vacuum furnace for 1 hour (inter-pass annealing, 40℃ lower than the third pass). It is then cooled in the furnace to below 80℃ and removed from the furnace. The rolling feed is no more than 4 mm, and the rolling speed is 70 strokes / min (5 strokes / min lower than the previous pass). It is rolled to Φ22×4.0 mm on a two-roll mill. The preceding tube is degreased and pickled, held at 720℃ in a vacuum furnace for 1 hour (inter-pass annealing, 20℃ lower than the fourth pass). It is then cooled in the furnace to below 80℃ and removed from the furnace. The rolling feed is no more than 4 mm, and the rolling speed is 70 strokes / min. It is rolled to Φ18×3.0 mm on a two-roll mill. The preceding tube is surface treated by pickling and degreasing, held at 680℃ in a vacuum furnace for 1 hour (inter-pass annealing, 40℃ lower than the fifth pass). It is then cooled in the furnace to below 80℃ and removed from the furnace. The rolling feed is no more than 3 mm. mm, rolling speed 65 times / min (5 times / min less than the previous pass), rolled to Φ16×1.5 mm on a two-roll mill.

[0060] Step 2 (Segmented Stress-Relief Vacuum Annealing): Several 190 mm samples were cut from Φ16×1.5 mm titanium alloy tubes, with both ends flush. After removing burrs from the ends and oil stains from the inner and outer surfaces of the tubes, they were placed in a vacuum tube annealing furnace. The entire process involved furnace-based heating and cooling. First, the temperature was raised to 480°C at 6°C / min and held for 10 min; then, it was raised to 500°C at 5°C / min and held for 90 min; finally, it was cooled to 300°C at 2°C / min and then cooled in the furnace to below 70°C before being removed from the furnace for inspection.

[0061] Step 3 (Flanging and Flattening Test): Two 24 mm × 2 flared samples and one 45 mm × 3 flattened sample were cut from the annealed finished pipe. The mandrel used for flaring was made of 304 stainless steel with a taper of 70-80°. The flattening plate was a 15 mm thick pure titanium plate. Flaring was performed to 1.2 times the measured outer diameter, and flattening was performed to 10 times the measured outer diameter. The pressing rate was the same for both tests, 3-5 mm / min. Testing showed that the titanium alloy finished pipe prepared in this embodiment had qualified flaring and flattening performance, and the inner and outer surfaces of the samples were smooth, without cracks or other defects. Figure 2 As shown.

[0062] Example 4

[0063] The titanium alloy finished tube prepared in this embodiment has a specification of Φ16×1.5 mm. The specific implementation steps are as follows:

[0064] Step 1 (vacuum annealing, tube rolling, surface treatment): Trim and straighten the Φ50 mm extruded tube, with both ends flush and free of burrs. The tube is held at 900℃ for 2 hours in a vacuum furnace (inter-pass annealing), with a feed rate not exceeding 4 mm and a rolling speed of 80 strokes / min, and rolled to Φ34×6.5 mm on a two-roll mill. Based on the surface condition of the preceding tube, it undergoes degreasing, polishing to remove micro-scratches, and internal and external pickling. After finishing, it is held at 860℃ for 2 hours in a vacuum furnace (inter-pass annealing, 40℃ lower than the first pass), cooled in the furnace to below 70℃, and then rolled to Φ26×5.0 mm on a two-roll mill, maintaining the same feed rate and rolling speed. The preceding tube undergoes surface treatment, is held at 830℃ for 2 hours in a vacuum furnace (inter-pass annealing, 30℃ lower than the second pass), cooled in the furnace to below 70℃, and then rolled to Φ23×4.0 mm on a two-roll mill, with a feed rate not exceeding 4 mm and a rolling speed of 75 strokes / min (5 strokes / min lower than the previous pass). mm; The preceding tube is surface treated, held at 780℃ in a vacuum furnace for 1 hour (inter-pass annealing, 50℃ lower than the third pass), cooled in the furnace to below 80℃ and then removed from the furnace. The rolling feed is no more than 4 mm, and the rolling speed is 70 times / min (5 times / min lower than the previous pass). It is rolled to Φ20×3.0 mm on a two-roll mill; The preceding tube is degreased and pickled, held at 760℃ in a vacuum furnace for 1 hour (inter-pass annealing, 20℃ lower than the fourth pass), cooled in the furnace to below 80℃ and then removed from the furnace. The rolling feed is no more than 4 mm, and the rolling speed is 70 times / min. It is rolled to Φ18×2.5 mm on a two-roll mill; The preceding tube is surface treated by pickling and degreasing, held at 710℃ in a vacuum furnace for 1 hour (inter-pass annealing, 50℃ lower than the fifth pass), cooled in the furnace to below 80℃ and then removed from the furnace. The rolling feed is no more than 3 mm. mm, rolling speed 65 times / min (5 times / min less than the previous pass), rolled to Φ16×1.5 mm on a two-roll mill.

[0065] Step 2 (Segmented Stress-Relief Vacuum Annealing): Several 190 mm samples were cut from Φ16×1.5 mm titanium alloy tubes, with both ends flush. After removing burrs from the ends and oil stains from the inner and outer surfaces of the tubes, they were placed in a vacuum tube annealing furnace. The entire process involved furnace-based heating and cooling. First, the temperature was raised to 440℃ at 6℃ / min and held for 10 min; then, it was raised to 460℃ at 5℃ / min and held for 90 min; finally, it was cooled to 300℃ at 2℃ / min and then cooled in the furnace to below 70℃ before being removed from the furnace for inspection.

[0066] Step 3 (Flanging and Flattening Test): Conduct the same flaring and flattening test as in Step 3 of Example 3. The titanium alloy finished pipe prepared in this example passed the flaring and flattening test, and the inner and outer surfaces of the sample were smooth, without cracks or other defects. Figure 2 As shown.

[0067] Comparative Example 1

[0068] The titanium alloy finished tube prepared in this comparative example has a specification of Φ22×2.0 mm. The specific implementation steps for preparing the finished tube are the same as step 1 of Example 1. The difference is that in step 2 of this comparative example, there is no need for segmented temperature and speed control. The prepared tube sample is placed in a vacuum furnace. After the vacuum degree meets the test requirements, it is directly heated to 520℃ at 6℃ / min, held at that temperature for 90 min, and then cooled to below 70℃ in the furnace before being removed for inspection. The same flaring and flattening test method as in step 3 of Example 1 is adopted. The test results show that cracks appear at the flaring end, and longitudinal cracks appear at the 6 o'clock position during flattening. Figure 3 As shown.

[0069] Comparative Example 2

[0070] The titanium alloy finished tube prepared in this comparative example has a specification of Φ22×2.0 mm. The specific implementation steps for preparing the finished tube are the same as step 1 of Example 2. The difference is that in step 2 of this comparative example, there is no need for segmented temperature and speed control. The prepared tube sample is placed in a vacuum furnace. After the vacuum degree meets the test requirements, it is directly heated to 480℃ at 6℃ / min, held at that temperature for 90 min, and then cooled to below 70℃ in the furnace before being removed for inspection. The same flaring and flattening test method as in step 3 of Example 2 is adopted. The test results show that cracks appear at the flaring end, and longitudinal cracks appear at the 6 o'clock position during flattening. Figure 3 As shown.

[0071] Comparative Example 3

[0072] The titanium alloy finished tube prepared in this comparative example has a specification of Φ16×1.5 mm. The specific implementation steps for preparing the finished tube are the same as step 1 of Example 3. The difference is that in step 2 of this comparative example, there is no need for segmented temperature and speed control. The prepared tube sample is placed in a vacuum furnace. After the vacuum degree meets the test requirements, it is directly heated to 500℃ at 6℃ / min, held at that temperature for 90 min, and then cooled to below 70℃ in the furnace before being removed for inspection. The same flaring and flattening test method as in step 3 of Example 3 is adopted. The test results show that cracks appear at the flaring end, and longitudinal cracks appear at the 6 o'clock position during flattening. Figure 4 As shown.

[0073] Comparative Example 4

[0074] The titanium alloy finished tube prepared in this comparative example has a specification of Φ16×1.5 mm. The specific implementation steps for preparing the finished tube are the same as step 1 of Example 4. The difference is that in step 2 of this comparative example, there is no need for segmented temperature and speed control. The prepared tube sample is placed in a vacuum furnace. After the vacuum degree meets the test requirements, it is directly heated to 460℃ at 6℃ / min, held at that temperature for 90 min, and then cooled to below 70℃ in the furnace before being removed for inspection. The same flaring and flattening test method as in step 3 of Example 4 is adopted. The test results show that cracks appear at the flaring end, and longitudinal cracks appear at the 6 o'clock position during flattening. Figure 4 As shown.

[0075] Flaring and flattening tests were conducted on the titanium alloy finished pipes of Examples 1-4 and Comparative Examples 1-4. The results are shown in Table 1 below:

[0076] Table 1

[0077]

[0078] Note: The flattening test value is the distance between two flat plates; it is considered qualified if no cracks appear when the flare is 1.2 times the measured outer diameter, and it is also considered qualified if no cracks appear when the flattening is less than 10 times the measured wall thickness.

[0079] Through Examples 1-4 ( Figures 1-2 ) and Comparative Examples 1-4 ( Figures 3-4 The comparison confirms that: Examples 1-4, which employ the technical solution of this invention, all passed the flaring and flattening test, with smooth inner and outer surfaces and no cracks; while Comparative Examples 1-4, which use one-stage direct heating annealing, all showed cracks in the flaring and longitudinal cracks in the flattening at the 6 o'clock position (or at both the 6 o'clock and 12 o'clock positions), and all failed. This fully demonstrates the reliability and effectiveness of the technical solution of this invention.

[0080] The flaring and flattening results of the titanium alloy finished pipe prepared by this invention meet the requirements of GB / T 242 and GB / T 244, and the bending deformation capacity meets the service requirements of current domestic and foreign aviation hydraulic pipelines.

[0081] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for preparing titanium alloy tubing, characterized in that, Includes the following steps: (1) The titanium alloy tube blank is subjected to multiple cold rolling passes, and vacuum annealing is performed between each pass after cold rolling to obtain the finished titanium alloy tube. (2) The finished titanium alloy tube is subjected to segmented stress-relief annealing, wherein the segmented stress-relief annealing includes: Heating stage: First, heat to the first temperature T1 at the first heating rate and hold at that temperature for the first holding time t1. Then heat to the second temperature T2 at the second heating rate and hold at that temperature for the second holding time t2; Cooling stage: Cool to the third temperature at the first cooling rate, and then cool to room temperature with the furnace; Wherein, the first temperature T1 is 440~500℃, the second temperature T2 is 460~520℃, and the first temperature is 20~60℃ lower than the second temperature; the first heating rate is 5~7℃ / min, the second heating rate is 4~6℃ / min, and the first heating rate is greater than the second heating rate; the third temperature is 200~350℃, and the first cooling rate is 1~4℃ / min.

2. The method according to claim 1, characterized in that, The vacuum annealing temperature decreases successively between passes, with the annealing temperature ranging from 680 to 900°C, and the annealing temperature difference between adjacent passes is 20 to 80°C.

3. The method according to claim 1, characterized in that, In step (1), the multi-pass cold rolling adopts two-roll rolling, with 3 to 8 passes, and the feed amount per pass is no more than 4 mm. As the cold rolling passes advance, the rolling speed of each pass decreases in a stepwise manner, and the rolling speed of any adjacent subsequent pass is no higher than that of the previous pass. The rolling speed of the first pass is 75 to 80 times / min, and the rolling speed of the last pass is 65 to 70 times / min.

4. The method according to claim 1, characterized in that, The first heating rate is 6℃ / min, the second heating rate is 5℃ / min, and the first cooling rate is 2℃ / min.

5. The method according to claim 1, characterized in that, The first heat preservation time is 5~20 min, the second heat preservation time is 60~120 min, and the third temperature is 300℃.

6. The method according to claim 1, characterized in that, The titanium alloy tubing is a near-α titanium alloy, which, by weight percentage, comprises: 2.60%~3.40% Al, 1.50%~2.50% V, 0.08%~0.13% Fe, 0.05%~0.15% O, ≤0.04% C, ≤0.015% N, ≤0.004% H, with the balance being Ti.

7. The method according to any one of claims 1 to 6, characterized in that, The manufacturing process of the titanium alloy tube includes, in sequence: composition design, melting and casting, forging, hot extrusion, intermediate annealing, tube rolling, surface treatment, and segmented stress-relieving vacuum annealing.

8. The method according to claim 7, characterized in that, The melting and casting of the ingot adopts a three-stage vacuum self-consumption melting process. The melting mold specifications are Φ500~800 mm, and the height of the pressing electrode block is 170~220 mm. The forging temperature is 800~1200℃, and the holding time is 1~2 h. After 8~10 forging cycles, the initial billet of titanium alloy tube is obtained by hot extrusion.

9. A titanium alloy tube prepared by the method according to any one of claims 1 to 8.

10. The titanium alloy tubing according to claim 9, characterized in that, During the flaring test, no cracks appeared when the flaring was 1.2 times the measured outer diameter; during the flattening test, no cracks appeared when the flattening was less than 10 times the measured wall thickness.

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