Preparation method of Ti35 alloy seamless elbow

Abstract

The invention discloses a preparation method of a Ti35 alloy seamless elbow. The method comprises the following steps: 1, drilling a through hole in the center of a Ti35 alloy bar blank, and performing hot extrusion; secondly, vacuum annealing treatment is conducted after multi-pass cold rolling large deformation; 3, cold pushing forming; and 4, after cleaning, annealing, machining and turning grooves, and detecting to obtain the Ti35 alloy seamless elbow. A through hole is drilled in the center of a Ti35 alloy bar blank, then hot extrusion is conducted, a Ti35 alloy seamless pipe blank uniform in wall thickness and good in plasticity is prepared through multi-pass cold rolling large deformation and recrystallization vacuum annealing, then cold pushing forming, annealing and machining are conducted, and the Ti35 alloy seamless elbow low in wall thickness reduction rate, good in wall thickness uniformity and smooth and flat in surface is prepared. And the requirements of Ti35 alloy pipe fittings in the nuclear chemical industry field are well met.

Description

Technical Field

[0001] The invention belongs to the field of nonferrous metal processing, and particularly relates to a method for preparing a Ti35 alloy seamless elbow. Background Art

[0002] Ti35 alloy material is used to manufacture core equipment in the nuclear chemical industry due to its excellent corrosion resistance. These equipment are connected by pipelines to form a complete nuclear chemical processing system, and elbows are an indispensable pipe fitting for pipeline connection.

[0003] The existing seamless elbow preparation technologies mainly include stamping, hot push and cold push. Stamping is to place the tube blank in a suitable mold and prepare it into a seamless elbow through upper and lower mold pressing. This method has no constraint in the tensile direction of the elbow during the forming process, resulting in serious thinning of the wall thickness after forming. At the same time, the local distortion of the tube surface is serious, and the tube rebounds severely after bending and forming, making it difficult to accurately control the forming size. The hot push method is to improve the plastic deformation ability of the tube blank by heating methods such as induction or flame, and then form the elbow by push. The elbow produced by this forming method has a relatively coarse and uneven structure. Improper heating is prone to overheating and overburning, and the surface quality is poor and the surface treatment is difficult due to deformation and oxidation. Cold push forming is mainly for materials with high plastic deformation ability. Since the forming process is sensitive to various process parameters, there are currently various problems such as unstable quality, uneven wall thickness after forming, and wrinkles and cracks in the appearance.

[0004] For Ti35 alloy elbows, the stamping forming method is not suitable because of the serious wall thickness reduction, which is extremely detrimental to the service life. After hot push forming, the structure is coarse and uneven, the various properties are unstable, and the wall thickness is significantly reduced after surface treatment. It is also not suitable for Ti35 alloy elbow forming. Cold push forming is a relatively ideal processing method for preparing Ti35 alloy, but it has high requirements on material plasticity, is sensitive to process parameters, and is prone to obvious wall thickness reduction, high wall thickness unevenness, surface wrinkles or cracks, etc. These phenomena have serious damage to the service life of Ti35 alloy elbows. Therefore, how to prepare Ti35 alloy seamless elbows with good appearance, no obvious wall thickness reduction and high uniformity, and a smooth and defect-free surface is an important technical problem that must be solved in the spent fuel reprocessing project. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing a Ti35 alloy seamless elbow in view of the deficiencies of the above-mentioned prior art. The method adopts multiple cold rolling large deformations in the process of preparing the tube billet, combined with vacuum recrystallization annealing, to prepare a Ti35 alloy seamless tube billet with uniform fine equiaxed structure, high plasticity and uniform wall thickness, and then adopts a cold push forming process to suppress the wall thickness thinning phenomenon, and combines with vacuum stress relief annealing to prepare a Ti35 alloy seamless elbow with low wall thickness thinning rate, good wall thickness uniformity, smooth and defect-free surface, which solves the problems of uneven wall thickness, serious wall thickness thinning and poor surface quality in the existing seamless elbow preparation technology.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing a Ti35 alloy seamless elbow, characterized in that the method comprises the following steps: Step 1: drilling a hole through the center of a Ti35 alloy rod blank, coating it with copper and then hot extruding it to prepare an extruded tube blank; Step 2: subjecting the extruded tube obtained in step 1 to multiple cold rolling and large deformation, and then subjecting the tube to vacuum annealing treatment to obtain a Ti35 alloy seamless tube with uniform wall thickness; Step 3, after the Ti35 alloy seamless tube obtained in step 2 is beveled, lubricant is applied to the inner and outer surfaces of the tube and the mandrel mold, and then placed in an elbow pusher for cold push forming to obtain an elbow blank; Step 4: Clean the oil and lubricant on the surface of the elbow blank obtained in step 3, and then anneal, machine bevel and test to obtain a Ti35 alloy seamless elbow.

[0007] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the wall thickness deviation of the Ti35 alloy billet after the center hole is drilled in step 1 does not exceed 0.5 mm. By controlling the wall thickness deviation range after drilling, the uniformity of the wall thickness during subsequent extrusion and rolling deformation is ensured.

[0008] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the temperature of the hot extrusion in step 1 is 105°C to 185°C below the phase transition point, and the unevenness of the wall thickness of the extruded tube billet does not exceed 1 mm. By controlling the extrusion forming temperature and the uniformity of the wall thickness of the extruded billet, a uniform wall thickness extruded tube billet with a softened structure that can be cold-rolled for large deformation is obtained.

[0009] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the rolling times of the multi-pass cold rolling with large deformation in step 2 are not less than 2 times, the total deformation amount is not less than 40%, and the Ti35 alloy seamless tube blank is a uniform equiaxed fine-grained structure. The rolling times and deformation amount of the multi-pass cold rolling with large deformation are controlled to fully break the grains and obtain a uniform equiaxed fine-grained structure.

[0010] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the temperature of the vacuum annealing treatment in step 2 is 200°C to 335°C below the phase transition point, and the holding time is 90min to 180min. By controlling the temperature and holding time of the vacuum annealing treatment, the tube blank is fully recrystallized to form a Ti35 alloy seamless tube blank with a fine, uniform, equiaxed structure and good plasticity.

[0011] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the wall thickness deviation of the Ti35 alloy seamless tube blank in step 2 does not exceed ±10% of the nominal wall thickness. Through this limitation, the wall thickness of the Ti35 alloy seamless tube blank is ensured to be uniform.

[0012] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the Ti35 alloy seamless tube blank described in step three is machined to have a bevel angle α of 48°~60° at both ends.

[0013] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the cold push forming in step 3 uses graphite powder as a lubricant, and the inner and outer gaps between the tube blank and the mold are 0.5mm~1.5mm, the thrust is applied to the tensile side of the tube blank elbow forming, and the pushing speed is 5mm / s~15mm / s. Preferably, the push forming is a cold push forming with a mandrel. By applying the thrust to the tensile side of the tube blank elbow forming, the forming tension is balanced to suppress the thinning of the wall thickness.

[0014] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the annealing in step 4 is vacuum stress relief annealing, and the cooling method is furnace cooling.

[0015] The above-mentioned method for preparing a Ti35 alloy seamless elbow is characterized in that the diameter of the Ti35 alloy seamless elbow in step 4 is Φ14mm~Φ114.3mm, and the wall thickness is 2mm~7mm.

[0016] Compared with the prior art, the present invention has the following advantages: 1. The present invention is based on the use demand and forming technology research of Ti35 alloy seamless elbows. First, hot extrusion tube blanks with good wall thickness uniformity and strong cold deformation ability are prepared by drilling hot extrusion. Then, a high-plasticity Ti35 alloy seamless tube blank with uniform wall thickness and uniform fine equiaxed structure is prepared by multiple cold rolling large deformation combined with vacuum annealing treatment process. Then, a cold push forming process is adopted to suppress the wall thinning phenomenon by applying thrust on the tensile side of the tube blank elbow forming, and combined with vacuum stress relief annealing, finally a Ti35 alloy seamless elbow with low wall thickness thinning rate, good wall thickness uniformity, smooth and clean surface (roughness Ra <1.6μm) and no defects is prepared, which meets the great demand for elbows for nuclear chemical pipelines.

[0017] 2. Compared with the stamping forming method, the present invention first controls the uniformity of the wall thickness and the plastic deformation capacity of the tube blank through multiple cold rolling large deformations, and adopts the cold pushing forming method, which effectively solves the problems of thinning of the elbow wall thickness and serious surface distortion. The prepared elbow has more uniform wall thickness and a smoother surface.

[0018] 3. Compared with the hot push forming method, the present invention adopts multiple cold rolling large deformation to control the plastic deformation ability of the tube blank, which effectively solves the problems of uneven structure and poor surface quality of hot push elbows. It is more energy-saving and efficient than hot push forming, and the comprehensive performance of the obtained elbows is better.

[0019] 4. The present invention combines cold stacking forming with vacuum stress relief annealing to avoid the problem of surface quality instability caused by severe oxidation of the elbow surface due to atmospheric annealing, significantly improves the quality of the elbow, and ensures that the wall thickness of the Ti35 alloy seamless elbow is uniform and the appearance is smooth and flat.

[0020] 5. The preparation process of the present invention is simple and has significant technical effects, and is suitable for extension to the field of titanium alloy and other non-ferrous metal elbow forming and preparation.

[0021] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The horizontal and vertical metallographic structures of the Ti35 alloy seamless tube prepared in Example 1 of the present invention.

[0023] Figure 2 This is a machining diagram of the Ti35 alloy seamless tube blank prepared in Example 1 of the present invention before push forming. DETAILED DESCRIPTION

[0024] Example 1 This embodiment includes the following steps: Step 1: Select a Ti35 alloy billet with a diameter of Φ214 mm, which has been fully broken into as-cast structures after repeated upsetting and forging deformation, has uniform composition, and has a polished surface. The phase transition point is 885°C~890°C, drill a through hole with a diameter of Φ91 mm in the center of the Ti35 alloy billet, and control the wall thickness deviation to be no more than 0.5 mm. Then, use copper to completely cover the inside of the through hole and the outer surface of the billet. After being kept at 780°C for 150 minutes, take it out of the furnace for extrusion to obtain an extruded tube billet with a diameter × wall thickness of Φ127 mm × 12 mm, and the unevenness of the wall thickness of the extruded tube billet does not exceed 1 mm. Step 2: The extruded tube obtained in step 1 is subjected to two cold rolling large deformations, with a total deformation of 49%, to obtain a tube with a diameter × wall thickness of Φ114.3mm×7mm, and then subjected to vacuum annealing at 685°C for 180min to obtain a Ti35 alloy seamless tube with uniform equiaxed fine grain structure, and the transverse and longitudinal metallographic structures of the Ti35 alloy seamless tube are both uniform equiaxed fine grain structures, such as Figure 1 As shown in the figure, after testing, the wall thickness deviation of the Ti35 alloy seamless tube billet does not exceed ±4% of the nominal wall thickness, and the plastic elongation of the tube billet reaches 44.5%, which is 78% higher than the standard value of Ti35 material (25%); Step 3: Draw dimension lines on the Ti35 alloy seamless tube obtained in step 2 and machine the tube to produce the following: Figure 2 The shape shown, the bevel angles α1 and α2 at both ends are 60° and 48° respectively, and the end of α2 is sharpened and flattened, lubricant graphite powder is applied on the inner and outer surfaces of the tube blank, the upper and lower molds, and the core rod tire mold, and then the tube blank is loaded into the mold, and the inner and outer clearance between the tube blank and the mold is 1.5mm. When loading, the side with the bevel angle α2=48° is placed on the push rod side, and placed in the elbow push machine for cold push molding, the thrust is applied to the tensile side of the tube blank elbow forming, and the pushing speed is 15mm / s to obtain the elbow blank; Step 4: Clean the surface oil and lubricant of the elbow blank obtained in step 3, then perform vacuum stress relief annealing at 650°C for 30 minutes, cool it to below 150°C with the furnace, and perform 100% PT inspection on the accessible surface of the elbow after machining and beveling. There are no defects, meeting the requirements of NB / T 47013.5-2015 Level Ⅰ, and obtain a Ti35 alloy seamless elbow with a diameter × wall thickness of Φ114.3mm×7mm; after appearance size inspection, the Ti35 alloy seamless elbow has a wall thickness reduction rate of 4.8%, a wall thickness unevenness of 8.5%, and a surface roughness Ra<1.6μm.

[0025] Example 2 This embodiment includes the following steps: Step 1: Select a Ti35 alloy billet with a diameter of Φ180 mm, which has been fully broken into as-cast structures after repeated upsetting and forging deformation, has uniform composition, and has a polished surface. The phase transition point is 885℃~890℃. Drill a through hole with a diameter of Φ64.5 mm in the center of the Ti35 alloy billet, and control the wall thickness deviation to be no more than 0.5 mm. Then, use copper to completely cover the inside of the through hole and the outer surface of the billet. After keeping the temperature at 750℃ for 120 minutes, take it out of the furnace for extrusion to obtain an extruded tube billet with a diameter × wall thickness of Φ80 mm × 10 mm. Step 2: The extruded tube obtained in step 1 is subjected to 3 cold rolling large deformations, with a total deformation of 68.5%, to obtain a tube with a diameter × wall thickness of Φ60.3mm × 3.91mm, and then subjected to vacuum annealing treatment at 600°C for 120min to obtain a Ti35 alloy seamless tube with uniform equiaxed fine grain structure. According to the test, the wall thickness deviation of the Ti35 alloy seamless tube does not exceed ±8% of the nominal wall thickness, and the plastic elongation of the tube reaches 36.5%, which is 46% higher than the standard value (25%) of Ti35 material; Step 3, draw dimension lines on the Ti35 alloy seamless tube obtained in step 2 and saw cut the material, the bevel angles α1 and α2 at both ends are 60° and 48° respectively, and the end of α2 is sharpened and flattened, lubricant graphite powder is applied on the inner and outer surfaces of the tube and the upper and lower molds and the core rod tire mold, and then the tube is loaded into the mold, and the inner and outer clearance between the tube and the mold is 1mm. When loading, the side with the bevel angle α2=48° is placed on the push rod side, and placed in the elbow push machine for cold push forming, the thrust is applied to the tensile side of the tube elbow forming, and the pushing speed is 8mm / s to obtain the elbow blank; Step 4: Clean the surface oil and lubricant of the elbow blank obtained in step 3, then perform vacuum stress relief annealing at 650°C for 30 minutes, cool it to below 150°C with the furnace, and perform 100% PT inspection on the accessible surface of the elbow after machining and beveling. There are no defects, meeting the requirements of NB / T 47013.5-2015 Level Ⅰ, and obtain a Ti35 alloy seamless elbow with a diameter × wall thickness of Φ60.3mm×3.91mm; after appearance size inspection, the Ti35 alloy seamless elbow has a wall thickness reduction rate of 4.8%, a wall thickness unevenness of 14.0%, and a surface roughness Ra<1.6μm.

[0026] Example 3 This embodiment includes the following steps: Step 1: Select a Ti35 alloy billet with a diameter of Φ145 mm, which has been fully broken into as-cast structures after repeated upsetting and forging deformation, has uniform composition, and has a polished surface. The phase transition point is 885℃~890℃, drill a through hole with a diameter of Φ35.5 mm in the center of the Ti35 alloy billet, and control the wall thickness deviation to be no more than 0.5 mm. Then, use copper to completely cover the inside of the through hole and the outer surface of the billet. After being kept at 760℃ for 120 minutes, take it out of the furnace for extrusion to obtain an extruded tube billet with a diameter × wall thickness of Φ49 mm × 10 mm. Step 2: The extruded tube obtained in step 1 is subjected to 4 cold rolling large deformations, with a total deformation of 77%, to obtain a tube with a diameter × wall thickness of Φ26.7mm × 3.91mm, and then subjected to vacuum annealing treatment at 600°C for 120min to obtain a Ti35 alloy seamless tube with uniform equiaxed fine grain structure. According to the test, the wall thickness deviation of the Ti35 alloy seamless tube does not exceed ±8% of the nominal wall thickness, and the plastic elongation of the tube reaches 45%, which is 80% higher than the standard value (25%) of Ti35 material; Step 3, draw dimension lines on the Ti35 alloy seamless tube obtained in step 2 and saw cut the material, the bevel angles α1 and α2 at both ends are 60° and 48° respectively, the end of α2 is sharpened and flattened, lubricant graphite powder is applied on the inner and outer surfaces of the tube and the upper and lower molds and the core rod tire mold, and then the tube is loaded into the mold, and the inner and outer clearance between the tube and the mold is 0.8mm. When loading, the side with the bevel angle α2=48° is placed on the push rod side, and placed in the elbow push machine for cold push forming, the thrust is applied to the tensile side of the tube elbow forming, and the pushing speed is 5mm / s to obtain the elbow blank; Step 4: Clean the surface oil and lubricant of the elbow blank obtained in step 3, then perform vacuum stress relief annealing at 650°C for 30 minutes, cool it to below 150°C with the furnace, and perform 100% PT inspection on the accessible surface of the elbow after machining and beveling. There are no defects, meeting the requirements of NB / T 47013.5-2015 Level Ⅰ, and obtain a Ti35 alloy seamless elbow with a diameter × wall thickness of Φ26.7mm×3.91mm; after appearance size inspection, the Ti35 alloy seamless elbow has a wall thickness thinning rate of 5.1%, a wall thickness unevenness of 10.4%, and a surface roughness Ra<1.6μm.

[0027] Example 4 This embodiment includes the following steps: Step 1: Select a Ti35 alloy billet with a diameter of Φ145 mm, which has been fully broken into as-cast structures after repeated upsetting and forging deformation, has uniform composition, and has a polished surface. The phase transition point is 885℃~890℃, drill a through hole with a diameter of Φ35.5 mm in the center of the Ti35 alloy billet, and control the wall thickness deviation to be no more than 0.5 mm. Then, use copper to completely cover the inside of the through hole and the outer surface of the billet. After keeping the temperature at 700℃ for 120 minutes, take it out of the furnace for extrusion to obtain an extruded tube billet with a diameter × wall thickness of Φ49 mm × 10 mm. Step 2: The extruded tube obtained in step 1 is subjected to 4 cold rolling large deformations, with a total deformation of 93.8%, to obtain a tube with a diameter × wall thickness of Φ14mm × 2mm, and then subjected to vacuum annealing at 550°C for 90 minutes to obtain a Ti35 alloy seamless tube with uniform equiaxed fine grain structure. According to the test, the wall thickness deviation of the Ti35 alloy seamless tube does not exceed ±4% of the nominal wall thickness, and the plastic elongation of the tube reaches 34.5%, which is 38% higher than the standard value (25%) of Ti35 material; Step 3, draw dimension lines on the Ti35 alloy seamless tube obtained in step 2 and saw cut the material, the bevel angles α1 and α2 at both ends are 60° and 48° respectively, and the end of α2 is sharpened and flattened, lubricant graphite powder is applied on the inner and outer surfaces of the tube and the upper and lower molds and the core rod tire mold, and then the tube is loaded into the mold, and the inner and outer clearance between the tube and the mold is 0.5mm. When loading, the side with the bevel angle α2=48° is placed on the push rod side, and placed in the elbow push machine for cold push forming, the thrust is applied to the tensile side of the tube elbow forming, and the pushing speed is 5mm / s to obtain the elbow blank; Step 4. Clean the surface oil and lubricant of the elbow blank obtained in step 3, then keep it at 650°C for 30 minutes for vacuum stress relief annealing, cool it to below 150°C with the furnace and take it out of the furnace. After machining and beveling, perform 100% PT inspection on the accessible surface of the elbow. There is no defect phenomenon, which meets the requirements of NB / T 47013.5-2015 Level Ⅰ inspection, and obtain a Ti35 alloy seamless elbow with a diameter × wall thickness of Φ14mm×2mm; after appearance size inspection, the Ti35 alloy seamless elbow has a wall thickness reduction rate of 3.2%, a wall thickness unevenness of 7.8%, and a surface roughness Ra<1.6μm.

[0028] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a Ti35 alloy seamless elbow, characterized in that: The method comprises the following steps: Step 1: drilling a hole through the center of a Ti35 alloy rod blank, coating it with copper and then hot extruding it to prepare an extruded tube blank; Step 2: subjecting the extruded tube obtained in step 1 to multiple cold rolling and large deformation, and then subjecting the tube to vacuum annealing treatment to obtain a Ti35 alloy seamless tube with uniform wall thickness; Step 3, after the Ti35 alloy seamless tube obtained in step 2 is beveled, lubricant is applied to the inner and outer surfaces of the tube and the mandrel mold, and then placed in an elbow pusher for cold push forming to obtain an elbow blank; Step 4: Clean the oil and lubricant on the surface of the elbow blank obtained in step 3, and then anneal, machine bevel and test to obtain a Ti35 alloy seamless elbow.

2. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The wall thickness deviation of the Ti35 alloy bar blank after the center hole is drilled in step 1 does not exceed 0.5 mm.

3. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The temperature of the hot extrusion in step 1 is 105° C. to 185° C. below the phase transition point, and the unevenness of the wall thickness of the extruded tube billet does not exceed 1 mm.

4. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The number of rolling times of the multi-pass cold rolling with large deformation in step 2 is not less than 2 times, the total deformation is not less than 40%, and the Ti35 alloy seamless tube billet has a uniform equiaxed fine-grained structure.

5. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The temperature of the vacuum annealing treatment in step 2 is 200° C. to 335° C. below the phase transition point, and the holding time is 90 min to 180 min.

6. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The wall thickness deviation of the Ti35 alloy seamless tube in step 2 does not exceed ±10% of the nominal wall thickness.

7. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The Ti35 alloy seamless tube blank described in step 3 is machined to have a bevel angle α of 48° to 60° at both ends.

8. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The cold pushing molding described in step three uses graphite powder as a lubricant, and the inner and outer gaps between the tube blank and the mold are 0.5mm~1.5mm, the thrust is applied to the tensile side of the tube blank elbow forming, and the pushing speed is 5mm / s~15mm / s.

9. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The annealing in step 4 is vacuum stress relief annealing, and the cooling method is furnace cooling.

10. The method for preparing a Ti35 alloy seamless elbow according to claim 1, characterized in that: The diameter of the Ti35 alloy seamless elbow described in step 4 is Φ14mm~Φ114.3mm, and the wall thickness is 2mm~7mm.

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