A method for preparing small-bore high-precision thin-wall GH738 pipe

Abstract

The present application relates to the technical field of pipe preparation, and specifically discloses a small-diameter high-precision thin-wall GH738 pipe preparation method, which comprises the following steps: step one, obtaining GH738 rod stock; step two, processing the rod stock into GH738 pipe blank; step three, obtaining semi-finished GH738 pipe; step four, cleaning the semi-finished GH738 pipe and then performing a vacuum heat treatment process to adjust the organization and performance of the pipe; step five, rounding and straightening pretreatment; and step six, obtaining small-size high-precision thin-wall GH738 pipe with an outer diameter of 5-8 mm, an outer diameter tolerance of -0.01 to -0.03 mm, a wall thickness of 0.5-1.0 mm, a wall thickness tolerance of ±0.05 mm, and a straightness of less than 0.3 mm / m. The small-size high-precision thin-wall GH738 pipe prepared by the method has higher size precision, better straightness, lower surface residual stress, and higher eddy current flaw detection qualification rate, and the pipe yield is significantly increased. The preparation process is simple, has strong repeatability, is suitable for large-scale industrial production, and has wide popularization value and significant economic benefits.

Description

Technical Field

[0001] This invention relates to the field of high-temperature alloy pipe manufacturing technology, specifically a method for manufacturing small-diameter, high-precision, thin-walled GH738 pipes with a diameter ≤6mm and a wall thickness ≤1mm. Background Technology

[0002] GH738 alloy is a high-performance nickel-based superalloy, a γ' phase precipitation-strengthened nickel-based superalloy with excellent high-temperature strength, creep resistance, thermal fatigue resistance, and oxidation resistance. Its chemical composition is shown in Table 1. Due to its superior performance, it is widely used in aerospace, shipbuilding, petrochemical, energy, and automotive industries. In aircraft engines, this alloy is processed into many structural components, such as combustion chambers, discs, blades, guide vanes, and casings.

[0003] Table 1 Chemical composition of conventional GH738 alloy

[0004]

[0005] Due to its excellent high-temperature performance, corrosion resistance, and fatigue resistance, GH738 alloy is also processed into an important type of structural component—seamless tubing. This seamless tubing is commonly used in high-temperature, high-pressure, and corrosive environments as a crucial conduit for conducting working media, such as GH738 tubing used in hot air engines. Since these tubing components are often manufactured using brazing processes, strict control of the gaps between the welded parts, brazing filler metal, and the tubing is necessary to ensure welding quality. Therefore, these tubing components require high dimensional accuracy, straightness, and ovality. For example, some φ6x1.0 tubing requires an outer diameter tolerance of 0.02mm, a wall thickness tolerance of ±0.05mm, and a straightness of <0.5mm / m.

[0006] Currently, traditional pipe manufacturing methods often face problems such as low dimensional accuracy, poor straightness, and low eddy current testing pass rate when producing small-diameter, high-precision, thin-walled GH738 pipes, resulting in a generally low yield. The main reasons for this problem include: 1. Rolling difficulties. GH738 material has high yield strength and is prone to springback after deformation, making rolling difficult and controlling the finished product dimensions challenging. 2. Straightening difficulties. GH738 has high resistance to deformation during rolling, making deformation difficult and resulting in high residual stress. After heat treatment, once the residual stress is released, the pipe is prone to bending with a small radius of curvature. According to the pipe straightening principle, the straightening effect is affected by straightening process parameters such as the original bending curvature, the distance between the straightening rollers, and the lead. When using a multi-roll straightener to straighten pipes, at least three reverse bending deformations need to be completed in one spiral phase. Therefore, the distance P between adjacent rollers is usually an odd multiple of half the lead t, i.e., P = k(t / 2), where k is an odd number. If the original bending radius of the pipe is too small, the straightener cannot apply sufficient reverse bending deformation due to the limitations of the rotary multi-roller straightening machine structure. The deformation is insufficient to break through the material's yield strength, ultimately forming an unavoidable bend, commonly known in engineering production as a "dead bend." This type of bend not only severely affects the straightness of the pipe but also makes polishing difficult, affecting the dimensional accuracy of the finished product. Residual bends can also affect the assembly accuracy of the pipe (e.g., hydraulic pipes, structural pipes) or fluid transmission efficiency. Thirdly, eddy current testing easily produces defect signals, resulting in low pass rates, low yields, and material waste. Because long, small-diameter, thin-walled pipes have poor rigidity, large deflection, and are prone to bending, straightening pipes with small radii of curvature are easily straightened with spiral straightening marks. These straightening marks are spiral grooves that, on the one hand, create an uneven lift-off effect between the eddy current probe and the pipe surface, causing false defect signals during eddy current testing; on the other hand, to ensure the straightness requirements of the pipe, multiple straightening passes are usually used when straightening is not ideal. Excessive straightening passes can generate uneven residual stress on the outer surface of GH738 pipes. Due to the piezoresistive effect, uneven electrical conductivity will occur in areas of uneven stress, increasing noise during eddy current testing. When straightening marks are severe, both of these factors can easily cause false alarms, reducing the eddy current testing pass rate and resulting in material waste. Summary of the Invention

[0007] To address the shortcomings of current technologies, this invention provides a method for preparing small-diameter, high-precision, thin-walled GH738 tubing. GH738 thin-walled tubing prepared using this method exhibits high dimensional accuracy, good straightness, low eddy current testing noise, high flaw detection pass rate, high yield, and a grain size level of 4-5. This method meets the technical requirements of high precision, high yield, and high structural stability for GH738 nickel alloy tubing used in the hot air engine field, and has broad application value and significant economic benefits.

[0008] The technical solution adopted in this invention is: a method for preparing GH738 nickel-based high-temperature alloy tubing, comprising the following steps:

[0009] Step 1: Hot-roll and polish the GH738 nickel-based alloy ingots obtained by vacuum induction melting → electroslag remelting or vacuum arc remelting to obtain GH738 bar billets.

[0010] Step 2: The GH738 bar obtained in Step 1 is sawed and cut into blanks, and then machined into GH738 tube blanks.

[0011] Step 3: The tube blank obtained in Step 2 is subjected to multiple cold rolling processes, annealing and recrystallization heat treatment of the semi-finished tube, straightening and polishing processes to obtain the semi-finished GH738 tube; among them, the annealing heat treatment of the semi-finished tube is carried out in a vacuum quenching furnace, the cooling gas is argon, the heating temperature is 1020~1080℃ and the holding time is 20~60 minutes, and the argon gas is used for rapid cooling.

[0012] Step 4: After cleaning the semi-finished GH738 pipe, perform a vacuum heat treatment process to adjust the structure and properties of the pipe. The annealing heat treatment of the finished pipe uses a vacuum quenching furnace with argon as the cooling gas. The heating temperature is 1020-1080℃ and the holding time is 20-60 minutes, followed by rapid cooling with argon.

[0013] Step 5: Perform pre-treatment on the pipes obtained in Step 4 to achieve roundness and straightness;

[0014] Step Six: The finished tube is then straightened using a multi-roller straightening machine, followed by polishing to meet the dimensional tolerances and surface roughness requirements, resulting in a small-sized, high-precision, thin-walled GH738 tube. The small-sized, high-precision, thin-walled GH738 tube has an outer diameter of 5–8 mm, an outer diameter tolerance of -0.01–-0.03 mm, a wall thickness of 0.5–1.0 mm, a wall thickness tolerance of ±0.05 mm, and a straightness of <0.5 mm / m.

[0015] The machined tube blank in step two has an inner and outer surface roughness of less than 1.6 and a coaxiality of less than 0.1 mm.

[0016] In step three, the deformation per pass of the multi-pass cold rolling is 10% to 30%, the rolling feed is 1 to 3 mm, and the rolling speed is 50 to 80 times / minute; the annealing and recrystallization heat treatment temperature of the semi-finished pipe is 1020℃ to 1080℃, and the holding time is 0.5t hours, where t is the pipe wall thickness; the straightness of the pipe after straightening is less than 1.5 mm / m.

[0017] In step four, the vacuum heat treatment temperature of the finished pipe is 1020-1080℃; the heat treatment time is 0.5t hours, where t is the pipe wall thickness.

[0018] In step five, a custom tooling mold is made according to the diameter of the heat-treated pipe. A drawing method is used to perform a rounding and straightening pretreatment on the pipe, ensuring that the outer surface of the pipe undergoes only slight plastic deformation without significant work hardening. The bending radius of the pipe after rounding and straightening pretreatment is greater than the minimum allowable bending radius of the straightening equipment. Simultaneously, the mold diameter B for rounding and straightening pretreatment is equal to the finished pipe diameter D - (0.03~0.10) mm, the ovality of the bore is <0.5%, the α angle is 6~30°, and the β angle is 60~90°.

[0019] In step six, the straightness of the finished pipe after straightening should be <0.02mm / 100mm, and the ellipticity of the pipe should be <0.5%; the diameter removal amount of the polishing process is 0.03~0.10mm.

[0020] Compared to traditional pipe manufacturing processes, this invention adds a rounding and straightening pretreatment process before pipe straightening. This removes small-radius initial bends in the pipe, ensuring that the bending radius of the pretreated pipe is greater than the minimum bending radius that the straightening equipment can straighten. This results in less straightening marks on the outer surface of the straightened pipe, lower residual stress, higher surface quality, better straightness, and less impact on subsequent eddy current testing. Combined with subsequent precision straightening and polishing processes, this allows for more effective control over the dimensional tolerances and straightness of the pipe.

[0021] Advantages of this invention:

[0022] This invention addresses the problems of the prior art by employing a precision rolling + rounding and straightening pretreatment + precision straightening + polishing process to produce small-sized, high-precision, thin-walled GH738 pipes with higher dimensional accuracy, higher eddy current testing pass rate, and significantly increased pipe yield. The manufacturing process of this invention is simple, highly repeatable, suitable for large-scale industrial production, meets the technical requirements for hot air engine pipes, and has broad application value and significant economic benefits. Attached Figure Description

[0023] Figure 1 Flowchart of the preparation process of this invention;

[0024] Figure 2 A schematic diagram of the pre-treatment mold structure for rounding and straightening according to the present invention;

[0025] Figure 3 Eddy current detection noise signal of Φ5 (outer diameter) × 0.75mm (wall thickness) GH738 seamless tube in Embodiment 1 of the present invention;

[0026] Figure 4 Eddy current detection noise signal of Φ6 (outer diameter) × 1.0mm (wall thickness) GH738 seamless tube in Embodiment 2 of the present invention;

[0027] Figure 5 The present invention describes the eddy current detection noise signal of the Φ5 (outer diameter) × 0.75mm (wall thickness) GH738 seamless tube in Comparative Example 1. Detailed Implementation

[0028] To make the technical means, creative features, objectives, and effects of this invention easier to understand, the invention is further described below in conjunction with specific embodiments. Figure 1 The process flow of the implementation method is shown. Figure 2 The pre-treatment mold structure for rounding and straightening is adopted.

[0029] Example 1

[0030] The method for preparing small-diameter, high-precision, thin-walled GH738 tubing in this embodiment includes the following steps:

[0031] Step 1: The GH738 nickel-based alloy ingot, which is made by vacuum induction melting and electroslag remelting, is hot rolled and polished to obtain GH738 bar billet with a diameter of φ32mm. Its main chemical composition is shown in Table 2.

[0032] Table 2 Chemical composition of GH738 bar billet with a diameter of φ32mm

[0033] C Cr Ni Co Mo Al Ti Zr 0.037 19.53 Remain 13.5 4.32 1.41 3.20 0.053 Fe B Mn Si P S Cu 0.32 0.006 ＜0.02 0.039 ＜0.005 ＜0.0005 ＜0.02

[0034] Step 2: The GH738 bar obtained in Step 1 is sawn to a length of 100-150mm and then processed into GH738 tube blanks with an outer diameter of φ30 × 3.0mm (wall thickness) by machining or other methods. The inner and outer surface roughness is less than 1.6 and the coaxiality is less than 0.1mm.

[0035] Step 3: The tube blank obtained in Step 2 is subjected to multiple cold rolling processes, vacuum annealing and recrystallization heat treatment of the semi-finished tube, straightening and polishing processes to obtain a semi-finished GH738 tube with a diameter of φ5 + 0.05 (outer diameter) × 0.75 mm. The deformation amount of each cold rolling process is 15%, the rolling feed is 1.5 mm, and the rolling speed is 50-80 times / minute. The annealing and recrystallization heat treatment temperature of the semi-finished tube is 1020℃-1080℃, and the holding time is 0.5-0.7 hours. The straightness of the tube after straightening is less than 1.5 mm / m.

[0036] Step 4: After cleaning the semi-finished GH738 pipe, perform a vacuum heat treatment process to adjust the structure and properties of the pipe; the vacuum heat treatment temperature is 1040℃ and the temperature is maintained for 20 minutes.

[0037] Step 5: Perform rounding and straightening pretreatment on the pipe obtained in Step 4. Custom-made tooling molds are used, and the pipe is rounded and straightened using a drawing method. The tooling mold hole diameter is B = φ5 ± 0.01 mm, angle α is 20°, and angle β is 60°.

[0038] Step Six: The finished pipe is then straightened using a multi-roller straightener, followed by a polishing process. The polishing diameter removal is 0.05mm to meet the dimensional tolerance and surface roughness requirements. The resulting small-sized, high-precision, thin-walled GH738 pipe has an outer diameter tolerance of φ5 of -0.01 to -0.03mm, a wall thickness of 0.75mm, a wall thickness tolerance of ±0.05mm, a straightness of <0.3mm / m, a dimensional pass rate of 90%, and a maximum eddy current noise signal value of 26mV. Figure 3 As shown, the eddy current noise is low, the pass rate is high at 95%, and the yield is 65%. Pipe samples were subjected to standard aging heat treatment (840℃ / 24h air cooling, 760℃ / 16h air cooling), and the high-temperature tensile strength of the pipe was measured at 815℃ to be 732MPa.

[0039] Example 2

[0040] The method for preparing small-diameter, high-precision, thin-walled GH738 tubing in this embodiment includes the following steps:

[0041] Step 1: The GH738 nickel-based alloy ingot, which was obtained by vacuum induction melting and vacuum consumable remelting, is hot rolled and polished to obtain GH738 bar billet with a diameter of φ30mm. Its chemical composition is shown in Table 3.

[0042] Table 3 Chemical composition of GH738 bar billet with a diameter of φ30mm

[0043] C Cr Ni Co Mo Al Ti Zr 0.045 19.3 Remain 13.1 4.31 1.51 3.07 0.060 Fe B Mn Si P S Cu 0.90 0.004 0.02 0.091 ＜0.005 ＜0.0005 ＜0.02

[0044] Step 2: Cut the GH738 bar obtained in Step 1 into a length of 100-150mm and process it into a GH738 tube blank with an outer diameter of φ28 × 2.5mm (wall thickness) by machining. The inner and outer surface roughness is less than 1.6 and the coaxiality is less than 0.1mm.

[0045] Step 3: The tube blank obtained in Step 2 is subjected to multiple cold rolling processes, vacuum annealing and recrystallization heat treatment of the semi-finished tube, straightening and polishing processes to obtain a semi-finished GH738 tube with a diameter of φ6 + 0.06 (outer diameter) × 1.0 mm. The deformation of each cold rolling process is 25%, the rolling feed is 3 mm, and the rolling speed is 50-80 times / minute. The vacuum annealing and recrystallization heat treatment temperature of the semi-finished tube is 1020℃-1080℃, and the holding time is 0.5-0.7 hours. The straightness of the tube after straightening is less than 1.5 mm / m.

[0046] Step 4: After cleaning the semi-finished GH738 pipe, perform a vacuum heat treatment process to adjust the structure and properties of the pipe; the vacuum heat treatment temperature is 1080℃ and the temperature is maintained for 30 minutes.

[0047] Step 5: Perform a rounding and straightening pretreatment on the pipe obtained in Step 4. This means using the rounding and straightening pretreatment process to remove pipes with a bending radius smaller than the minimum allowable radius of the straightening equipment. A custom-made tooling mold is used, and the rounding and drawing method is employed to perform the straightening pretreatment on the pipe. The tooling mold aperture size is B = φ6 ± 0.01 mm, angle α is 26°, and angle β is 70°.

[0048] Step Six: The finished pipe is then straightened using a multi-roller straightener, followed by a polishing process. The polishing diameter removal is 0.06mm to meet the dimensional tolerance and surface roughness requirements. The resulting small-sized, high-precision, thin-walled GH738 pipe has an outer diameter tolerance of -0.01 to -0.03mm, a wall thickness of 1.0mm, a wall thickness tolerance of ±0.05mm, a straightness of <0.3mm / m, a dimensional pass rate of 92%, and a maximum eddy current noise of 23mV. Figure 4 As shown, the eddy current noise signal is low, the pass rate is high at 97%, and the yield is 70%. Pipe samples were subjected to standard aging heat treatment (840℃ / 24h air cooling, 760℃ / 16h air cooling), and the high-temperature tensile strength of the pipe was measured at 815℃ to be 745MPa.

[0049] Comparative Example 1

[0050] The preparation method of this comparative example of small-diameter, high-precision, thin-walled GH738 pipe is the same as that in Example 1, except that the pre-treatment of rounding and straightening in step five is omitted, and step six is ​​performed directly. The finished pipe is straightened using a multi-roller straightener, followed by a polishing process. The polishing diameter removal is 0.03–0.05 mm, meeting the requirements for dimensional tolerance and surface roughness. The resulting small-diameter, high-precision, thin-walled GH738 pipe has an outer diameter tolerance of φ5 of -0.01 to -0.03 mm, a wall thickness of 0.75 mm, a wall thickness tolerance of ±0.05 mm, a straightness of less than 0.5 mm / m, a dimensional pass rate of 80%, and a maximum eddy current noise signal value of 87 mV. Figure 5 As shown, the eddy current noise signal is slightly high, and the eddy current interference signal is large in some areas of the pipe. The pass rate is slightly low at 79%, and the yield is 40%. After the pipe sample is subjected to standard aging heat treatment (840℃ / 24h air cooling, 760℃ / 16h air cooling), the high-temperature tensile strength of the pipe is measured to be 731MPa at 815℃.

[0051] The yield and grain size data of the small-diameter, high-precision, thin-walled GH738 pipe prepared in this comparative example are shown in Table 4.

[0052] Table 4. Performance test comparison data of the small-diameter, high-precision, thin-walled GH738 pipes prepared according to the present invention.

[0053]

[0054]

[0055] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

[0056] Matters not covered in this invention are common knowledge.

[0057] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their likenesses.

Claims

1. A method for manufacturing small-diameter, high-precision, thin-walled GH738 pipes, characterized in that: Includes the following steps: Step 1: Hot-roll and polish the GH738 nickel-based alloy ingots obtained by vacuum induction melting → electroslag remelting or vacuum arc remelting to obtain GH738 bar billets. Step 2: The GH738 bar obtained in Step 1 is sawed and cut into blanks, and then processed into GH738 tube blanks by machining. Step 3: The tube blank obtained in Step 2 is subjected to multiple cold rolling processes, annealing and recrystallization heat treatment of the semi-finished tube, straightening and polishing processes to obtain the semi-finished GH738 tube; among them, the annealing heat treatment of the semi-finished tube is carried out in a vacuum quenching furnace, the cooling gas is argon, the heating temperature is 1020~1080℃ and the holding time is 20~60 minutes, and the argon gas is used for rapid cooling. Step 4: After cleaning the semi-finished GH738 pipe, perform a vacuum heat treatment process to adjust the structure and properties of the pipe. The annealing heat treatment of the finished pipe uses a vacuum quenching furnace with argon as the cooling gas. The heating temperature is 1020-1080℃ and the holding time is 20-60 minutes, followed by rapid cooling with argon. Step 5: Perform pre-treatment on the pipes obtained in Step 4 to achieve roundness and straightness; Step Six: Then, the finished pipe is straightened using a multi-roller straightening machine, followed by a polishing process to meet the dimensional tolerance and surface roughness requirements, resulting in a small-diameter, high-precision, thin-walled GH738 pipe. The small-diameter, high-precision, thin-walled GH738 pipe has an outer diameter of 5~8mm, an outer diameter tolerance of -0.01~-0.03mm, a wall thickness of 0.5~1.0mm, a wall thickness tolerance of ±0.05mm, and a straightness of <0.3mm / m. In step five, tooling molds are customized according to the diameter of the finished heat-treated pipe. The pipe is pre-treated by drawing to achieve roundness and straightness. The bending radius of the pipe after roundness and straightness pre-treatment is greater than the minimum bending radius allowed by the straightening equipment. At the same time, the diameter of the mold for roundness and straightness pre-treatment is 0.03~0.05mm smaller than the diameter of the finished pipe, and the ovality of the hole is <0.5%.

2. The method for manufacturing small-diameter, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In step two, the machined tube blank has an inner and outer surface roughness of less than 1.6 and a coaxiality of less than 0.1 mm.

3. The method for manufacturing small-diameter, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In step three, the deformation per pass of the multi-pass cold rolling is 15%~30%, the rolling feed is 1~3mm, and the rolling speed is 50~80 times / minute; the vacuum annealing and recrystallization heat treatment temperature of the semi-finished pipe is 1020℃~1080℃, and the holding time is 0.5t hours, where t is the pipe wall thickness; the straightness of the pipe after straightening is less than 1.5mm / m.

4. The method for manufacturing small-diameter, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In step four, the vacuum heat treatment temperature of the finished pipe is 1020~1080℃; the heat treatment time is 0.5t hours, where t is the pipe wall thickness.

5. The method for manufacturing small-diameter, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In step six, the straightness of the finished pipe after straightening should be <0.02mm / 100mm, and the ellipticity of the pipe should be <0.5%; the diameter removal amount of the polishing process is 0.03~0.10mm.

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