Preparation method of small-caliber high-precision thin-wall GH738 pipe

Abstract

The invention relates to the technical field of pipe preparation, and particularly provides a small-caliber high-precision thin-wall GH738 pipe preparation method which comprises the following steps: step 1, obtaining a GH738 bar blank; secondly, the bar is machined into a GH738 pipe blank; 3, a semi-finished product GH738 pipe is obtained; 4, after the semi-finished GH738 pipe is cleaned, a finished product vacuum heat treatment procedure is conducted, and the structure and performance of the pipe are adjusted; step 5, rounding and straightening pretreatment; and 6, the small-size high-precision thin-wall GH738 pipe is obtained. The outer diameter is 5-8 mm, the outer diameter tolerance is-0.01--0.03 mm, the wall thickness is 0.5-1.0 mm, the wall thickness tolerance is + / -0.05 mm, and the straightness is smaller than 0.3 mm / m. The method has the advantages that the prepared small-size high-precision thin-wall GH738 pipe is higher in size precision, better in straightness, lower in surface residual stress and higher in eddy current flaw detection qualification rate, and the pipe yield is remarkably increased. And the preparation process is simple, has strong repeatability, is suitable for industrial large-scale production, and has wide popularization value and remarkable economic benefits.

Description

Technical Field

[0001] The present invention relates to the technical field of high-temperature alloy pipe preparation, and in particular to a method for preparing a small-caliber, high-precision, thin-walled GH738 pipe with a diameter of ≤6 mm and a wall thickness of ≤1 mm. Background Art

[0002] GH738 alloy is a high-performance nickel-based superalloy strengthened by γ' precipitation. It exhibits excellent high-temperature strength, creep resistance, thermal fatigue resistance, and oxidation resistance. Its chemical composition is shown in Table 1. Due to its excellent properties, it is widely used in the aerospace, marine, petrochemical, energy, and automotive sectors. In aircraft engines, this alloy is processed into numerous 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 outstanding high-temperature performance, corrosion resistance, and fatigue resistance, GH738 alloy is also processed into a key structural component—seamless pipe. This seamless pipe is typically used in high-temperature, high-pressure, and corrosive environments as a crucial conduit for conducting working media, such as the GH738 pipes used in steam turbines. Because these pipes are often brazed into equipment components, the gaps between the welded parts, the brazing filler metal, and the pipe must be strictly controlled to ensure weld quality. Consequently, these pipes require high dimensional accuracy, straightness, and ovality. For example, the outer diameter tolerance of some φ6x1.0 pipes must reach 0.02mm, the wall thickness tolerance must be ±0.05mm, and the straightness must be less than 0.5mm / m.

[0006] At present, traditional pipe preparation methods often face problems such as low dimensional accuracy, poor straightness, and low eddy current testing pass rate when preparing small-diameter, high-precision, thin-walled GH738 pipes, resulting in a generally low yield rate. The main reasons for this problem include: 1. Difficulty in rolling. The GH738 material has a high yield strength and is prone to rebound after deformation, making rolling difficult and difficult to control the size of the finished product. 2. Difficulty in straightening. The GH738 has a high resistance to deformation during rolling, is difficult to deform, and has large residual stress. After heat treatment, when the residual stress is released, the pipe is very likely to bend with a small curvature radius. According to the principle of pipe straightening, the straightening effect of the pipe is affected by the straightening process parameters such as the original bending curvature, the pitch of the straightening rollers, and the lead. When using a multi-roller straightening machine to straighten the pipe, at least three reverse bending deformations need to be completed in one spiral phase. Therefore, the spacing P between two adjacent rollers is usually an odd multiple of half the lead t, that is, P = k(t / 2), where k is an odd number. If the original bend radius of the pipe is too small, the mechanical limitations of the rotary multi-roller straightening machine prevent it from applying sufficient reverse bending deformation. Consequently, the deformation is insufficient to break the material's yield strength, resulting in an irreversible bend, commonly known in engineering production as a "dead bend." This type of bend not only seriously affects the straightness of the pipe but also makes polishing difficult, affecting the dimensional accuracy of the finished product. The residual bend can also affect the assembly accuracy of pipes (such as hydraulic and structural pipes) or fluid transmission efficiency. Third, it is prone to appearing as a defect signal during eddy current testing, resulting in low eddy current testing pass rates and yield rates, and resulting in material waste. Long, small-diameter, thin-walled pipes have poor rigidity, large deflection, and are prone to bending. Straightening pipes with small curvature radii can easily cause localized spiral straightening marks. These straightening marks are spiral grooves that, on the one hand, produce uneven lift-off between the eddy current probe and the pipe surface, causing false defect signals during eddy current testing. On the other hand, to ensure pipe straightness, multiple straightening passes are often used when straightening is unsatisfactory. Excessive straightening passes can create uneven residual stress on the outer surface of GH738 pipes. This piezoresistive effect can lead to uneven electrical conductivity in these areas, increasing noise during eddy current testing. Severe straightening marks can lead to false alarms, lowering the eddy current testing pass rate and causing material waste. Summary of the Invention

[0007] To address the shortcomings of current technology, the present invention provides a method for preparing small-diameter, high-precision, thin-walled GH738 pipes. The GH738 thin-walled pipes prepared by this method have high dimensional accuracy, good straightness, low eddy current detection noise, high flaw detection pass rate, high yield rate, and a grain size grade of 4 to 5. These methods meet the technical requirements of high precision, high yield rate, and high structural stability for GH738 nickel alloy pipes used in the field of thermal engines, and have broad promotion value and significant economic benefits.

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

[0009] Step 1: hot rolling and polishing the GH738 nickel-based alloy ingot obtained by vacuum induction melting, electroslag remelting or vacuum consumable remelting to obtain GH738 bar blanks;

[0010] Step 2: sawing the GH738 bar obtained in step 1, and processing the bar into GH738 tube blanks by machining;

[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 a semi-finished GH738 tube; wherein the semi-finished tube annealing heat treatment adopts a vacuum gas quenching furnace, the cooling gas is argon, the heating temperature is 1020-1080°C, the heat is maintained for 20-60 minutes, and the argon is rapidly cooled;

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

[0013] Step 5: Perform rounding and straightening pretreatment on the pipe obtained in step 4;

[0014] Step 6: The finished pipe is then straightened using a multi-roll straightening machine and then polished to meet the dimensional tolerance and surface roughness requirements of the finished product, resulting in a small-sized, high-precision, thin-walled GH738 pipe. The small-sized, high-precision, thin-walled GH738 pipe has an outer diameter of 5-8mm, an outer diameter tolerance of -0.01 to -0.03mm, a wall thickness of 0.5-1.0mm, a wall thickness tolerance of ±0.05mm, and a straightness of <0.5mm / m.

[0015] After the tube blank is machined in step 2, the inner and outer surface roughness is less than 1.6, and the coaxiality is less than 0.1 mm.

[0016] In step 3, the deformation 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° C. to 1080° C., 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 4, the vacuum heat treatment temperature of the finished pipe is 1020-1080° C.; the holding time is 0.5t hours, where t is the pipe wall thickness.

[0018] In step five, a custom tooling die is created based on the diameter of the finished heat-treated pipe. The pipe is then pre-straightened using a drawing process to achieve just slight plastic deformation on the outer surface, without significant work hardening. The bending radius of the pipe after pre-straightening is greater than the minimum allowed by the straightening equipment. Furthermore, the die diameter (B) for pre-straightening is equal to the finished pipe diameter (D) minus (0.03 to 0.10) mm, with an aperture ellipticity of less than 0.5%, an α angle of 6 to 30°, and a β angle of 60 to 90°.

[0019] In step 6, the straightness of the finished pipe after straightening should be less than 0.02mm / 100mm, and the ovality of the pipe should be less than 0.5%; the diameter removal amount of the polishing process is 0.03~0.10mm.

[0020] Compared to traditional pipe preparation processes, this method adds a rounding straightening pretreatment process before straightening. This eliminates the small initial bend radius of the pipe, ensuring that the bending radius of the pretreated pipe is larger than the minimum bend radius that the straightening equipment can straighten. This reduces straightening marks on the outer surface of the straightened pipe, reduces residual stress, improves outer surface quality, improves pipe straightness, and minimizes the impact on subsequent eddy current testing of the pipe. Combined with subsequent precision straightening and polishing processes, this method achieves more effective control of the pipe's dimensional tolerance and straightness.

[0021] Advantages of the present invention:

[0022] This invention addresses the challenges of the prior art by utilizing precision rolling, rounding and straightening pretreatment, precision straightening, and polishing to produce small, high-precision, thin-walled GH738 pipes with enhanced dimensional accuracy, a higher eddy current flaw detection pass rate, and a significantly increased pipe quality. The invention's simple and highly repeatable production process makes it suitable for large-scale industrial production, meeting the technical requirements for pipes used in hot gas turbines. It has broad application value and significant economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Preparation process flow chart of the present invention;

[0024] Figure 2 Schematic diagram of the rounding and straightening pretreatment die structure of the present invention;

[0025] Figure 3 The eddy current detection noise signal of the GH738 seamless pipe with a diameter of Φ5 (outer diameter) × 0.75 mm (wall thickness) in Example 1 of the present invention;

[0026] Figure 4 The eddy current detection noise signal of the GH738 seamless pipe with a diameter of Φ6 (outer diameter) × 1.0 mm (wall thickness) in Example 2 of the present invention;

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

[0028] In order to make the technical means, creative features, objectives and effects of the present invention easier to understand, the present invention will be further described below in conjunction with specific implementation methods. Figure 1 , showing the process flow of the implementation method, Figure 2 It is the rounding straightening pretreatment mold structure adopted.

[0029] Example 1

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

[0031] Step 1: hot rolling and polishing the vacuum induction melting → electroslag remelting GH738 nickel-based alloy ingot to obtain φ32mm GH738 bar blank, the main chemical composition of which is shown in Table 2;

[0032] Table 2 Chemical composition of GH738 bar blank with diameter 32 mm

[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: Saw and cut the GH738 rod obtained in step 1 into pieces with a length of 100-150 mm and process the rod into a GH738 tube blank with a diameter of φ30 (outer diameter) × 3.0 mm (wall thickness) by machining, etc., with an inner and outer surface roughness of less than 1.6 and a coaxiality of less than 0.1 mm.

[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 of φ5+0.05 (outer diameter)×0.75mm; the deformation amount of each cold rolling process is 15%, the rolling feed amount is 1.5mm, and the rolling speed is 50-80 times / min; the annealing and recrystallization heat treatment temperature of the semi-finished tube is 1020°C to 1080°C, and the holding time is 0.5-0.7 hours; the straightness of the tube after straightening is less than 1.5mm / m.

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

[0037] Step 5: Pre-process the tube obtained in Step 4 by straightening it back into a round shape. Customize the tooling and use the drawing method to pre-process the tube by straightening it back into a round shape. The tooling aperture size is B = φ5 ± 0.01 mm, the α angle is equal to 20°, and the β angle is equal to 60°.

[0038] Step 6: Then use a multi-roll straightening machine to straighten the finished pipe, and then perform a polishing process. The polishing diameter removal in the polishing process is 0.05mm, meeting the finished product size tolerance and surface roughness requirements. The obtained small-size high-precision thin-walled GH738 pipe φ5 has an outer diameter tolerance 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 qualification rate of 90%, and a maximum eddy current noise signal of 26mV. Figure 3 As shown, the eddy current noise is low, the inspection pass rate is high at 95%, and the yield rate is 65%. After taking the pipe sample and undergoing the standard aging heat treatment process (840℃ / 24h air cooling, 760℃ / 16h air cooling), the high-temperature tensile strength of the pipe was measured at 815℃ to be 732MPa.

[0039] Example 2

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

[0041] Step 1: hot rolling and polishing the GH738 nickel-based alloy ingot obtained by vacuum induction melting → vacuum consumable remelting to obtain 30 mm φ GH738 bar blanks, the chemical composition of which is shown in Table 3;

[0042] Table 3 Chemical composition of GH738 bar blank with diameter 30 mm

[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: Saw and cut the GH738 rod obtained in step 1 into pieces with a length of 100-150 mm and machine the rod into a GH738 tube blank with a diameter of φ28 (outer diameter) × 2.5 mm (wall thickness), with an inner and outer surface roughness of less than 1.6 and a coaxiality of less than 0.1 mm.

[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 of φ6+0.06 (outer diameter)×1.0 mm; the deformation amount of each cold rolling process is 25%, the rolling feed amount is 3 mm, and the rolling speed is 50 to 80 times / min; the vacuum annealing and recrystallization heat treatment temperature of the semi-finished tube is 1020° C. to 1080° C., and the holding time is 0.5 to 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 performance of the pipe; the vacuum heat treatment temperature of the finished product is 1080°C and the temperature is kept for 30 minutes.

[0047] Step 5: The tube obtained in Step 4 undergoes a rounding and straightening pretreatment. This process eliminates bends that fall below the minimum allowable radius of the straightening equipment. A custom tooling die is used to pre-straighten the tube using the rounding and drawing method. The tooling aperture dimensions are B = φ6 ± 0.01 mm, with an α angle of 26° and a β angle of 70°.

[0048] Step 6: Then use the multi-roll straightening machine to straighten the finished pipe, and then perform the polishing process. The polishing diameter removal in the polishing process is 0.06mm, which meets the finished product size tolerance and surface roughness requirements. The obtained small-size high-precision thin-walled GH738 pipe φ6 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 qualification rate of 92%, and a maximum eddy current noise of 23mV. Figure 4 As shown, the eddy current noise signal is low, the inspection pass rate is as high as 97%, and the yield rate is 70%. After taking the pipe sample and undergoing the standard aging heat treatment process (840℃ / 24h air cooling, 760℃ / 16h air cooling), the high-temperature tensile strength of the pipe was measured at 815℃ to be 745MPa.

[0049] Comparative Example 1

[0050] The preparation method of the small-diameter, high-precision, thin-walled GH738 pipe in this comparative example is the same as that in Example 1, except that the rounding and straightening pretreatment in step 5 is not performed, and step 6 is performed directly. The finished pipe is straightened using a multi-roll straightening machine, and then a polishing process is performed. The polishing process removes 0.03 to 0.05 mm of diameter, meeting the requirements of the finished product size tolerance and surface roughness. The obtained small-size, high-precision, thin-walled GH738 pipe has an outer diameter tolerance 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 qualification rate of 80%, and a maximum eddy current noise signal 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 inspection pass rate is slightly low at 79%, and the yield rate is 40%. After the pipe samples were subjected to a standard aging heat treatment process (840℃ / 24h air cooling, 760℃ / 16h air cooling), the high-temperature tensile strength of the pipe was measured at 815℃ to be 731MPa.

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

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

[0053]

[0054]

[0055] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any changes or modifications made in accordance with the spirit and essence of the present invention are intended to be included within the scope of protection of the present invention.

[0056] Matters not covered by the present invention are known technologies.

[0057] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and alterations may be made to the embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing small-caliber, high-precision, thin-walled GH738 pipes, characterized by: The following steps are involved: Step 1: hot rolling and polishing the GH738 nickel-based alloy ingot obtained by vacuum induction melting, electroslag remelting or vacuum consumable remelting to obtain GH738 bar blanks; Step 2: sawing the GH738 bar obtained in step 1, and processing the bar into GH738 tube blanks by mechanical processing; 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 a semi-finished GH738 tube; wherein the semi-finished tube annealing heat treatment adopts a vacuum gas quenching furnace, the cooling gas is argon, the heating temperature is 1020-1080°C, the heat is maintained for 20-60 minutes, and the argon is rapidly cooled; Step 4: After cleaning the semi-finished GH738 pipe, perform a vacuum heat treatment process to adjust the structure and performance of the pipe. The finished pipe is annealed in a vacuum quenching furnace with argon as the cooling gas. The heating temperature is 1020-1080℃ and the temperature is kept for 20-60 minutes, followed by rapid cooling with argon. Step 5: Perform rounding and straightening pretreatment on the pipe obtained in step 4; Step 6: The finished pipe is then straightened using a multi-roll straightening machine, and then polished to meet the finished product size tolerance and surface roughness requirements, thereby obtaining a small-size, high-precision, thin-walled GH738 pipe; the small-size, high-precision, thin-walled GH738 pipe has an outer diameter of 5 to 8 mm, an outer diameter tolerance of -0.01 to -0.03 mm, a wall thickness of 0.5 to 1.0 mm, a wall thickness tolerance of ±0.05 mm, and a straightness of <0.3 mm / m.

2. The method for preparing a small-sized, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In the step 2, the roughness of the inner and outer surfaces of the tube blank after machining is less than 1.6, and the coaxiality is less than 0.1 mm.

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

4. The method for preparing a small-size, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In the step 4, the vacuum heat treatment temperature of the finished pipe is 1020-1080° C.; the holding time is 0.5t hours, where t is the pipe wall thickness.

5. The method for preparing a small-size, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In the step 5, a tooling die is customized according to the diameter of the finished pipe after heat treatment, and the pipe is pre-treated by rounding and straightening by drawing. The bending radius of the pipe after rounding and straightening pre-treatment is greater than the minimum bending radius allowed by the straightening equipment; at the same time, the diameter of the die for rounding and straightening pre-treatment is 0.03 to 0.05 mm smaller than the diameter of the finished pipe, and the aperture ellipticity is less than 0.5%.

6. The method for preparing a small-sized, high-precision, thin-walled GH738 pipe according to claim 1, characterized in that: In step 6, the straightness of the finished pipe after straightening should be less than 0.02 mm / 100 mm, and the ovality of the pipe should be less than 0.5%. The diameter removal amount in the polishing process is 0.03 to 0.10 mm.

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