625 alloy tube for a tower type photo-thermal power plant heat absorber

Abstract

The application discloses a 625 alloy pipe for a heat absorber of a tower type photo-thermal power station, which contains the following elements in percentage by mass: Cr=21.0-23.0%, Mo=8.5-10.0%, Nb+Ta=3.50-4.15%, Fe=1-5%, C=0.01-0.03%, N<=0.015%, P<=0.01%, H<=0.008%, O<=0.003%, S<=0.003%, and the rest is Ni and inevitable impurity elements. The 625 alloy seamless precision welded pipe prepared by the application has a single austenite structure, no continuous or semi-continuous net-shaped carbide is precipitated on the grain boundary, no strip-shaped structure appears in the grain, and fine dispersed carbide and gamma prime phase are precipitated in the grain. The structure state has stability at a working condition of 565-650 DEG C, and can ensure excellent comprehensive mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of high-performance special alloy material manufacturing technology, specifically involving a 625 alloy tube for use in the absorber of a tower solar thermal power plant. Background Technology

[0002] Concentrated solar power (CSP) is a reliable and clean energy source, characterized by safety, cleanliness, zero emissions, and abundant resources. Furthermore, CSP power generation integrates peak shaving, energy storage, and power generation, offering advantages that other clean energy sources cannot replace. As of the end of 2023, my country's cumulative installed capacity of solar thermal power generation reached 588MW, accounting for 7.8% of the global total. There are 43 CSP projects under construction in my country. Among the operating and under-construction CSP plants in my country, tower-type CSP plants account for over 60%.

[0003] One of the core pieces of equipment in a tower-type concentrated solar power (CSP) plant is the receiver at the top of the tower, and the core component of the receiver is N06625 nickel-based alloy welded pipe. This welded pipe has a normal operating temperature range of 290-565℃, a maximum operating temperature of 650℃, and a minimum non-operating temperature of -20℃. The working medium is a highly corrosive binary molten salt of sodium nitrate and potassium nitrate, and the operating pressure is 2MPa. Under these operating conditions, if the pipe ruptures, the high-temperature molten salt will leak, causing the entire power plant to shut down and potentially leading to dangerous accidents such as fires and explosions. The 625 alloy welded pipe operates for extended periods in these harsh environments of high temperature, high pressure, and strong corrosion, placing high demands on the pipe's chemical composition, purity, microstructure, mechanical properties, and surface quality. Furthermore, the 625 alloy welded pipe must be welded to the upper and lower headers during installation, requiring high dimensional accuracy. Summary of the Invention

[0004] The purpose of this invention is to overcome at least one deficiency of the prior art and to provide a 625 alloy tube for a tower-type solar thermal power plant receiver.

[0005] The technical solution adopted in this invention is:

[0006] This invention provides a 625 alloy tube for a tower-type solar thermal power plant absorber. The 625 alloy tube, by mass percentage, contains the following elemental composition: Cr = 21.0–23.0%, Mo = 8.5–10.0%, Nb+Ta = 3.50–4.15%, Fe = 1–5%, C = 0.01–0.03%, N ≤ 0.015%, P ≤ 0.01%, H ≤ 0.008%, O ≤ 0.003%, S ≤ 0.003%, with the remainder being Ni and unavoidable impurity elements.

[0007] In some instances, the composition of the 625 alloy tubing, by weight percentage, also includes the following elements: Co≤1.0%, Si≤0.50%, Mn≤0.50%, Al≤0.40%, Ti≤0.40%.

[0008] In some examples, the 625 alloy tubing is composed of: Cr = 21.0–23.0%, Mo = 8.5–10.0%, Nb+Ta = 3.50–4.15%, Fe = 1–5%, C = 0.01–0.03%, N ≤ 0.015%, P ≤ 0.01%, H ≤ 0.008%, O ≤ 0.003%, S ≤ 0.003%, Co ≤ 1.0%, Si ≤ 0.50%, Mn ≤ 0.50%, Al ≤ 0.40%, Ti ≤ 0.40%, with the remainder being Ni and unavoidable impurity elements.

[0009] In some instances, the method for preparing the 625 alloy welded pipe includes the following steps:

[0010] 1) The 625 alloy was subjected to hot rolling and cold rolling processes in sequence to obtain 625 alloy strip;

[0011] 2) Weld the 625 alloy strip from step 1) into a tube, and then perform bright annealing to obtain a 625 alloy tube blank.

[0012] 3) The 625 alloy tube blank after the bright annealing treatment in step 2) is subjected to cold rolling;

[0013] 4) Degrease and clean the 625 alloy pipe after the cold deformation treatment in step 3);

[0014] 5) Perform a bright heat treatment on the 625 alloy finished tube after degreasing and cleaning in step 4).

[0015] 6) The 625 alloy finished tube after the final bright heat treatment in step 5) is straightened and its outer surface is polished.

[0016] 7) Perform surface treatment on the 625 alloy finished pipe obtained in step 6) to obtain the 625 alloy welded pipe.

[0017] In some instances, the bright annealing treatment in step 2) is performed at a temperature of 1050–1100°C for a holding time of 5–15 minutes.

[0018] In some instances, the deformation amount processed by the cold deformation process in step 3) ranges from 30% to 60%.

[0019] In some instances, the temperature of the bright heat treatment in step 5) is 950–1100°C, and the holding time is 5–15 min.

[0020] In some instances, the surface roughness Ra of the 625 alloy welded pipe after surface treatment in step 7) is ≤1.60μm.

[0021] The non-metallic inclusion rating of the 625 alloy pipe provided by this invention shall meet the following requirements: the coarse and fine inclusions of categories A, B, C, and D shall not exceed grade 1.0, and the sum of the coarse and fine inclusions of categories A, B, C, and D shall not exceed grade 4.0; nitride inclusions shall be rated with reference to categories B and D, the coarse inclusions of categories B and D shall not exceed grade 1.0, the fine inclusions of category B shall not exceed grade 1.5, and the fine inclusions of category D shall not exceed grade 2.0.

[0022] The 625 alloy pipe provided by this invention has a room temperature yield strength ≥414MPa, a room temperature tensile strength ≥827MPa, a room temperature elongation ≥35%, a 650℃ yield strength ≥210MPa, a 650℃ tensile strength ≥550MPa, and a 650℃ elongation ≥40%.

[0023] The 625 alloy pipe provided by this invention should have a base material grain size greater than 5.0, a weld structure of austenitic structure, a weld grain size of not less than 6.0, and no continuous or semi-continuous carbide precipitation at the grain boundaries.

[0024] The beneficial effects of this invention are:

[0025] This invention controls the carbon content to 0.01-0.03% and the chromium content to the middle limit, with a target range of 21.0-22.5%, to ensure that the alloy has good corrosion resistance and structural stability, while also taking into account certain mechanical properties.

[0026] This invention strictly controls the content of gaseous elements such as N, H, and O, which can significantly reduce the possibility of non-metallic inclusions and ensure good high-temperature performance and corrosion resistance while maintaining the high purity of the alloy.

[0027] The basic microstructure of the 625 alloy seamless precision welded pipe prepared by this invention is a single austenitic structure. There are no continuous or semi-continuous network carbide precipitations at the grain boundaries, no banded structure appears in the grains, and fine dispersed carbides and γ′ phase precipitation are present in the grains. This microstructure exhibits stable performance under operating conditions of 565-650℃ and can guarantee excellent comprehensive mechanical properties. Attached Figure Description

[0028] Figure 1 This refers to the microstructure of the weld seam of the welded pipe.

[0029] Figure 2 The microstructure and IPF image of the weld seam after annealing of cold-rolled welded pipe. Detailed Implementation

[0030] The present invention will now be described in detail with reference to embodiments, comparative examples, and experimental data.

[0031] For the target product of 625 alloy seamless precision welded pipe with specifications of Φ44.45*1.45mm, the specific implementation process is as follows. Other specifications of 625 alloy seamless precision welded pipe can be implemented with reference to this method, and the specific deformation process and heat treatment process can be adjusted accordingly.

[0032] Example 1

[0033] 1) Smelting and heat treatment

[0034] The raw materials are smelted using a vacuum induction and electroslag remelting method. The vacuum induction electrode is 430mm in diameter, the electroslag ingot is 550mm in shape, and the ingot weighs approximately 3 tons. The electroslag ingot undergoes homogenization heat treatment, with a holding time of at least 24 hours at 1200℃. After the holding time, it is forged. It is then hot-forged into a slab with a thickness of 150mm, with an initial forging temperature of at least 1100℃ and a final forging temperature of at least 800℃.

[0035] Two samples were taken from the hot-forged slab, one from each end of the ingot (equivalent to the head and tail), and their chemical composition was tested according to ASTM E2594. The test results are shown in Table 1 below, and all results meet the requirements for chemical composition control. Two samples were also taken from the hot-forged slab, one from each end of the electroslag ingot (equivalent to the head and tail), for longitudinal non-metallic inclusion testing. The non-metallic inclusion rating was performed according to ASTM E45 standard method A. The non-metallic inclusion rating results are shown in Table 2. The inclusions of fine series A, B, C, and D in the forged bar were all ≤0.5 grade, the nitrides of fine series B were ≤1.0 grade, and the nitrides of fine series D were ≤1.0 grade. The inclusion content was low and met the design requirements.

[0036] Table 1 Chemical composition of slab (wt.%)

[0037] head 0.015 0.001 0.09 0.02 0.006 21.7 60.5 1.40 tail 0.018 0.001 0.08 0.02 0.006 21.6 59.9 1.47 element Mo Al Co Ti Nb+Ta N H O head 8.86 0.10 0.04 0.10 3.72 0.005 0.0002 0.0025 tail 8.91 0.10 0.04 0.07 3.69 0.006 0.0003 0.0024

[0038] Table 2. Rating of Non-metallic Inclusions in Slabs

[0039]

[0040] 2) Cold-rolled strip

[0041] The cold-rolled strip has a thickness of 2.2 mm, with a thickness deviation of ±0.05 mm, and a width of 156.6 mm, with a width deviation of ±0.20 mm. The surface condition is 2B. The strip has a grain size of 6.0-7.0, is pure austenitic, and exhibits no carbide banding.

[0042] 3) Laser welding

[0043] The welding was performed using a fully automatic CNC laser welding machine with a welding power of 2400W and a welding speed of 3.0m / min. The protective atmosphere was high-purity argon with a purity of not less than 99.99%, with an external shielding gas flow rate of 10L / min and an internal shielding gas flow rate of 6L / min. The weld reinforcement was 0.10mm. The welded tube blank specifications were Ф50.8*2.2mm.

[0044] 4) Tube blank annealing

[0045] The tube blanks were annealed in a continuous bright annealing furnace at a temperature of 1100℃ and a holding time of 8 minutes.

[0046] 5) Final cold rolling

[0047] The LG60 type two-weld-seam rolling mill was used for cold rolling. The cold working deformation process is detailed in Table 3 below.

[0048] Table 3 Cold Working Deformation Process

[0049]

[0050] Sufficient lubrication must be maintained on both the inner and outer surfaces during rolling. The outer surface quality must be inspected for each pipe during rolling, and no rolling defects such as cracks, folds, rolling marks, or dents are permitted on either the inner or outer surfaces. The dimensions and surface quality of the pipes should be frequently checked during the rolling process. If any quality issues are found, the wear of the tools and dies must be promptly inspected.

[0051] Finished pipe size control requirements: outer diameter and tolerance 44.45±0.20mm, wall thickness and tolerance 1.45±0.12mm.

[0052] 6) Degreasing and cleaning

[0053] The finished cold-rolled tubes are degreased and cleaned using an ultrasonic degreasing and cleaning device. The degreasing solution is an alkaline cleaning solution with a concentration of not less than 3%. After cleaning, the tubes are inspected for cleanliness. The inner and outer surfaces of the tubes are clean, dry, and free of oil and residue.

[0054] 7) Final heat treatment

[0055] The final heat treatment after cold rolling and degreasing and cleaning adopts a protective atmosphere bright annealing process with an annealing temperature of 980℃ and a holding time of 10min. The protective atmosphere is hydrogen with a purity greater than 99.95%, which ensures that the finished pipe has high mechanical properties and that no oxidation or discoloration occurs on the inner and outer surfaces of the pipe, thus ensuring that the pipe has good surface quality.

[0056] 8) Finishing of finished products

[0057] Each finished tube after final heat treatment is straightened. After straightening, the surface of the finished tube must not have serious straightening defects such as dents, scratches, or indentations. Ensure that the curvature does not exceed 1.0 mm / m and the total curvature does not exceed 0.1% of the length.

[0058] 9) Non-destructive testing

[0059] Ultrasonic testing

[0060] Each finished tube after finishing undergoes ultrasonic testing. The blind zone at the tube end should not exceed 100mm, and this blind zone should be completely removed during the final cutting of the finished product.

[0061] The ultrasonic testing comparison tube has longitudinal and transverse grooves (rectangular) machined on both the inner and outer walls. Any two grooves should be separated to distinguish them and ensure that the echoes can be clearly distinguished. Groove dimensions: depth ≤ 0.10mm, width ≤ 0.20mm, length ≤ 12.7mm.

[0062] Acceptance criteria: If the echo signal of the defect in the inspected pipe is equal to or exceeds the height of the echo signal of any defect in the comparison sample pipe, then the pipe is unqualified.

[0063] hydrostatic test

[0064] Each finished pipe that passes the ultrasonic testing shall undergo a hydrostatic test. The hydrostatic test pressure shall be 4 MPa (gauge pressure), and the pressure holding time shall be no less than 10 seconds. No leakage is allowed. After the hydrostatic test, immediately dry the inner and outer surfaces of the pipe with clean, oil-free, and dry compressed air, and blow dry the inner surface of the pipe with a dry, clean white cloth strip to ensure that the inner surface of the pipe is completely dry.

[0065] 10) Finished product cutting

[0066] For pipes that pass the water pressure test, they shall be cut, have their ends flattened, and be deburred as required. The cut length shall be in accordance with the customer's drawings, with a length deviation of 0 to +5.0 mm.

[0067] 11) Dimensional and surface inspection

[0068] Each finished pipe after cleaning is inspected for size and surface.

[0069] The outer diameter was measured using an outside micrometer at the beginning, middle, and end, and the maximum and minimum values ​​were recorded. The wall thickness was measured using a wall thickness micrometer at the beginning and end, and the maximum and minimum values ​​were recorded. The length of the finished tube was measured using a tape measure and recorded. The curvature of the finished tube was measured using a straightness straightedge, and the maximum value was recorded. All measuring instruments must be calibrated and within their validity period. Upon inspection, the dimensions and deviations conform to the following specifications:

[0070] Outer diameter deviation: 44.5 ± 0.20 mm

[0071] Wall thickness deviation: 1.45±0.12mm

[0072] Length deviation: L(0∽+5.0)mm

[0073] Bending: The bending shall not exceed 1.0 mm / m, the bending of the whole length shall not exceed 0.1% of the length, and the bending within 200 mm of the pipe end shall not exceed 0.25 mm.

[0074] Ovality: not exceeding 0.32mm

[0075] Wall thickness unevenness: not exceeding 0.20mm

[0076] The surface quality of the finished pipes is inspected visually. The inner and outer surfaces of the pipes should be smooth, free from cracks, folds, twists, undercut, incomplete penetration, concave welds, and other defects that affect usability. These defects should be completely removed, and the actual wall thickness at the removed area should not be less than the minimum allowable wall thickness, with a smooth transition at the removal point. The inner and outer surfaces of the pipes should be bright and clean, free from oil stains, dust, water stains, fingerprints, oxidation discoloration, etc.

[0077] 12) Cleaning, labeling and packaging

[0078] The inner and outer surfaces of the finished pipes that have passed inspection shall be cleaned and their cleanliness checked. The inner and outer surfaces of the pipes shall be clean, dry, and free of rust, dust, and other contaminants. Each finished pipe shall be marked with an identification mark, which shall include the standard number, alloy grade, specifications, furnace number, batch number, and pipe number.

[0079] For pipes that pass the final inspection, each pipe is bagged and placed in a special wooden box. The pipes are required to be laid flat in a single layer inside the wooden box, with one end aligned, and each layer is separated by pearl cotton or foam to prevent bumps or scratches.

[0080] 13) Organization and performance testing

[0081] When the finished product is cut, samples are taken for testing according to the test items in Table 4. All performance indicators meet the ASTM B704 standard and design requirements.

[0082] Table 4 Organization and Performance Testing

[0083] 1 Finished product ingredients 1 piece / batch ASTM E2594, ASTM E1019, ASTM B880 2 Non-metallic inclusions 1 piece / batch ASTM E45 3 room temperature stretching 2 per batch ASTM E8 4 650℃ high temperature stretching 2 per batch ASTM E21 5 Grain size 1 piece / batch ASTM E3 ASTM E112 6 Flattening test 2 per batch ASME SB751 7 curling test 2 per batch ASME SB751

[0084] The chemical composition test results of the finished pipe are shown in Table 5, which meet the design requirements.

[0085] Table 5 Chemical composition of finished pipes (wt.%)

[0086]

[0087] The non-metallic inclusion test results of the finished pipe are shown in Table 6. They meet the design requirements, have high purity, and play an important role in improving the high temperature fatigue resistance and corrosion resistance.

[0088] Table 6. Rating of Non-metallic Inclusions in Finished Pipes

[0089]

[0090] The room temperature tensile properties of the finished tubes are shown in Table 7, which meet the steel grade 1 standard and the design requirements.

[0091] Table 7. Room temperature tensile properties of 625 alloy tubing

[0092]

[0093] The tensile properties of the finished pipe at 650℃ are shown in Table 8, which meet the design requirements. The high high-temperature strength ensures that the pipe has high safety and good fatigue resistance under the working conditions.

[0094] Table 8 High-Temperature Tensile Properties of 625 Alloy Tubes

[0095]

[0096] The finished pipe base material has a grain size of 7.0 grade, and the weld has a grain size of 9.0 grade. Both are austenitic structures with no continuous or semi-continuous carbide precipitation at the grain boundaries, which meets the design requirements.

[0097] The flattened and rolled samples showed no cracks and met the design requirements.

[0098] Example 2

[0099] 1) Smelting and heat treatment

[0100] The raw materials are smelted using a vacuum induction and electroslag remelting method. The vacuum induction electrode is 510mm in size, the electroslag ingot is 660mm in shape, and the ingot weighs approximately 5 tons. The electroslag ingot undergoes homogenization heat treatment, which involves holding at 1200℃ for 48 hours. After the homogenization heat treatment, the ingot is removed from the furnace and forged. It is then hot-forged into a slab with a thickness of 150mm. The initial forging temperature is not lower than 1100℃, and the final forging temperature is not lower than 800℃.

[0101] One sample was taken from each end of the hot-forged slab, corresponding to the head and tail of the steel ingot, and the chemical composition was tested according to ASTM E2594. The test results are shown in Table 9 below.

[0102] Table 9 Chemical composition of slab (wt%)

[0103] head 0.023 0.001 0.10 0.05 0.007 21.8 60.3 3.50 tail 0.025 0.001 0.11 0.05 0.008 21.5 60.1 3.60 element Mo Al Co Ti Nb+Ta N H O head 8.90 0.12 0.04 0.13 3.65 0.007 0.0002 0.0022 tail 9.10 0.12 0.04 0.11 3.73 0.008 0.0003 0.0025

[0104] Table 10 Rating of Non-metallic Inclusions in Slabs

[0105]

[0106] 2) Cold-rolled strip

[0107] The cold-rolled strip has a thickness of 2.5 mm, with a thickness deviation of ±0.10 mm, and a width of 196.5 mm, with a width deviation of ±0.20 mm. The surface condition is 2B. The strip has a grain size of 6.0-7.0, is pure austenitic, and exhibits no carbide banding.

[0108] 3) Laser welding

[0109] The welding was performed using a fully automatic CNC laser welding machine with a welding power of 2450W and a welding speed of 3.0m / min. The protective atmosphere was high-purity argon with a purity of not less than 99.99%, with an external shielding gas flow rate of 10L / min and an internal shielding gas flow rate of 8L / min. The weld reinforcement was 0.11mm. The welded tube blank specifications were Ф63.5*2.5mm.

[0110] 4) Tube blank annealing

[0111] The tube blanks were annealed in a continuous bright annealing furnace at a temperature of 1100℃ and a holding time of 10 min.

[0112] 5) Final cold rolling

[0113] The LG60 type two-weld-seam rolling mill was used for cold rolling. The cold working deformation process is detailed in Table 11 below.

[0114] Table 11 Cold Working Deformation Process

[0115]

[0116] Sufficient lubrication must be maintained on both the inner and outer surfaces during rolling. The outer surface quality must be inspected for each pipe during rolling, and no rolling defects such as cracks, folds, rolling marks, or dents are permitted on either the inner or outer surfaces. The dimensions and surface quality of the pipes should be frequently checked during the rolling process. If any quality issues are found, the wear of the tools and dies must be promptly inspected.

[0117] Finished pipe size control requirements: outer diameter and tolerance 44.45±0.20mm, wall thickness and tolerance 1.45±0.12mm.

[0118] 6) Degreasing and cleaning

[0119] The finished cold-rolled tubes are degreased and cleaned using an ultrasonic degreasing and cleaning device. The degreasing solution is an alkaline cleaning solution with a concentration of not less than 3%. After cleaning, the tubes are inspected for cleanliness. The inner and outer surfaces of the tubes are clean, dry, and free of oil and residue.

[0120] 7) Final heat treatment

[0121] The final heat treatment after cold rolling and degreasing and cleaning adopts a protective atmosphere bright annealing process with an annealing temperature of 1100℃ and a holding time of 10min. The protective atmosphere is hydrogen with a purity greater than 99.95%, which ensures that the finished pipe has high mechanical properties and that no oxidation or discoloration occurs on the inner and outer surfaces of the pipe, thus ensuring that the pipe has good surface quality.

[0122] 8) Finishing of finished products

[0123] The same finishing, non-destructive testing, finished product cutting, dimensional and surface inspection, cleaning, labeling and packaging, and tissue property testing were performed on this embodiment as on Example 1.

[0124] The chemical composition test results of the finished pipe are shown in Table 12, and they meet the design requirements.

[0125] Table 12 Chemical composition of finished pipes (wt.%)

[0126]

[0127] The results of non-metallic inclusion testing of the finished pipe are shown in Table 13. They meet the design requirements, have high purity, and play an important role in improving high-temperature fatigue resistance and corrosion resistance.

[0128] Table 13 Rating of Non-metallic Inclusions in Finished Pipes

[0129]

[0130] The room temperature tensile properties of the finished tube are shown in Table 14, which meet the requirements of steel grade 2 and conform to the design requirements of steel grade 2.

[0131] Table 14. Room Temperature Tensile Properties of 625 Alloy Tubes

[0132]

[0133] The tensile properties of the finished pipe at 650℃ are shown in Table 15, which meet the design requirements. The high high-temperature strength ensures that the pipe has high safety and good fatigue resistance under the working conditions.

[0134] Table 15. High-Temperature Tensile Properties of 625 Alloy Tubes

[0135]

[0136] The finished pipe base material has a grain size of 6.0 grade, and the weld has a grain size of 8.0 grade, both of which are austenitic structures with no continuous or semi-continuous carbide precipitation at the grain boundaries, which meets the design requirements.

[0137] The flattened and rolled samples showed no cracks and met the design requirements.

[0138] Figure 1 The image shows the microstructure of the weld seam in the welded state of the pipe. As can be seen from the image, the weld seam structure is characterized by coarse columnar crystals.

[0139] Figure 2 The image shows the microstructure and IPF (Integrated Photoforming Facility) of the weld seam after annealing of cold-rolled welded pipe. As can be seen from the image, rolling and heat treatment effectively refine the weld seam microstructure.

[0140] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions without departing from the concept of the present invention are all within the protection scope of the present invention.

Claims

1. A 625 alloy tube for use in a tower-type solar thermal power plant receiver, characterized in that, The 625 alloy pipe, by mass percentage, contains the following elemental composition: Cr = 21.0–23.0%, Mo = 8.5–10.0%, Nb+Ta = 3.50–4.15%, Fe = 1–5%, C = 0.01–0.03%, N ≤ 0.015%, P ≤ 0.01%, H ≤ 0.008%, O ≤ 0.003%, S ≤ 0.003%, Co ≤ 1.0%, Si ≤ 0.50%, Mn ≤ 0.50%, Al ≤ 0.40%, Ti ≤ 0.40%, with the remainder being Ni and unavoidable impurity elements. The base metal grain size of the 625 alloy pipe should be greater than grade 5.0, the weld microstructure should be austenitic with a weld grain size not less than grade 6.0, and there should be no continuous or semi-continuous carbide precipitation at the grain boundaries. The non-metallic inclusion rating should meet the following requirements: A The coarse and fine grades of categories A, B, C, and D do not exceed grade 1.0, and the sum of the coarse and fine grades of categories A, B, C, and D does not exceed grade 4.

0. Nitride inclusions are graded with reference to categories B and D. The coarse grades of categories B and D do not exceed grade 1.0, the fine grades of category B do not exceed grade 1.5, and the fine grades of category D do not exceed grade 2.

0. The 625 alloy is smelted by vacuum induction + electroslag remelting. The electroslag ingot is subjected to homogenization heat treatment. The homogenization heat treatment regime is 1200℃ and the holding time is not less than 24 hours. After the holding time, it is taken out of the furnace and hot forged into slabs. The initial forging temperature is not less than 1100℃ and the final forging temperature is not less than 800℃. The preparation method of the 625 alloy welded pipe includes the following steps: 1) Cold rolling is performed on 625 alloy to obtain 625 alloy strip; 2) Weld the 625 alloy strip from step 1) into a tube, and then perform bright annealing to obtain a 625 alloy tube blank. The bright annealing temperature is 1050~1100℃ and the holding time is 5~15min. 3) Perform cold deformation treatment on the 625 alloy tube blank after the bright annealing treatment in step 2); 4) Degrease and clean the 625 alloy pipe after the cold deformation treatment in step 3); 5) Perform bright heat treatment on the 625 alloy finished tube after degreasing and cleaning in step 4). The temperature of bright heat treatment is 950-1100℃ and the holding time is 5-15min. 6) The 625 alloy finished tube after the final bright heat treatment in step 5) is straightened and its outer surface is polished. 7) Perform surface treatment on the 625 alloy finished pipe obtained in step 6) to obtain the 625 alloy welded pipe.

2. The 625 alloy pipe according to claim 1, characterized in that, The deformation range of the cold deformation process in step 3) is 30-60%.

3. The 625 alloy pipe according to claim 1, characterized in that, The surface roughness Ra of the 625 alloy welded pipe after surface treatment in step 7) is ≤1.60μm.

4. The 625 alloy pipe according to claim 1, characterized in that, The 625 alloy pipe has a room temperature yield strength ≥414MPa, a room temperature tensile strength ≥827MPa, a room temperature elongation ≥30%, a 650℃ yield strength ≥210MPa, a 650℃ tensile strength ≥550MPa, and a 650℃ elongation ≥40%.

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