Stainless steel infiltrated layer pipe containing welded joint and preparation method of stainless steel infiltrated layer pipe

Abstract

The invention relates to a stainless steel infiltrated layer pipe containing a welded joint and a preparation method thereof, and belongs to the technical field of heat-resistant stainless steel. According to the preparation method, the pipe bodies are connected through welding; the welded pipe body is placed in the slurry to be soaked; the slurry comprises an aluminizing agent; the soaked pipe body is subjected to heat treatment, and a permeation layer is obtained on the inner wall of the pipe body; and the inner wall of the pipe body subjected to heat treatment is cleaned, and the stainless steel infiltrated layer pipe is obtained. The obtained stainless steel infiltrated layer pipe has high high-temperature steam oxidation resistance, the overall performance of the pipe body is improved, and the steam oxidation weight increment at 650 DEG C / 10000 h is smaller than or equal to 0.2 mg / cm < 2 >.

Description

Technical Field

[0001] This invention relates to the field of heat-resistant stainless steel technology, and in particular to a stainless steel diffusion-coated pipe with welded joints and its preparation method. Background Technology

[0002] Superheaters and reheaters in ultra-supercritical power plants consist of multiple rows of small-diameter stainless heat-resistant steel tubes, including straight and bent sections, welded together. S30432 stainless heat-resistant steel is a primary candidate material for superheaters and reheaters in ultra-supercritical units due to its excellent high-temperature performance and economic efficiency. However, given the frequent start-ups and shutdowns of thermal power units in modern power systems, seamless tubes made of S30432 stainless heat-resistant steel are prone to creep-fatigue-oxidation cracks under the combined effects of high temperature, stress, and steam corrosion, especially in the weak welded joints, severely impacting their service life.

[0003] To improve overall performance, the main approach is to optimize the composition ratio of S30432 stainless heat-resistant steel to increase material strength. Alternatively, shot peening of the inner wall can improve the seamless pipe's resistance to steam oxidation, thereby slowing down the oxidation rate and preventing cracks from further propagating into the matrix.

[0004] However, S30432 stainless heat-resistant steel may undergo martensitic transformation during service. Due to the significant difference in thermal expansion coefficients between the martensitic structure and the matrix (austenite), frequent temperature changes under deep peak-shaving conditions are more likely to lead to stress and crack initiation. Studies have shown that residual stress significantly accelerates stress-induced martensitic transformation, and existing processes such as pipe bending, shot peening, and welding generate a large amount of residual stress, for which there are currently no effective means to mitigate this.

[0005] Therefore, the present invention aims to provide a method that can simultaneously improve the strength and steam corrosion resistance of S30432 stainless steel pipes with welded joints in superheaters and reheaters of ultra-supercritical units. Compared with the traditional shot-peened pipe assembly welding process, it can significantly reduce the residual stress of the steel pipe and greatly improve its service life. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a stainless steel seepage-layered tube with welded joints and its preparation method, in order to solve the problems of insufficient steam oxidation resistance and easy cracking under deep peak shaving conditions of existing S30432 stainless steel tubes used in boiler superheaters / reheaters.

[0007] On one hand, the present invention provides a method for preparing a stainless steel diffusion-coated pipe containing a welded joint, comprising the following steps:

[0008] S1: Connect the pipe bodies by welding;

[0009] S2: Immerse the welded pipe in the slurry;

[0010] The slurry includes an aluminizing agent;

[0011] S3: After soaking, the tube body is heat-treated to obtain a seepage layer on the inner wall of the tube body;

[0012] S4: Clean the inner wall of the heat-treated tube to obtain a stainless steel coated tube.

[0013] Further, in step S1, the chemical composition of the stainless steel diffusion layer tube is as follows: C: 0.075-0.085%, Si: 0.20-0.25%, Mn: 0.7-0.9%, Cr: 17.5-18.0%, Ni: 8.5-9.0%, Mo: 0.20-0.60%, Nb: 0.40-0.45%, Cu: 3.0-3.5%, N: 0.08-0.10%, Al≤0.008%, B: 0.002-0.004%, with the balance being Fe.

[0014] Furthermore, in step S1, the pipe body includes a straight pipe and / or a bent pipe.

[0015] Further, in step S1, the composition of the welding material is: C: 0.08-0.10%, Si: 0.1-0.2%, Mn: 2.8-3.2%, Cr: 17.5-19.0%, Ni: 12.0-15.0%, Mo: 0.5-0.8%, Cu: 2.8-3.2%, Nb: 0.8-1.0%, N: 0.18-0.22%, Zr: 0.010-0.015%.

[0016] Furthermore, the aluminizing agent comprises: 90%–98% iron-aluminum powder; 1%–10% activator; and 0.2%–2% aluminizing agent.

[0017] Wherein, the iron and aluminum powder has a mesh size ≤ 300 mesh, the activator is NH4Cl or NH4F, and the penetration accelerator is La2O3 or Y2O3.

[0018] Furthermore, the slurry also includes silica water-based sol;

[0019] The weight ratio of the aluminizing agent to the silica water-based sol is (0.8-1.0):1.

[0020] Furthermore, in step S2, the thickness of the slurry on the inner wall of the tube after soaking is 0.5 to 2.0 mm.

[0021] Further, in step S3, the heating temperature of the heat treatment is 700-800°C, the heat treatment time is 3-5 hours, and then the furnace is cooled to room temperature at a rate not exceeding 120°C / h.

[0022] Further, in step S4, two cleaning cycles are performed. The first cleaning cycle uses an aqueous solution of 15-25 g / L sodium hydroxide and 5-7 g / L sodium gluconate, with a cleaning temperature of 60-70°C and a cleaning time of 8-11 min.

[0023] The second cleaning uses 5-8% ethylenediaminetetraacetic acid and 0.1-0.5% N2H4 corrosion inhibitor. The pH of the solution is adjusted to 8.5-9.5 using ammonia water, and the solution is circulated and rinsed at 60-80℃ for 15-30 minutes.

[0024] On the other hand, the present invention provides a stainless steel diffusion-coated tube containing a welded joint, obtained by the preparation method described in the present invention, wherein the stainless steel diffusion-coated tube exhibits a weight gain of ≤0.2 mg / cm³ during steam oxidation at 650℃ / 10000h. 2 .

[0025] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0026] 1. In this invention, pipe bodies are connected using welding technology. The welded pipe bodies are then immersed in a slurry containing an aluminizing agent, forming a slurry layer of a certain thickness on the inner wall of the pipe. During subsequent heat treatment, a diffusion layer is formed on the inner wall of the pipe. After cleaning, a stainless steel diffusion-coated pipe with welded joints is obtained. The resulting stainless steel diffusion-coated pipe exhibits high resistance to high-temperature steam oxidation, improving the overall performance of the pipe body. The weight gain during steam oxidation at 650℃ / 10000h is ≤0.2mg / cm³. 2 ;

[0027] 2. In this invention, adjusting the composition of the stainless steel diffusion tube and the welding material significantly improves the toughness and strength of the welded joint, which helps to improve the service life of the material under deep peak shaving conditions.

[0028] 3. This invention achieves a penetration layer on the inner wall of the pipe through heat treatment, replacing the traditional shot peening method. This results in stronger resistance to high-temperature steam oxidation and avoids the residual stress generated during shot peening, which accelerates stress-induced martensitic transformation. This improves the steel pipe's resistance to creep-fatigue cracking, preventing crack initiation and propagation, and reducing the risk of pipe rupture caused by oxide scale peeling. Furthermore, the heat treatment process eliminates or reduces residual stress at bends and welds, improving overall performance.

[0029] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0030] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0031] Figure 1 The image shows the microstructure and oxygen distribution of a stainless steel tube obtained using existing methods after steam oxidation at 600℃ for 10,000 hours.

[0032] Figure 2 The image shows the microstructure and oxygen distribution of the stainless steel diffusion-coated tube obtained in Example 1 after steam oxidation at 600°C for 10,000 hours. Detailed Implementation

[0033] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0034] Superheaters and reheaters in ultra-supercritical power plants consist of multiple rows of small-diameter stainless heat-resistant steel tubes, including straight and bent sections, welded together. S30432 stainless heat-resistant steel is a primary candidate material for superheaters and reheaters in ultra-supercritical units due to its excellent high-temperature performance and economic efficiency. However, given the frequent start-ups and shutdowns of thermal power units in modern power systems, seamless tubes made of S30432 stainless heat-resistant steel are prone to creep-fatigue oxidation cracks under the combined effects of high temperature, stress, and steam corrosion, especially in the weak welded joints, severely impacting their service life.

[0035] To improve overall performance, the main approach is to optimize the composition ratio of S30432 stainless heat-resistant steel to increase material strength. Alternatively, shot peening of the inner wall can improve the seamless pipe's resistance to steam oxidation, thereby slowing down the oxidation rate and preventing cracks from further propagating into the matrix.

[0036] However, S30432 stainless heat-resistant steel may undergo martensitic transformation during service. Due to the significant difference in thermal expansion coefficients between the martensitic structure and the matrix (austenite), frequent temperature changes under deep peak-shaving conditions are more likely to lead to stress and crack initiation. Studies have shown that residual stress significantly accelerates stress-induced martensitic transformation, and existing processes such as pipe bending, shot peening, and welding generate a large amount of residual stress, for which there are currently no effective means to mitigate this.

[0037] Therefore, the present invention provides a method for preparing a stainless steel diffusion-coated pipe containing a welded joint, comprising the following steps:

[0038] S1: Connect the pipe bodies by welding;

[0039] S2: Immerse the welded pipe in the slurry;

[0040] The slurry includes an aluminizing agent;

[0041] S3: After soaking, the tube body is heat-treated to obtain a seepage layer on the inner wall of the tube body;

[0042] S4: Clean the inner wall of the heat-treated tube to obtain a stainless steel coated tube.

[0043] Compared with existing technologies, this invention connects the pipe bodies using welding technology, then immerses the welded pipe bodies in a slurry containing an aluminizing agent, forming a slurry layer of a certain thickness on the inner wall of the pipe. During subsequent heat treatment, a diffusion layer is formed on the inner wall of the pipe. After cleaning, a stainless steel diffusion-coated pipe with welded joints is obtained. The resulting stainless steel diffusion-coated pipe exhibits high resistance to high-temperature steam oxidation, improving the overall performance of the pipe body. The weight gain during steam oxidation at 650℃ / 10000h is ≤0.2mg / cm³. 2 .

[0044] Specifically, in step S1, the pipe body includes a straight pipe and / or a bent pipe.

[0045] It should be noted that during the welding connection process, straight pipes can be welded to bends, straight pipes to straight pipes, and bends to bends.

[0046] In the existing process, pipe bends are usually made by cold bending of straight pipes. Tensile stress exists in the inner arc of the bend section. On the one hand, it may cause the propagation of the original microcracks in the matrix. On the other hand, it will also accelerate the stress-induced martensitic phase transformation and increase the probability of crack nucleation.

[0047] In addition, the shot peening and welding processes generate a large amount of residual stress, which can also accelerate the induction of martensitic phase transformation and increase the probability of crack nucleation.

[0048] This invention achieves a permeation layer on the inner wall of the pipe through heat treatment, replacing the traditional shot peening method. This results in stronger resistance to high-temperature steam oxidation and avoids the residual stress generated during shot peening, which accelerates stress-induced martensitic transformation. This improves the steel pipe's resistance to creep-fatigue cracking, preventing crack initiation and propagation, and reducing the risk of pipe rupture caused by oxide scale peeling. Furthermore, the heat treatment process eliminates or reduces residual stress at bends and welds, improving overall performance.

[0049] Specifically, in step S1, the chemical composition of the stainless steel diffusion layer tube is as follows: C: 0.075-0.085%, Si: 0.20-0.25%, Mn: 0.7-0.9%, Cr: 17.5-18.0%, Ni: 8.5-9.0%, Mo: 0.20-0.60%, Nb: 0.40-0.45%, Cu: 3.0-3.5%, N: 0.08-0.10%, Al≤0.008%, B: 0.002-0.004%, with the balance being Fe.

[0050] It should be noted that, in order to meet the performance requirements of seamless steel pipes under deep peak-shaving conditions, this invention optimizes the composition of the S30432 stainless steel diffusion-coated pipe within the standard range. On the one hand, the Cr content needs to be controlled at 17.5-18.0%, and the Nb / C ratio needs to be controlled at ≥4.5 to improve the stability of the austenitic matrix and reduce the risk of cracking due to martensitic phase transformation. Through surface diffusion, the seamless pipe can be effectively protected for a long time. On the other hand, 0.2-0.6% Mo is added to the matrix to improve the material strength. In addition, the Al content is required to be below 0.008% to avoid coarsening of the primary Nb(C,N) precipitate phase due to Al2O3 inclusions.

[0051] Specifically, in step S1, the composition of the welding material is: C: 0.08-0.10%, Si: 0.1-0.2%, Mn: 2.8-3.2%, Cr: 17.5-19.0%, Ni: 12-15%, Mo: 0.5-0.8%, Cu: 2.8-3.2%, Nb: 0.8-1.0%, N: 0.18-0.22%, Zr: 0.010-0.015%.

[0052] It should be noted that the optimized welding material composition, based on S30432 steel, increases the content of elements such as Ni, Mn, and N to expand the austenite phase region, ensuring that the weld is a fully austenitic structure. Simultaneously, a certain amount of Mo is added to improve strength, and 0.8-1.0% Nb is added to refine the weld solidification structure. Furthermore, 0.01-0.015% Zr is added to reduce the size of primary precipitated Nb (C,N), thereby improving the weld impact energy.

[0053] Specifically, the aluminizing agent comprises: 90%–98% iron-aluminum powder; 1%–10% activator; and 0.2%–2% aluminizing agent.

[0054] Wherein, the iron and aluminum powder has a mesh size ≤ 300 mesh, the activator is NH4Cl or NH4F, and the penetration accelerator is La2O3 or Y2O3.

[0055] It should be noted that the main reactions in the heat treatment process are shown in equations (1) to (5) (taking NH4Cl activator as an example). Using iron-aluminum powder as the main raw material for aluminizing significantly reduces the reactivity, resulting in a FeAl aluminizing layer with better plasticity and avoiding the formation of brittle FeAl3, Fe2Al5, and other aluminizing layers. The activator decomposes to produce hydrogen chloride or hydrogen fluoride, causing the aluminum in the iron-aluminum powder to become gaseous and deposited on the surface of the steel pipe. The aluminizing agent is used to improve the morphology of the aluminizing layer, forming a dense FeAl aluminizing layer and avoiding defects and cracks in the aluminizing layer.

[0056] Meanwhile, when the mesh size of the iron and aluminum powder is higher than 300 mesh, the effective reaction area after the slurry is coated is too low, the reaction rate is unstable, and discontinuous seepage layers appear.

[0057]

[0058] 2FeAl+6HCl→AlCl3+FeCl3+3H2 (2)

[0059] AlCl3 + 2Al → 3AlCl (3)

[0060] 3AlCl→2[Al]+AlCl3 (4)

[0061] [Al]+[Fe]→FeAl (5)

[0062] Specifically, the slurry also includes silica water-based sol; the ratio of the aluminizing agent to the silica water-based sol is (0.8-1.0):1.

[0063] It should be noted that silica water-based sol is a stable colloidal system formed by the uniform dispersion of nano-sized silica particles in water. It acts as a solvent to disperse the aluminizing agent, preventing rapid sedimentation and agglomeration in the slurry and ensuring uniform slurry composition. Simultaneously, the silica sol transforms into a robust SiO2 network during subsequent high-temperature diffusion processing, providing stable skeletal support.

[0064] Specifically, in step S2, the thickness of the slurry on the inner wall of the tube after soaking is 0.5 to 2.0 mm.

[0065] Specifically, in step S3, the heating temperature of the heat treatment is 700-800℃, the heat treatment time is 3-5h, and then the furnace is cooled to room temperature at a rate not exceeding 120℃ / h.

[0066] It should be noted that in this invention, the welded pipe body is immersed in a slurry containing an aluminizing agent, and then a layer of slurry with a thickness of 0.5 to 2.0 mm is attached to the inner wall of the pipe body. When heated at a temperature of 700 to 800°C, reactions (1) to (5) occur, forming a diffusion layer on the inner wall of the pipe body, thereby improving the resistance to high-temperature steam oxidation.

[0067] The heating temperature for heat treatment can be 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, or 800℃. When the heat treatment heating temperature is higher than 800℃, it causes coarsening of the tube grains, resulting in mixed crystals and thus reducing the tube's performance. When the heat treatment heating temperature is lower than 700℃, the aluminizing efficiency is too low, and an effective aluminized layer cannot be formed.

[0068] Furthermore, due to the difference in thermal expansion coefficients between the diffusion layer and the pipe body, it is necessary to furnace cool to room temperature at a rate not exceeding 120℃ / h. Cooling rates exceeding 120℃ / h can cause cracks in the diffusion layer, affecting its protective performance. Simultaneously, the heat treatment process can eliminate or reduce residual stress at bends and welds, improving overall performance.

[0069] Specifically, the thickness of the infiltrated layer is 3–10 μm.

[0070] Specifically, in step S4, two cleaning cycles are performed. The first cleaning cycle uses an aqueous solution of 15-25 g / L sodium hydroxide and 5-7 g / L sodium gluconate, with a cleaning temperature of 60-70°C and a cleaning time of 8-11 minutes.

[0071] The second cleaning uses 5-8% ethylenediaminetetraacetic acid and 0.1-0.5% N2H4 corrosion inhibitor. The pH of the solution is adjusted to 8.5-9.5 using ammonia water, and the solution is circulated and rinsed at 60-80℃ for 15-30 minutes.

[0072] It should be noted that the first flushing is mainly to remove the residual slurry from the inner wall of the pipe or stainless steel infiltration pipe, while the second flushing is to form a dense Al2O3 oxide layer on the inner surface of the pipe or stainless steel infiltration pipe through pre-oxidation, so as to avoid the rapid consumption of the FeAl infiltration layer in the early stage of service and extend the protection life.

[0073] To more clearly describe the present invention, the following embodiments and comparative examples are provided for further illustration.

[0074] Example 1

[0075] A method for preparing a stainless steel diffusion-coated pipe containing a welded joint includes the following steps:

[0076] S1: Connect straight pipes and bends by welding;

[0077] The composition of the straight and bent pipes is as follows: C: 0.075%, Si: 0.22%, Mn: 0.8%, Cr: 17.7%, Ni: 8.9%, Mo: 0.45%, Nb: 0.36%, Cu: 3.2%, N: 0.09%, Al: 0.006%, B: 0.003%, with the balance being Fe.

[0078] Welding material composition: C: 0.092%, Si: 0.18%, Mn: 3.0%, Cr: 17.7%, Ni: 14%, Mo: 0.7%, Cu: 3.0%, Nb: 0.9%, N: 0.2%, Zr: 0.012%.

[0079] S2: Immerse the welded pipe in the slurry; the thickness of the slurry adhering to the inner wall of the pipe is 0.5-2.0 mm;

[0080] The slurry consists of an aluminizing agent and a silica water-based sol in a weight ratio of 1:1.

[0081] The aluminizing agent consists of 95 wt% iron and aluminum powder, 4 wt% NH4Cl, and 1 wt% La2O3; with a mesh size of 200 mesh.

[0082] S3: Heat-treat the soaked tube at 750℃ for 4 hours; then cool it to room temperature in a furnace at a rate of 100℃ / h.

[0083] S4: Clean the inner wall of the heat-treated pipe to obtain a stainless steel coated pipe. Two cleaning processes are used. The first cleaning uses an aqueous solution of 18 g / L sodium hydroxide and 6 g / L sodium gluconate at 65℃ for 9 minutes. The second cleaning uses a solution of 5% ethylenediaminetetraacetic acid and 0.3% N2H4 corrosion inhibitor, with the pH adjusted to 9.2 using ammonia, and is circulated and rinsed at 60-80℃ for 20 minutes.

[0084] Example 2

[0085] The preparation process of Example 2 is largely the same as that of Example 1, except that the aluminizing agent in Example 2 includes 92 wt% iron and aluminum powder, 7 wt% NH4Cl, and 1 wt% La2O3.

[0086] Example 3

[0087] The preparation process of Example 3 is largely the same as that of Example 1, except that the heat treatment temperature in Example 3 is 720°C and the time is 3 hours.

[0088] Example 4

[0089] The preparation process of Example 4 is largely the same as that of Example 1, except that the aluminizing agent in Example 4 includes 92 wt% iron and aluminum powder, 7 wt% NH4F, and 1 wt% La2O3.

[0090] Example 5

[0091] The preparation process of Example 5 is largely the same as that of Example 1, except that the aluminizing agent in Example 5 includes 96 wt% iron and aluminum powder, 3 wt% NH4Cl, and 1 wt% Y2O3.

[0092] Comparative Example 1

[0093] The preparation process of Comparative Example 1 is largely the same as that of Example 1, except that the chemical composition of the straight and bent pipes in Comparative Example 1 is C: 0.075%, Si: 0.22%, Mn: 0.8%, Cr: 17.7%, Ni: 8.9%, Nb: 0.36%, Cu: 3.2%, N: 0.09%, Al: 0.006%, B: 0.003%, with the balance being Fe.

[0094] Comparative Example 2

[0095] The preparation process of Comparative Example 2 is largely the same as that of Example 1, except that the composition of the welding material in Comparative Example 2 is C: 0.092%, Si: 0.18%, Mn: 3.0%, Cr: 17.7%, Ni: 14%, Mo: 0.7%, Cu: 3.0%, Nb: 0.9%, N: 0.2%.

[0096] Comparative Example 3

[0097] The preparation process of Comparative Example 3 is largely the same as that of Example 1, except that the aluminizing agent of Comparative Example 3 includes 95 wt% aluminum powder, 4 wt% NH4Cl, and 1 wt% La2O3.

[0098] Comparative Example 4

[0099] The preparation process of Comparative Example 4 is largely the same as that of Example 1, except that the aluminizing agent of Comparative Example 4 includes 65 wt% iron-aluminum powder, 30 wt% aluminum powder, 4 wt% NH4Cl, and 1 wt% La2O3.

[0100] Comparative Example 5

[0101] The preparation process of Comparative Example 5 is largely the same as that of Example 1, except that the heat treatment temperature in Comparative Example 5 is 680°C and the time is 5 hours.

[0102] Comparative Example 6

[0103] The preparation process of Comparative Example 6 is largely the same as that of Example 1, except that the heat treatment temperature in Comparative Example 6 is 850°C and the time is 4 hours.

[0104] Comparative Example 7

[0105] The preparation process of Comparative Example 7 is largely the same as that of Example 1, except that Comparative Example 7 uses only 18 g / L sodium hydroxide and 6 g / L sodium gluconate aqueous solution, and performs a cleaning once at a temperature of 65°C for 9 minutes.

[0106] Performance testing

[0107] The above embodiments and comparative examples were subjected to performance testing, mainly including mechanical property testing and antioxidant property testing. The test results are shown in Table 1 and Table 2.

[0108] Table 1 Performance Test Results

[0109]

[0110]

[0111] *In Table 1, the matrix refers to the pipe body with a seepage layer.

[0112] Table 2 Results of Antioxidant Performance Test

[0113]

[0114] As can be seen from Examples 1-5 and Comparative Examples 1-7, and with reference to Tables 1 and 2, the stainless steel diffusion-coated pipe obtained by the preparation method provided in the embodiments of the present invention not only possesses high mechanical properties—with a tensile strength of the matrix exceeding 785 MPa and a yield strength exceeding 440 MPa; and a weld tensile strength exceeding 720 MPa and a yield strength exceeding 535 MPa, and an impact energy exceeding 98 J—but also exhibits uniform diffusion layer, high resistance to steam oxidation, a diffusion layer thickness of 3–10 μm, and a weight gain of ≤0.2 mg / cm³ under steam oxidation at 650℃ / 10000h. 2 .

[0115] Reference Figure 1 and Figure 2 ,in Figure 1 The stainless steel pipes are obtained using existing technology. They are first shot-peened and then the straight and bent pipes are welded together. Figure 2 To obtain a stainless steel diffusion layer pipe by means of the method described in Example 1.

[0116] from Figure 1 As can be seen, cracks and oxide scale formed between the weld and the substrate, indicating poor resistance to high-temperature stress-vapor corrosion. From... Figure 2As can be seen, a permeation layer with a thickness of 3 to 10 μm is basically formed, and the steam oxidation performance is relatively high.

[0117] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a stainless steel diffusion-coated pipe containing a welded joint, characterized in that, Includes the following steps: S1: Connect the pipe bodies by welding; S2: Immerse the welded pipe in the slurry; The slurry includes an aluminizing agent; S3: After soaking, the tube body is heat-treated to obtain a seepage layer on the inner wall of the tube body; S4: Clean the inner wall of the heat-treated tube to obtain a stainless steel coated tube.

2. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, In step S1, the chemical composition of the stainless steel diffusion layer tube is as follows: C: 0.075-0.085%, Si: 0.20-0.25%, Mn: 0.7-0.9%, Cr: 17.5-18.0%, Ni: 8.5-9.0%, Mo: 0.20-0.60%, Nb: 0.40-0.45%, Cu: 3.0-3.5%, N: 0.08-0.10%, Al≤0.008%, B: 0.002-0.004%, with the balance being Fe.

3. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, In step S1, the pipe body includes straight pipes and / or bends.

4. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, In step S1, the composition of the welding material is: C: 0.08-0.10%, Si: 0.1-0.2%, Mn: 2.8-3.2%, Cr: 17.5-19.0%, Ni: 12.0-15.0%, Mo: 0.5-0.8%, Cu: 2.8-3.2%, Nb: 0.8-1.0%, N: 0.18-0.22%, Zr: 0.010-0.015%.

5. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, The aluminizing agent comprises: 90%–98% iron and aluminum powder; 1%–10% activator; and 0.2%–2% aluminizing agent. Wherein, the iron and aluminum powder has a mesh size ≤ 300 mesh, the activator is NH4Cl or NH4F, and the penetration accelerator is La2O3 or Y2O3.

6. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, The slurry also includes silica water-based sol; The weight ratio of the aluminizing agent to the silica water-based sol is (0.8-1.0):

1.

7. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, In step S2, the thickness of the slurry on the inner wall of the tube after soaking is 0.5 to 2.0 mm.

8. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, In step S3, the heating temperature of the heat treatment is 700-800℃, the heat treatment time is 3-5h, and then the furnace is cooled to room temperature at a rate not exceeding 120℃ / h.

9. The method for preparing a stainless steel diffusion-coated pipe containing a welded joint according to claim 1, characterized in that, In step S4, two cleaning cycles are performed. The first cleaning cycle uses an aqueous solution of 15-25 g / L sodium hydroxide and 5-7 g / L sodium gluconate at a temperature of 60-70°C for 8-11 minutes. The second cleaning uses 5-8% ethylenediaminetetraacetic acid and 0.1-0.5% N2H4 corrosion inhibitor. The pH of the solution is adjusted to 8.5-9.5 using ammonia water, and the solution is circulated and rinsed at 60-80℃ for 15-30 minutes.

10. A stainless steel diffusion pipe containing a welded joint, characterized in that, The stainless steel diffusion-coated tube, obtained by the preparation method according to any one of claims 1-9, exhibits a weight gain of ≤0.2 mg / cm³ during steam oxidation at 650℃ / 10000h. 2 .

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