Cold-rolled composite seamless steel pipe for boiler

Abstract

This invention belongs to the field of metal composite pipe manufacturing technology and discloses a cold-rolled composite seamless steel pipe for boilers, including a seamless base pipe and a seamless cladding pipe. The seamless cladding pipe covers the outside of the seamless base pipe, forming a continuous and uninterrupted metallurgical bonding interface between the seamless base pipe and the seamless cladding pipe. A gradient alloy transition diffusion layer is formed at the metallurgical bonding interface. The seamless base pipe is made of boiler-specific low-alloy heat-resistant steel seamless pipe, and the seamless cladding pipe is made of corrosion-resistant austenitic stainless steel or duplex stainless steel seamless pipe. The composite seamless steel pipe adopts an integral seamless structure without longitudinal weld seams. Cr and Ni elements in the gradient alloy transition diffusion layer are continuously distributed in a gradient from the cladding to the base layer, realizing an integrated design of heat-resistant and pressure-bearing inner layer and corrosion-resistant and protective outer layer. The inner and outer layers form a stable and continuous metallurgical bonding interface. The pipe has no longitudinal weld seams and is suitable for long-term service in the high-temperature, high-pressure, and corrosive environment of boilers, significantly reducing the overall procurement and maintenance costs of the pipe.

Description

Technical Field

[0001] This invention relates to the field of metal composite pipe manufacturing technology, and in particular to a cold-rolled composite seamless steel pipe for boilers. Background Technology

[0002] Boiler water-cooled walls and superheater pipes operate under high temperature and high pressure conditions for a long time. The flue gas is rich in sulfides, chlorides and fly ash particles. The outer wall of the pipe is constantly subjected to high temperature sulfide corrosion, chlorine corrosion and solid particle erosion. The pressurized medium inside the pipe requires the pipe material to have excellent high temperature strength and creep resistance.

[0003] Current mainstream pipe materials used in engineering applications have significant shortcomings: single low-alloy heat-resistant steel pipes such as 20G, 15CrMoG, and 12Cr1MoVG have moderate material costs and meet the requirements for high-temperature pressure resistance, but their resistance to flue gas corrosion is insufficient. In boilers burning high-sulfur and high-chlorine fuels, the pipe wall thins rapidly, making it prone to local pitting corrosion and perforation, which can lead to pipe rupture and shutdown, resulting in high operation and maintenance costs. All-stainless steel or nickel-based alloy seamless steel pipes have excellent corrosion resistance, but the raw material prices are high, and large-scale applications require huge investments. At the same time, the high-temperature creep performance of these alloys is not as good as that of boiler-specific low-alloy heat-resistant steels, and there is a risk of plastic deformation under long-term high-pressure conditions.

[0004] Bimetallic composite pipes have emerged as a technological direction that balances pressure resistance and corrosion resistance. Current mainstream manufacturing processes for composite pipes include exploded composite welded pipes, hot-extruded composite pipes, and mechanically sleeved composite pipes. Exploded composite pipes, when rolled from sheet metal, have longitudinal welds, resulting in uneven microstructure and reduced corrosion resistance, making them unsuitable for the long-term service requirements of high-temperature, high-pressure boiler components. Hot-extruded composite pipes require ultra-large tonnage extrusion equipment, leading to significant fixed asset investment and limiting the range of production specifications. Uneven metal flow during the forming process can easily cause interfacial discontinuity and surface peeling defects. Conventional mechanically sleeved composite pipes rely solely on interference fits for physical bonding, lacking a metallurgical interface. During the hot and cold cycles generated by boiler start-up and shutdown, the difference in thermal expansion coefficients of dissimilar metals can cause interface debonding. Air intrusion into the interlayer can trigger interface oxidation, resulting in a high risk of delamination failure during long-term service.

[0005] Furthermore, most existing publicly available cold-rolled bimetallic composite pipes have an inner corrosion-resistant lining structure, primarily used for corrosion protection of internal media within pipelines. Composite pipes with an outer lining to resist external flue gas corrosion exhibit fundamentally different stress states, interface stress distributions, and forming difficulties compared to lining-lined composite pipes, making the lining composite process unsuitable for direct transfer. Moreover, conventional cold-rolled composite processes lack interlayer vacuum protection and pre-diffusion processes before cold rolling, making the interface highly susceptible to oxidation during heat treatment and hindering the formation of a continuous and stable element diffusion metallurgical interface. Therefore, this paper proposes a cold-rolled composite seamless steel pipe for boilers that features interface metallurgical bonding, seamless construction, high dimensional accuracy, and suitability for mass production, resolving the conflict between high-temperature corrosion and pressure-bearing costs. Summary of the Invention

[0006] The present invention aims to provide a cold-rolled composite seamless steel pipe for boilers to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A cold-rolled composite seamless steel pipe for boilers, characterized in that it comprises a seamless base tube and a seamless cladding tube, wherein the seamless cladding tube covers the outside of the seamless base tube, a continuous and uninterrupted metallurgical bonding interface is formed between the seamless base tube and the seamless cladding tube, and a gradient alloy transition diffusion layer is formed at the metallurgical bonding interface. The seamless base tube is made of boiler-specific low-alloy heat-resistant steel seamless pipe, and the seamless cladding tube is made of corrosion-resistant austenitic stainless steel or duplex stainless steel seamless pipe. The composite seamless steel pipe adopts an integral seamless structure without longitudinal welded joints, and the Cr and Ni elements in the gradient alloy transition diffusion layer are continuously gradient distributed from the cladding to the base tube.

[0008] Preferably, the wall thickness of the seamless base layer pipe accounts for 65% to 85% of the total wall thickness of the composite steel pipe, and the wall thickness of the seamless cladding pipe accounts for 15% to 35% of the total wall thickness of the composite steel pipe; the outer diameter of the composite steel pipe is φ25 to 76 mm, and the total wall thickness is 3.0 to 12.0 mm; the interfacial bonding strength of the metallurgical bonding interface is ≥220 MPa.

[0009] Preferably, the seamless base tube is made of 15CrMoG, 12Cr1MoVG or 20G boiler-specific heat-resistant seamless steel pipe; the seamless cladding tube is made of TP304L, TP316L or 2205 duplex stainless steel seamless pipe.

[0010] Preferably, the thickness of the gradient alloy transition diffusion layer is 5–25 μm.

[0011] Preferably, the outer surface roughness Ra of the composite steel pipe is ≤3.2μm, and the inner surface roughness Ra is ≤1.6μm.

[0012] A method for preparing a cold-rolled composite seamless steel tube for boilers, characterized by comprising the following steps performed sequentially: S1. Tube blank pretreatment: Machine seamless base tube blanks and seamless cladding tube blanks respectively; perform precision turning, grinding, degreasing and rust removal on the outer circle of the base tube blank and the inner hole of the cladding tube blank; S2. Cold nesting assembly: At room temperature, the seamless base tube blank is inserted into the seamless cladding tube blank, with welding allowance reserved at both ends; S3. Vacuum sealing: Perform annular sealing welding on both ends of the composite tube blank, extract the air between the two tube layers, the vacuum degree of the interlayer is ≤1×10⁻² Pa, and then seal the ends with welding. S4. Pre-diffusion heat treatment: Heat the end-sealed composite tube blank to 780-860℃, hold for 40-90 minutes, and then slowly cool it in the furnace. S5. Multi-pass cold rolling forming: At least two passes of cold rolling deformation are carried out using a periodic cold rolling mill, with a single pass section shrinkage rate of 25% to 40% and a total section shrinkage rate of ≥60%; a special high-temperature lubricating medium is used in the cold rolling process. S6. Solution diffusion heat treatment of finished product: After cold rolling, heat the pipe to 920-1020℃, hold for 20-60 minutes, and then cool rapidly. S7. Finishing and Inspection: Straightening, cutting, non-destructive testing, mechanical property testing, interface peeling test, metallographic inspection, to obtain finished cold-rolled composite seamless steel pipes for boilers.

[0013] Preferably, in step S5, an intermediate stress-relief annealing is added between two adjacent cold rolling passes, with an annealing temperature of 720-800℃ and a holding time of 25-50 minutes.

[0014] Preferably, after the interlayer is vacuum sealed in step S3, a trace amount of high-purity argon gas is introduced into the interlayer.

[0015] Preferably, the water pressure test pressure of the finished steel pipe is ≥ 1.5 times the boiler design working pressure, and there is no leakage after holding the pressure for 5 minutes.

[0016] The beneficial effects of this technical solution compared to existing technologies are as follows: (1) The pipe material in this scheme is an integral seamless structure, and there is no weak longitudinal weld area of ​​the explosive composite welded pipe, which has higher pressure safety and meets the requirements of boiler pressure components.

[0017] (2) This scheme adopts a periodic cold rolling forming route, which significantly reduces the investment in heavy equipment compared with hot extrusion process. The finished product has high precision in outer diameter and wall thickness, excellent surface finish, and stable heat exchange efficiency.

[0018] (3) This scheme forms a metallurgical interface by relying on gradient diffusion between the inner and outer layers, which is different from simple mechanical interference bonding. It significantly improves the resistance to thermal cycling peeling and effectively inhibits the precipitation of interfacial carbides.

[0019] (4) This solution uses low-cost boiler-specific low-alloy heat-resistant steel as the base material and only uses corrosion-resistant alloy on the outer layer, which reduces the material cost by 30% to 50% compared with all stainless steel pipes.

[0020] (5) This solution has strong process controllability and can mass-produce small and medium diameter boiler heat exchange tubes, which are suitable for various boiler corrosion conditions such as water-cooled walls, superheaters, and economizers. Attached Figure Description

[0021] Figure 1 This is a front cross-sectional view of the present invention; Figure label: Seamless base tube 1, seamless coated tube 2, gradient alloy transition diffusion layer 3. Detailed Implementation

[0022] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments: like Figure 1 As shown, a cold-rolled composite seamless steel pipe for boilers according to the present invention includes a seamless base tube 1 and a seamless cladding tube 2, with the seamless cladding tube 2 covering the outside of the seamless base tube 1; a continuous and uninterrupted metallurgical bonding interface is formed between the seamless base tube 1 and the seamless cladding tube 2, and a gradient alloy transition diffusion layer 3 is formed at the metallurgical bonding interface; the seamless base tube 1 is a boiler-specific low-alloy heat-resistant steel seamless pipe, and the seamless cladding tube 2 is a corrosion-resistant austenitic stainless steel or duplex stainless steel seamless pipe; the composite seamless steel pipe is an integral seamless structure without longitudinal welded joints; the Cr and Ni elements in the gradient alloy transition diffusion layer 3 are continuously gradient distributed from the cladding to the base tube.

[0023] The seamless base layer pipe 1 accounts for 65%–85% of the total wall thickness of the composite steel pipe, and the seamless cladding pipe 2 accounts for 15%–35% of the total wall thickness of the composite steel pipe; the outer diameter of the composite steel pipe is φ25–76 mm, and the total wall thickness is 3.0–12.0 mm; the interfacial bonding strength of the metallurgical interface is ≥220 MPa. The thickness of the gradient alloy transition diffusion layer 3 is 5–25 μm. The surface roughness Ra of the outer surface of the composite steel pipe is ≤3.2 μm, and the surface roughness Ra of the inner surface is ≤1.6 μm.

[0024] Example 1 Target finished product: Seamless composite steel pipe for boiler water-cooled walls, outer diameter φ38mm, wall thickness 6.0mm. Base tube blank: 12Cr1MoVG seamless tube, outer diameter of the blank is 26mm, and the wall thickness is 4.5mm; Coated tube blank: TP316L stainless steel seamless tube, blank outer diameter 42mm, wall thickness 1.5mm; Preparation steps: S1. The mating surfaces of the inner and outer tubes are precision machined and polished to remove oxide scale, and then degreased and cleaned with organic solvents. S2, ambient temperature cold state nested assembly, the base tube is inserted into the cladding tube; S3. Argon arc welding is used to seal the two ends in a ring. The interlayer is evacuated to 8×10⁻³ Pa and then sealed by welding. S4. Pre-diffusion heat treatment: Hold at 820℃ for 60 minutes, then cool with the furnace; S5, two-pass cold rolling, the first pass has a section shrinkage rate of 33%, and the intermediate annealing between passes is carried out at 760℃ for 35 minutes; the second pass has a section shrinkage rate of 30%, and the cold rolling is carried out to the target outer diameter φ38mm. S6. Solution diffusion heat treatment of finished product: hold at 960℃ for 40 minutes, then rapidly cool with water mist; S7. Straighten, cut the head, and then carry out ultrasonic flaw detection, interface peeling test, metallographic inspection and hydrostatic test in sequence.

[0025] Finished product inspection: The thickness of the interface gradient alloy transition diffusion layer is 12-18μm, and the interface bonding strength is 256MPa; the room temperature tensile strength of the steel pipe is ≥450MPa, and the high temperature creep performance meets the boiler design specifications; the high temperature sulfidation corrosion resistance of the outer wall is significantly better than that of a single 12Cr1MoVG steel pipe.

[0026] Example 2 The finished product has a diameter of φ51mm and a wall thickness of 7.0mm, and is used in biomass boiler superheaters. Base material: 15CrMoG; Coating material: 2205 duplex stainless steel; Preparation process flow: S1. Pre-treatment of tube blanks: 15CrMoG base tube blanks and 2205 duplex stainless steel coated tube blanks are processed separately, and the mating surfaces are precision machined, ground, degreased and rusted. S2, Cold Nested Assembly: The base tube blank is inserted into the cladding tube blank at room temperature; S3. Vacuum sealing: Both ends are sealed with annular welding, and the interlayer is evacuated to ≤1×10⁻² Pa before sealing. S4. Pre-diffusion heat treatment: Hold at 840℃ for 60 minutes, then slowly cool with the furnace; S5, multi-pass cycle cold rolling, with intermediate stress-relieving annealing between passes; S6. Solution diffusion heat treatment of finished product: Hold at 980℃ for 35 minutes, then cool rapidly; S7, straightening, head cutting and full performance testing.

[0027] The pipe material has better resistance to chlorine corrosion than the austenitic stainless steel coating solution, and is suitable for high-chlorine biomass fuel corrosion conditions.

[0028] The specific implementation process is as follows: In use, the finished composite seamless steel pipes are first cut and beveled according to the boiler piping layout requirements; then, the two ends of the pipes are welded and assembled with the headers and connecting pipes to form the heat exchange tube bundles of the water-cooled walls, superheaters, or economizers; after assembly, a whole-body water pressure leak test is carried out to confirm that there is no leakage; during boiler operation, the high-temperature and high-pressure working fluid flows inside the seamless base tube, and the seamless base tube 1 bears the medium pressure and high-temperature creep load; the outside of the pipe is in direct contact with the boiler flue gas, and the seamless cladding tube 2 resists the high-temperature sulfidation corrosion, chlorine corrosion, and fly ash erosion brought by the flue gas; the gradient alloy transition diffusion layer 3 buffers the interface stress generated by the cold and hot cycles during the start-up and shutdown process, prevents interface peeling failure, and ensures the long-term stable operation of the tube bundle.

[0029] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A cold-rolled composite seamless steel pipe for boilers, characterized in that: The system includes a seamless base tube (1) and a seamless cladding tube (2). The seamless cladding tube (2) covers the outside of the seamless base tube (1). A continuous and uninterrupted metallurgical interface is formed between the seamless base tube (1) and the seamless cladding tube (2). A gradient alloy transition diffusion layer (3) is formed at the metallurgical interface. The seamless base tube (1) is made of boiler-specific low-alloy heat-resistant steel seamless tube. The seamless cladding tube (2) is made of corrosion-resistant austenitic stainless steel or duplex stainless steel seamless tube. The composite seamless steel pipe adopts an integral seamless structure without longitudinal welding seams. The Cr and Ni elements in the gradient alloy transition diffusion layer (3) are continuously distributed in a gradient from the cladding to the base.

2. The cold-rolled composite seamless steel pipe for boilers according to claim 1, characterized in that: The wall thickness of the seamless base tube (1) accounts for 65% to 85% of the total wall thickness of the composite steel pipe, and the wall thickness of the seamless cladding tube (2) accounts for 15% to 35% of the total wall thickness of the composite steel pipe; the outer diameter of the composite steel pipe is φ25 to 76 mm, and the total wall thickness is 3.0 to 12.0 mm; the interfacial bonding strength of the metallurgical bonding interface is ≥220 MPa.

3. The cold-rolled composite seamless steel pipe for boilers according to claim 1, characterized in that: The seamless base tube (1) is made of 15CrMoG, 12Cr1MoVG or 20G boiler-specific heat-resistant seamless steel pipe; the seamless cladding tube (2) is made of TP304L, TP316L or 2205 duplex stainless steel seamless pipe.

4. The cold-rolled composite seamless steel pipe for boilers according to claim 1, characterized in that: The gradient alloy transition diffusion layer (3) has a thickness of 5 to 25 μm.

5. The cold-rolled composite seamless steel pipe for boilers according to claim 1, characterized in that: The outer surface roughness Ra of the composite steel pipe is ≤3.2μm, and the inner surface roughness Ra is ≤1.6μm.

6. The method for preparing a cold-rolled composite seamless steel pipe for boilers according to claim 1, characterized in that: This includes the following steps performed sequentially: S1. Pre-treatment of tube blanks: The seamless base tube (1) blank and the seamless coated tube (2) blank are processed respectively; the outer circle of the base tube blank and the inner hole of the coated tube blank are precision turned, ground, degreased and rusted. S2, Cold nested assembly: At room temperature, the seamless base tube (1) blank is inserted into the seamless cladding tube (2) blank, with welding allowance reserved at both ends; S3. Vacuum sealing: Perform annular sealing welding on both ends of the composite tube blank, extract the air between the two tube layers, the vacuum degree of the interlayer is ≤1×10⁻² Pa and then seal it with welding. S4. Pre-diffusion heat treatment: Heat the end-sealed composite tube blank to 780-860℃, hold for 40-90 minutes, and then slowly cool it in the furnace. S5. Multi-pass cold rolling forming: At least two passes of cold rolling deformation are carried out using a periodic cold rolling mill, with a single pass section shrinkage rate of 25% to 40% and a total section shrinkage rate of ≥60%; a special high-temperature lubricating medium is used in the cold rolling process. S6. Solution diffusion heat treatment of finished product: After cold rolling, heat the pipe to 920-1020℃, hold for 20-60 minutes, and then cool rapidly. S7. Finishing and Inspection: Straightening, cutting, non-destructive testing, mechanical property testing, interface peeling test, metallographic inspection, to obtain finished cold-rolled composite seamless steel pipes for boilers.

7. The method for preparing a cold-rolled composite seamless steel pipe for boilers according to claim 6, characterized in that: In step S5, an intermediate stress-relief annealing is added between two adjacent cold rolling passes, with an annealing temperature of 720-800℃ and a holding time of 25-50 minutes.

8. The method for preparing a cold-rolled composite seamless steel pipe for boilers according to claim 6, characterized in that: After vacuum sealing of the interlayer in step S3, a trace amount of high-purity argon gas is introduced into the interlayer.

9. The method for preparing a cold-rolled composite seamless steel pipe for boilers according to claim 6, characterized in that: The finished steel pipe should be tested under water pressure at a pressure ≥ 1.5 times the boiler's design working pressure, and there should be no leakage after holding the pressure for 5 minutes.

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