Metal welded pipe production and application

 Metal Welded Pipe Production and Application

Large-diameter thin-walled metal pipes generally refer to various metal pipes with a pipe diameter greater than 100 mm and a wall thickness less than 1 mm. Large-diameter thin-walled metal pipes have important uses, especially with the development and application of bimetallic composite pipe fittings and pipes, large-diameter thin-walled metal pipes of different materials can be combined with steel pipes or non-ferrous metal pipes to form different purposes Large diameter bimetallic tubing. In addition, in the field of deep processing of metal pipes, large-diameter thin-walled metal pipes can be processed by spinning, rolling and other processing methods to produce metal products such as corrugated pipes, simplified pipes, containers, and variable-diameter pipe fittings. The production of walled metal pipes is very difficult. If hot-rolling is used to produce large-diameter seamless metal pipes, since the wall thickness of the hot-rolled finished pipes is mostly between tens of millimeters and tens of millimeters, the amount of wall reduction is large, and multiple passes of cold rolling and cold drawing are required. Or spinning processing, high cost, low efficiency, large amount of metal loss, difficult to achieve mass production. In addition, the hot-rolled large-diameter seamless pipe production equipment has a large number and high quality, low equipment utilization rate, and low economic benefits. Large-diameter metal pipes can be produced directly by welding. Thin metal strips can be used without the need for wall-reducing processing of metal pipes. However, the current welding metal pipe production process equipment is difficult to meet the production of large-diameter thin-walled pipes.

According to the way of forming and welding, straight seam welded metal pipes can be divided into:
(1) Continuously formed welded pipe. The production method of metal continuous forming straight seam welded pipe is to use coiled metal strips, and then enter the continuous roll forming unit after unfolding and straightening. After continuous forming, high frequency welding, resistance welding, submerged arc welding and other welding methods are often used. Furnace welding can be used. After the tube blank is continuously welded into a metal pipe, the internal and external weld seams of the welded pipe can be processed by planing, and then cut into a fixed length by sawing machine. The continuous forming welding method is characterized by high production efficiency and can produce straight seam welded pipes of various specifications. However, due to the limitations of equipment capabilities and processes in forming, welding and sizing, it is not suitable for producing large-diameter and ultra-thin-walled welded pipes.
(2) Bending and forming welded pipes. The production method of metal bending forming straight seam welded pipe is to use a single piece of steel plate cut into a fixed length to form a single piece, and use a press or a three-roller rolling machine to gradually bend or roll it into a tubular shape, and then use a buried Arc welding, gas shielded welding and other welding methods are used to weld it into straight seam welded pipes. The characteristics of the bending forming welding method are: the process is simple and flexible, and the production equipment is small, which is suitable for the production of large-diameter thick-walled straight seam welded pipes. The disadvantage is that the production efficiency is low. Similarly, this method has limitations in forming and welding methods, and is not suitable for producing large-diameter, ultra-thin-walled and long-length welded pipes. To sum up, the key processes in the production of large ancient diameter thin-walled metal welded pipes are: forming, welding, cutting to length and seam treatment.

1 Production of large-diameter thin-walled metal welded pipes
Like ordinary straight seam welded pipes, large-diameter thin-walled metal welded pipes can be produced in two ways: press bending and continuous roll forming. The product requirements for large-diameter thin-walled metal welded pipes are: the shape and size of the pipe body are accurate and the weld quality meets the requirements. The metal at the weld must have good density and ductility. Due to the large diameter and thin wall of large-diameter thin-walled metal welded pipes, there are many differences in the production of ordinary straight seam welded pipes in terms of forming, welding and sizing. The large-diameter and extremely thin-walled straight seam welded pipes here generally refer to straight seam metal welded pipes with a diameter greater than 100mm and a wall thickness less than 1mm. The technical difficulties of its production are: the plate and strip are too thin, the bending radius is large, the simplified shape is unstable, the springback is large, and the forming is difficult; the pipe diameter is large, the pipe wall is thin, and it is difficult to weld by extrusion butt welding and welding wire The seam is difficult to handle, the tube wall is thin, and the tube is easily crushed and elastic
Permanent deformation, it is difficult to withstand the pressure of sawing or shearing to length. The following technical measures can be used to solve the above problems, namely: flexible bending of strips, pre-sizing of strips, pre-welding of formed pipes, multi-directional cutting to lengths, automatic argon arc welding, weld seam pressing, etc.

1.1 Press bending production
It is a complete set of special equipment that can produce large-diameter thin-walled metal welded pipes by pressing and bending. The technical key points in the production process of this set of equipment are:
(1) Crop. According to the expanded size of the welded pipe diameter, it is accurately cut to ensure that the size error is within +0.15 mm; the fracture has no burrs and flying burrs, and the edge, parallelism and straightness error of the plate is less than or equal to 0.10 mm.
(2) Pre-bending. To avoid the formation of "peach shape" and ensure the welding quality, it is necessary to pre-bend the metal sheet. For the pre-bending of the thin plate, special treatment is required for the indenter to ensure the accurate bending shape of the edge of the plate.
(3) Forming. Forming can be bent multiple times by a press, or continuous rolling by a three-roller rolling machine. In order to ensure the stress state at the weld, the bending curvature of the forming should be greater than the curvature of the finished pipe.
(4) Welding. Thin-walled pipes should be welded by argon tungsten arc welding. Place the pre-welded tube blank on the specially developed thin-walled tube automatic welding table and fix it. By adjusting the current and welding speed, continuous welding is carried out, so that the molten pool runs through the entire wall thickness to form a double-sided weld.
(5) Weld treatment. Due to the thin wall thickness, the weld height needs to be processed for subsequent processing. Different from the commonly used planing method, the weld seam treatment of thin-walled welded pipe adopts the rolling method. After welding, the pipe is placed on a specially developed weld seam rolling machine (Fig. 1b), and the moving roller and the mandrel inside the pipe are jointly The function is to flatten the weld seam, make the thickness of the weld seam consistent with the wall thickness, and at the same time make the structure of the weld seam dense and have a certain degree of ductility to facilitate subsequent plastic forming processing. Due to the welding without welding wire, the protrusion at the weld seam is very small, and the metal at the weld seam has both extension and width when rolling, so its longitudinal elongation is very small, which is not enough to have an uneven influence on the pipe length .

1.2 Continuous Roll Forming Production
For mass production of large-diameter thin-walled welded pipes of general specifications, continuous roll forming is still required. In order to adapt to this production process, it is necessary to improve the existing longitudinal welded pipe production process equipment, mainly including forming, welding, cutting to length and welding seam treatment. Like the bending forming process, the roll shape design and roll surface treatment of the forming roll should ensure accurate forming. In order to eliminate the stress at the weld, the bending curvature of the forming should be greater than the curvature of the finished pipe. Tungsten argon arc welding is used for welding. By adjusting the current and the running speed of the pipe, the weld seam is guaranteed to be continuous, and the molten pool runs through the entire wall thickness to form a double-sided weld seam. The technical difficulty in the production of large-diameter thin-walled welded pipes is the sizing of the pipes. Due to the large diameter and thin wall thickness of metal welded pipes, the sizing flying saw used in the production of ordinary welded pipes cannot meet the needs. The same as the production process of bending forming, the weld seam of thin-walled welded pipe is processed by rolling. Place the thin-walled welded pipes of fixed length on the weld seam rolling machine to flatten the weld seam to ensure that the weld seam has good flatness, compactness and ductility.

2 Application of large-diameter thin-walled metal welded pipe
Mass-produced large-diameter thin-walled metal welded pipes are used in fluid transportation (pipelines), liquid containers (cylinders) and light metal structures (grid structures)
It is of great significance in fields such as structure) and is mainly used for the processing and production of bimetallic pipe fittings. Large-diameter thin-walled stainless steel welded pipes produced by bending have been used in the processing and manufacturing of pressure-bearing bimetallic cylinders and bimetallic cylinders. 

Large-diameter thin-walled metal welded pipes of different materials combined with metal pipes of different materials and different wall thicknesses have the following significant advantages when used in various engineering fields: 
①The material selection is convenient, and steel, stainless steel and non-ferrous metal materials can be used; 
②The pipe can be made The walls have different physical and chemical properties; 
③ flexible size specifications, accurate diameter and wall thickness; 
④ suitable for the production of large-diameter bimetallic pipe fittings; 
⑤ compared with metallurgical bonding, metal recovery is convenient, and the two metals can be separated by a simple method.

The methods of producing metal composite pipes by using large-diameter thin-walled metal welded pipes mainly include hydraulic bulging method and rolling method. The hydraulic bulging method uses liquid pressure to make the inner tube produce diameter-expanding plastic deformation, eliminate the bonding gap, and at the same time make the outer tube elastically deform in diameter, so that an elastic bonding force is generated between the inner and outer tubes to realize bimetallic composite. . The rolling method is to use rolling composite equipment, use the outer tube as the outer tool, and the rolling head as the inner tool, and roll the inner wall of the inner tube
Pressing process causes the inner tube to produce plastic deformation of wall reduction and extension, and the outer surface of the inner tube and the inner surface of the outer tube are mechanically occluded, thereby generating plastic bonding force and realizing bimetallic composite. The plastic deformation of the inner tube wall reduction extension, the bonding strength of the mechanical engagement with the outer tube is greater than the bonding strength produced by the bulging method, but due to the plastic deformation, the weld strength, compactness and ductility of the inner tube are required Can meet the plastic deformation requirements.

In addition, the interference fit of the inner and outer pipes can also be realized through the diameter reduction deformation treatment of the outer steel pipe, so that the inner and outer steel pipes are tightly combined. Due to the greater wall thickness of the outer tube, greater deformation force is required.

3 Epilogue
Large-diameter thin-walled metal pipe is an economical metal material, which is difficult to produce by conventional seamless pipe production process and welded pipe production process. With the development of deep processing technology of metal materials, the use of large-diameter thin-walled metal pipes is becoming more and more extensive, and it is very necessary to develop corresponding production processes and technical equipment.