Stainless steel has good comprehensive performance and good appearance and surface characteristics, and is widely used in all walks of life. Likewise, stainless steel pipes are no exception. Stainless steel pipe is a kind of steel with hollow section, which is generally divided into stainless steel seamless pipe and welded pipe. There are also some differences in their processing methods and properties, the differences are as follows:

1. The difference in the production process

Stainless steel welded pipe is welded by steel plate or steel strip after being rolled and formed by the unit and die. Generally, there is a welding seam on the inner wall of the pipe; while the seamless pipe uses a round tube blank as a raw material for perforation, through cold rolling, cold drawing or Manufactured by the hot extrusion production process, there are no welding points on the tube.

2. Differences in appearance of steel pipes

Stainless steel welded pipe, the tolerance of its wall thickness is very small, and the wall thickness of the entire circumference is very uniform; the steel pipe has high precision, high brightness on the inner and outer surfaces of the pipe, and can be arbitrarily determined; it can be used as a thin-walled pipe. The steel pipe of the seamless pipe has low precision, uneven wall thickness, low brightness on the inner and outer surfaces of the pipe, high cost of sizing, and there are pits and black spots on the inner and outer surfaces that are not easy to remove. Therefore, seamless pipes are usually produced with thicker walls.

3. Differences in performance and price

Seamless pipes are much higher than welded pipes in terms of corrosion resistance and pressure and high temperature resistance. With the improvement of the welding process, the mechanical properties and mechanical properties are gradually approaching the seamless pipe. The production process of seamless pipe is more complicated, and its price is more expensive than that of welded pipe.

Based on the characteristics and differences between stainless steel seamless pipes and welded pipes, reasonable choices should be made during application to achieve economical, beautiful and reliable results:

(1) When used as decorative pipes, product pipes, and prop pipes, it is generally required to have a good surface effect, and stainless steel welded pipes are usually used;

(2) For general low-pressure fluid transportation, such as low-pressure systems such as water, oil, gas, air, and heating hot water or steam, stainless steel welded pipes are usually used;

(3) For the pipelines used to transport fluids in industrial engineering and large-scale equipment, as well as the pipelines for transporting fluids in power plants and nuclear power plant boilers that require high temperature, high pressure and high strength, stainless steel seamless pipelines should be selected;

(4) Stainless steel welded pipes are generally used for liquid transportation below 0.8MPa, and seamless pipes can be used to withstand liquid transportation above 0.8MPa. If the pressure requirements are not high, it is more economical to use welded pipes.

4. Advantages and disadvantages of stainless steel seamless pipe

advantage:

The forming speed is fast and the output is high, and it can be made into a variety of cross-sectional forms to meet the needs of the use conditions; cold rolling can cause a large plastic deformation of the steel, thereby increasing the yield point of the steel. Hot rolling can destroy the casting structure of the ingot, refine the grain of the steel, and eliminate the defects of the microstructure, so that the steel structure is dense and the mechanical properties are improved.

shortcoming:

(1) Metal delamination - When the steel pipe is cold rolled, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and delamination (interlayer) occurs. Delamination greatly deteriorates the mechanical properties of the steel through the thickness, and there is a possibility of interlaminar tearing as the weld expands.

(2) Uneven wall thickness - We know that metal has thermal expansion and cold contraction. Since the cold-rolled steel pipe reaches the standard in terms of length and thickness at the end of rolling, there will still be a certain negative difference after cooling. The greater this negative difference, the worse the uniformity of the wall thickness. Therefore, the wall thickness, length, straightness and ovality of the cold-rolled seamless steel pipe cannot be too correct.

(3) Residual stress - due to uneven cooling, steel pipes of various sections have residual stress. The larger the section size of the steel, the greater the residual stress, which will have a certain impact on the performance under the action of external force. For example, it may have adverse effects on deformation, stability, and fatigue resistance.

(4) Poor surface finish - the tensile marks on the inner surface of the steel pipe are longitudinally distributed, showing symmetrical or single straight folds, some of which exist in the whole length, and some exist locally.

in conclusion:

Due to the complexity of the production process, the uniformity of the wall thickness and the surface roughness of the inner and outer walls of the seamless pipe are poor. In actual operation, it is easy to make chloride ion (or sulfur ion) substances (such as organic matter) adhere to the stainless steel pipe wall. , forming a local oxygen-deficient acidic environment, so that the inner wall passivation film dissolves and loses its protective effect on the metal. The metal directly contacts the medium in the acidic environment, loses electrons as an anode and forms hydroxide with water, and the hydrogen in the hydroxide Oxygen radicals are replaced by chloride radicals and dissolve in water, reducing anode passivation. The binder acts as a cathode to capture the hydrogen ions generated by the inner wall to form hydrogen, and discharge it together with the reaction products of the inner wall metal. . When the temperature rises above 50 degrees, the pitting corrosion accelerates, and the higher the temperature, the faster the pitting corrosion. Stress corrosion will also occur under the condition of large stress, showing the characteristics of stress corrosion expansion. In addition, the seamless pipe will produce various unstable factors during the welding process, which is not easy to weld firmly and is not fully welded.

5. Advantages of stainless steel welded steel pipe

Stainless steel welded steel pipe - stainless steel strip with uniform wall thickness is welded by one-time coil forming without adding solder under gas shielding.

advantage:

(1) Uniform wall thickness - the base material is a forming strip, with good wall thickness consistency and high surface finish, reaching industrial clean surface level 2B.

(2) Low residual stress - After forming, the stainless steel tube is brightly annealed at a high temperature higher than 1040 degrees to eliminate stress.

(3) High strength of the weld - fusion welding is used for welding, and the material composition remains unchanged. After high temperature heat treatment, the weld has the same intergranular structure as the base metal, and the weld is destroyed by flattening, reverse bending, and flaring. During the experiment, there will be no cracks or cracks, burrs and other problems at the weld. In addition to this, eddy current testing and water pressure or gas-gas tests are also required to ensure the quality of the pipes.

(4) Good consistency - the outer diameter, wall thickness, length and straightness of the pipe are consistent, and the machining accuracy is high.