Stainless steel has good comprehensive performance and good appearance and surface characteristics, and is widely used in various industries. Similarly, stainless steel pipes are no exception.
Stainless steel pipe is a kind of steel with a hollow section, generally divided into stainless steel
seamless pipe and welded pipe. There are also certain differences in their processing methods and performance, the differences are as follows:
1) Difference in the production process
Stainless steel welded pipes are welded by steel plates or steel strips after being crimped and formed by the machine unit and molds. There is generally a weld on the inner wall of the pipes; while the seamless pipes use round tube blanks as raw materials for perforation, through cold rolling, cold drawing or Made of hot extrusion production process, there are no welding points on the tube.
2) Difference in the appearance of steel pipes
The stainless steel welded pipe has a very small wall thickness tolerance, and the wall thickness of the entire circumference is very uniform; the steel pipe has high precision, the inner and outer surfaces of the pipe have high brightness, and can be arbitrarily scaled; thin-walled pipes can be made. The steel pipe of the seamless pipe has low precision, uneven wall thickness, low brightness of 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, the walls of seamless pipes usually produced are thicker.
3) Differences in performance and price
Seamless pipes are much higher than welded pipes in terms of corrosion resistance, pressure resistance and high temperature resistance. With the improvement of the welded pipe manufacturing process, the mechanical properties and mechanical properties are gradually approaching the seamless pipe. The seamless pipe is more complicated in the production process, and its price is more expensive than the welded pipe. Based on the characteristics and differences of stainless steel seamless pipes and welded pipes, reasonable selection should be made during application to achieve economical, beautiful and reliable results:
1. When making 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 lower pressure fluid transportation, such as water, oil, gas, air and low-pressure systems such as heating hot water or steam, stainless steel welded pipes are usually used;
3. Stainless steel seamless pipes should be used for pipes used in industrial engineering and large-scale equipment to convey fluids, as well as pipes for conveying fluids that require high temperature, high pressure and high strength on boilers in power plants and nuclear power plants;
4. Stainless steel welded pipes are generally used for liquid transportation below 0.8MPa, and seamless pipes can be used for liquid transportation above 0.8MPa. If the pressure requirements are not high, it will be more economical to use welded pipes.
Advantages and disadvantages of stainless steel seamless pipe:
Advantage:
The forming speed is fast, 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 large plastic deformation of the steel, thereby increasing the yield point of the steel. Hot rolling can destroy the casting structure of the steel ingot, refine the grains 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 stratification - when the steel pipe is cold-rolled, the non-metallic inclusions (mainly sulfides and oxides, and silicates) inside the steel are pressed into thin sheets, and stratification (interlayer) occurs. Delamination greatly deteriorates the mechanical properties of steel along the thickness direction, and may cause interlaminar tearing when the weld expands.
2. Uneven wall thickness—we are well aware that metals expand with heat and contract with cold. Even if the cold-rolled steel pipe meets the standard in length and thickness at the end of rolling, there will still be a certain negative difference after cooling. The greater the negative difference, the worse the uniformity of the wall thickness. Therefore, the wall thickness, length, straightness and ellipticity of cold-rolled seamless steel pipes cannot be too accurate.
3. Residual stress - Due to uneven cooling, steel pipes of various cross-sections have residual stress. The larger the cross-sectional size of the steel, the greater the residual stress, which has a certain impact on performance under the action of external force. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.
4. Poor surface finish - the inner surface of the steel pipe is longitudinally distributed, showing symmetrical or single linear folds, some of which exist all the way, and some exist locally.
In Conclusion:
Due to the complexity of the production process of the seamless pipe, the uniformity of the wall thickness is poor, and the surface roughness of the inner and outer walls is poor. In actual operation, it is easy to make the substances (such as organic matter) of chloride ions (or sulfide ions) stick to the stainless steel pipe wall. , forming a local anoxic acidic environment, so that the passive film on the inner wall dissolves and loses the protective effect on the metal. The metal directly contacts the medium in the acidic environment, and as an anode, it loses electrons and forms hydroxide with water. The hydrogen in the hydroxide Oxygen is replaced by chloride and dissolved in water, reducing the anode passivation. The binder acts as a cathode to capture the hydrogen ions generated on the inner wall to form hydrogen gas, and discharges it together with the metal reaction product on the inner wall. As the reaction progresses, the metal on the pipe wall is continuously corroded to form pitting corrosion, and the pores will gradually deepen and increase. . Pitting corrosion accelerates when the temperature rises above 50 degrees, and the higher the temperature, the faster the pitting corrosion speed. Under the condition of greater stress, stress corrosion will also occur, 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 impenetrable.
Advantages of stainless steel welded pipe:
Stainless steel
welded steel pipe - the stainless steel strip with uniform wall thickness is welded and welded once under gas protection without adding solder and rolled into shape.
Advantage:
1. Uniform wall thickness - the base material is a formed strip, with good wall thickness consistency and high surface finish, reaching industrial grade 2B.
2. Low residual stress - after forming, the steel pipe is bright annealed at a high temperature higher than 1040 degrees to eliminate stress.
3. High strength of the weld seam - fusion welding is used for welding, and the material composition remains unchanged. After high-temperature heat treatment, the weld seam has the same intergranular structure as the base metal, and the weld seam is subjected to destructive tests such as flattening, reverse bending, and flaring. , there will be no cracks or cracks, burrs and other problems at the weld. In addition, eddy current testing and water pressure or gas test are also required to ensure the quality of the pipe.
4. Good consistency - the outer diameter, wall thickness, length and straightness of the pipe are consistent, and the processing precision is high.
