The difference between stainless steel seamless pipe and welded pipe

Stainless steel has good comprehensive performance and good appearance and surface properties, and is widely used in various industries. Likewise, stainless steel pipes are no exception. Stainless steel pipe is a steel material with a hollow section, generally divided into stainless steel seamless pipes and welded pipes. There are also certain differences in their processing methods and performance, the differences are as follows:

　　1. Differences in production technology

　　Stainless steel welded pipes are made of steel plates or steel strips that are welded after being curled and formed by a unit and a mold. There is generally a weld on the inner wall of the pipe; while seamless pipes use round tube blanks as raw materials for perforation, which are processed by cold rolling, cold drawing or Made by hot extrusion production process, there are no welding spots on the tube.

　　2. Differences in appearance of steel pipes

　　Stainless steel welded pipes have very small wall thickness tolerances and are very uniform throughout the circumference; the steel pipes have high precision, high brightness on the inner and outer surfaces of the pipes, and can be arbitrarily cut to length; thin-walled pipes can be made. Seamless steel pipes have low precision, uneven wall thickness, low brightness of the inner and outer surfaces of the pipe, high cost of sizing, and pitting and black spots on the inner and outer surfaces that are difficult to remove. Therefore, seamless pipes are usually produced with thicker walls.

　　3. Differences in performance and price

　　Seamless pipes are much better than welded pipes in terms of corrosion resistance and pressure and high temperature resistance. With the improvement of welded pipe manufacturing technology, the mechanical properties and mechanical properties are gradually approaching seamless pipes. The production process of seamless pipes is more complicated, and its price is more expensive than welded pipes. Based on the characteristics and differences between stainless steel seamless pipes and welded pipes, reasonable selection should be made during application to achieve economical, beautiful and reliable effects: 1. When used as decorative pipes, product pipes, and prop pipes, good surface effects are generally required. , stainless steel welded pipes are usually used; 2. For generally 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 industrial engineering and large-scale Pipes for transporting fluids on equipment, as well as pipelines for transporting fluids on boilers in power stations and nuclear power plants that require resistance to high temperature, high pressure, and high strength, should use stainless steel seamless pipes; 4. Stainless steel welded pipes are generally used for liquid transportation below 0.8MPa. Seamless pipes It can be used to withstand liquid transportation above 0.8MPa. If the pressure requirements are not high, it will be more economical to use welded pipes.

　　4. Advantages and Disadvantages of Stainless Steel Seamless Pipes

　　Advantages: fast forming speed, high output, and can be made into a variety of cross-sectional forms to adapt to the needs of use conditions; cold rolling can cause large plastic deformation of 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 defects in the microstructure, thereby making the steel structure dense and improving the mechanical properties. Disadvantages: 1. Metal delamination - when the steel pipe is cold rolled, the non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are pressed into thin sheets, resulting in delamination (sandwiching). Delamination greatly deteriorates the mechanical properties of the steel along the thickness direction, and may cause interlaminar tearing when the weld expands. 2. Uneven wall thickness - We all know that metal expands with heat and contracts with cold. Even if the cold-rolled steel pipe reaches the standard in length and thickness at the end of rolling, there will still be a certain negative difference after final cooling. This The larger the negative difference, the worse the wall thickness uniformity. Therefore, the wall thickness, length, straightness and ovality 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-section size of the steel, the greater the residual stress, which will have a certain impact on performance under the action of external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc. 4. Poor surface finish - the stretch marks on the inner surface of the steel pipe are distributed longitudinally, showing symmetrical or single linear folds, some of which exist throughout the entire length, and some of which exist partially.

　　in conclusion:

　　Due to the complexity of the production process, seamless pipes have poor wall thickness uniformity and poor inner and outer wall surface roughness. In actual operation, it is easy for chloride ion (or sulfide ion) substances (such as organic matter) to bond to the stainless steel pipe wall. , forming a local anoxic acidic environment, causing the inner wall passivation film to dissolve and lose 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 chlorine radicals and dissolved in water, reducing the anode passivation effect. The adhesive acts as a cathode to capture the hydrogen ions generated on the inner wall to form hydrogen gas, and discharges them outward together with the reaction products of the metal on the inner wall. As the reaction progresses, the metal on the pipe wall is continuously corroded away to form pitting corrosion, and the holes will gradually deepen and increase. . When the temperature rises above 50 degrees, pitting corrosion accelerates. The higher the temperature, the faster the pitting corrosion rate. Under conditions of large stress, stress corrosion will also occur and show stress corrosion expansion characteristics. In addition, seamless pipes will produce various unstable factors during the welding process, making it difficult to weld firmly and without penetration.

　　4. Advantages of stainless steel welded steel pipes

　　Stainless steel welded steel pipe - stainless steel strips with uniform wall thickness are welded and rolled in one go under gas protection without adding solder.

　　Advantages: 1. Uniform wall thickness - the base material is formed strip, with good wall thickness consistency and high surface finish, reaching industrial clean surface grade 2B. 2. Low residual stress - the formed steel pipe is bright annealed at a high temperature higher than 1040 degrees to eliminate stress. 3. High strength of the weld - welding adopts fusion welding and the material composition remains unchanged. After high-temperature heat treatment, the weld and the base material have the same intergranular structure. The weld is subjected to damage experiments such as flattening, reverse bending, and flaring. There will be no cracks, cracks, burrs or other problems at the weld. In addition, eddy current testing and water pressure or gas-gas testing are also required to ensure the quality of the pipes. 4. Good consistency - the outer diameter, wall thickness, length and straightness of the pipe have good consistency and high processing accuracy.