Stainless steel is widely used in all walks of life because of its good comprehensive performance and good appearance and surface characteristics. Similarly, stainless steel pipe is no exception. Stainless steel pipe is a kind of steel with hollow section, generally divided into stainless steel seamless pipe and welded pipe. There are also some differences in their processing methods and properties, as follows:
1、 Differences in manufacturing process
Stainless steel welded pipe is welded by steel plate or steel strip after being curled and formed by the unit and mold, and there is generally a weld on the inner wall of the pipe; The seamless tube is made of round tube blank as the raw material for piercing and through the production process of cold rolling, cold drawing or hot extrusion. There is no welding point on the tube.
2、 Differences in the appearance of steel pipes
The wall thickness tolerance of stainless steel welded pipe is very small, and the wall thickness of the whole circumference is very uniform; The precision of steel pipe is high, the brightness of the inner and outer surface of the pipe is high, and the length can be determined arbitrarily; Able to make thin-walled tubes. The steel pipe of seamless pipe has low precision, uneven wall thickness, low brightness of internal and external surfaces, high cost of sizing, and it is difficult to remove pits and black spots on the internal and external surfaces. Therefore, the seamless pipe wall is usually thicker.
3、 Differences in performance and price
Seamless pipe is much better than welded pipe in corrosion resistance, pressure resistance and high temperature resistance. With the improvement of welded pipe manufacturing process, the mechanical properties and mechanical properties are slowly approaching to seamless pipe. Seamless pipe is more complicated in manufacturing process, and its price is more expensive than welded pipe.
Based on the characteristics and differences between stainless steel seamless pipe and welded pipe, reasonable selection shall be made in application to achieve economic, beautiful and reliable effects:
1. When it is used as decorative pipe, product pipe and prop pipe, it is generally required to have a good surface effect, and stainless steel welded pipe is usually selected;
2. Generally, stainless steel welded pipes are used for fluid transmission at low pressure, such as low pressure systems for water, oil, gas, air, heating hot water or steam;
3. Seamless stainless steel pipes shall be selected for pipes used to transport fluid in industrial projects and large equipment, as well as fluid pipes required to be resistant to high temperature, high pressure and high strength in power plants and nuclear power plants;
4. Stainless steel welded pipe is generally used for liquid transportation below 0.8MPa, and seamless pipe can be used for liquid transportation above 0.8MPa. If the pressure requirement is not high, it will be more economical to use welded pipe.
4、 Advantages and disadvantages of stainless steel seamless pipe
With fast molding speed and high output, it can be made into a variety of cross section forms to meet the needs of the use conditions; Cold rolling can make steel produce great plastic deformation, thus improving the yield point of 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.
1. Metal delamination - when the steel pipe is cold rolled, the non-metallic inclusions (mainly sulfide, oxide, and silicate) inside the steel are pressed into thin sheets, and delamination (interlayer) occurs. 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 are familiar with the thermal expansion and cold contraction of metal. Even if the length and thickness of the cold-rolled steel pipe meet the standard when rolling to the end, a certain negative difference will appear after the final cooling. The greater the negative difference, the worse the uniformity of wall thickness. Therefore, the wall thickness, length, straightness and ovality of cold rolled seamless steel tubes cannot be required too correctly.
3. Residual stress - due to uneven cooling, steel pipes of various sections have residual stress. The larger the steel section size is, the greater the residual stress is, which has a certain impact on the performance under the 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 linear folds, some of which are full-length and some of which are local.
Due to the complexity of the production process, poor wall thickness uniformity and poor surface roughness of the inner and outer walls of seamless tubes, it is very easy to make chloride (or sulfur ion) substances (such as organic matter) stick to the stainless steel tube wall in actual operation, forming a local anoxic acidic environment, making the inner wall passive film dissolve and lose its protection against metal. The metal directly contacts the medium in the acidic environment, loses electrons as an anode and forms hydroxide with water, The hydroxyl radical in the hydroxide is replaced by chlorine radical and dissolved in water, reducing the anode passivation. The binder, as the cathode, captures the hydrogen ions generated on the inner wall to form hydrogen, which is discharged outward together with the metal reaction products on the inner wall. With the deepening of the reaction, the metal on the tube wall is continuously corroded to form pitting corrosion, and the pores will gradually deepen and increase. The higher the temperature is, the faster the pitting speed will be when the temperature rises above 50 ℃. Under the condition of large stress, stress corrosion will occur, showing the characteristics of stress corrosion expansion. In addition, various unstable factors will occur during the welding process of seamless pipes, which are not easy to weld firmly and are not fully welded.
4、 Advantages of stainless steel welded steel pipe
Stainless steel welded steel pipe - the stainless steel strip with uniform wall thickness is rolled and formed by fusion welding without added solder under gas protection.
1. Uniform wall thickness - the base metal is formed 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 pipe shall be bright annealed at a temperature higher than 1040 ℃ to relieve the stress.
3. High weld strength - fusion welding is adopted for welding, and the material composition is unchanged. After high temperature heat treatment, the weld and the base metal have the same intergranular structure. When flattening, reverse bending, flaring and other destructive tests are carried out on the weld, no cracking, cracks, burrs and other problems will occur at the weld. In addition, eddy current testing and hydraulic or pneumatic testing shall be conducted 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 accuracy is high.