Stainless steel has good comprehensive performance and good appearance surface characteristics, in all walks of life has a wide range of applications. Similarly, stainless steel pipe is no exception. Stainless steel pipe is a kind of hollow section of steel, generally divided into stainless steel seamless pipe and welded pipe. Their processing methods and performance also have some differences, the differences are as follows:
First, the production process is different
Stainless steel welded pipe is made of steel plate or steel strip after the machine and mold curling forming, and the inner wall of the pipe generally has a weld; The seamless pipe uses round tube billet as raw material for perforation, through the production process of cold rolling, cold drawing or hot extrusion, there is no welding point on the pipe.
Two, the appearance of the steel pipe is different
Stainless steel welded pipe, the wall thickness tolerance is very small, the whole circumference of the wall thickness is very uniform; The steel pipe has high precision, high brightness of inside and outside surface, and can be arbitrarily measured. You can make thin-walled tubes. And seamless pipe steel pipe precision is low, the wall thickness is not uniform, the inside and outside of the pipe surface brightness is low, the size cost is high, and the inside and outside of the pitting and black spots are not easy to remove. As a result, seamless tubes are usually produced with thicker walls.
Three, the performance and price of the difference
Seamless pipe in corrosion resistance and pressure resistance to high temperature and other properties are much higher than welded pipe. With the improvement of welding process, mechanical properties and mechanical properties are also slowly close to seamless pipe. Seamless pipe in the production process is more complex, its price is more expensive than welded pipe.
Based on the characteristics and differences of stainless steel seamless pipe and welded pipe, the application should be reasonably selected to achieve economic, beautiful and reliable results:
1, used for decorative pipe, product pipe, props pipe, generally required to have a good surface effect, usually choose stainless steel welded pipe;
2, for the general low pressure fluid transportation, such as conveying water, oil, gas, air and heating hot water or steam and other low pressure system, usually choose stainless steel welded pipe;
3, for the use of industrial engineering and large equipment on the transmission of fluid pipeline, as well as power station, nuclear power plant boiler requirements of high temperature and high pressure, high strength fluid pipeline, stainless steel seamless pipe should be selected;
4, stainless steel welded pipe is generally used for liquid transportation below 0.8MPa, seamless pipe can be used to withstand liquid transportation above 0.8MPa. If the pressure requirement is not high, it is more economical to use welded pipe.
Four, stainless steel seamless pipe advantages and disadvantages
Advantages:
The molding speed is fast, the yield is high, can be made into a variety of cross section forms, in order to meet the needs of the conditions of use; Cold rolling can make the steel produce great plastic deformation, thus 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 microstructure of the steel is dense and the mechanical properties are improved.
Disadvantages:
1, metal stratification - steel tube after cold rolling, the non-metallic inclusions inside the steel (mainly sulfide and oxide, and silicate) are pressed into thin sheets, stratification (sandwich) phenomenon. Delamination greatly worsens the mechanical properties of steel along the thickness direction and may lead to interlayer tearing during weld expansion.
2, uneven wall thickness - we are familiar with the metal with thermal expansion and cold contraction, because the cold rolled steel pipe even in length and thickness are up to standard when rolling to the end, there will be a certain negative difference after cooling, the greater the negative difference, the worse the uniformity of wall thickness. Therefore, the wall thickness, length, straightness and ellipticity of cold rolled seamless steel pipe cannot be too correct.
3, residual stress - because of uneven cooling, various sections of steel pipe have residual stress, steel section size is larger, the greater the residual stress, under the action of external force has a certain impact on the performance. Such as deformation, stability, fatigue resistance and other aspects may have adverse effects.
4. Poor surface finish -- the tensile marks on the inner surface of the steel pipe are longitudinal distribution, showing a symmetrical or single straight fold, some through length exists, but also local existence.
Conclusion:
Seamless tube due to the complexity of the production process, the wall thickness uniformity is poor, the inner and outer wall surface roughness is poor, the actual operation is very easy to make chloride ion (or sulfur ion) material (such as organic matter) bonded to the stainless steel pipe wall, forming a local anoxic acid environment, the inner wall passivation film dissolution lost the protective effect of metal, metal directly contact with the medium in the acidic environment, As the anode loses electrons and forms hydroxide with water, the hydroxide in hydroxide is replaced by chlorine and dissolved in water, reducing the anode passivation. As the cathode, the adhesive captures hydrogen ions produced by the inner wall to form hydrogen, which is discharged outward together with the reaction products of the inner wall metal. With the deepening of the reaction, the metal of the tube wall is constantly corroded to form pore corrosion, and the pore will gradually deepen and increase. When the temperature rises above 50 degrees, pitting accelerates, and the higher the temperature, the faster the pitting speed. Under high stress conditions, stress corrosion will also occur and show the characteristics of stress corrosion expansion. In addition, seamless pipe in the welding process will produce a variety of unstable factors, not easy to solder and solder opaque.
Four, stainless steel welded steel pipe advantages
Stainless steel welded steel pipe -- stainless steel strip with uniform wall thickness is fused and welded without adding solder under gas protection.
Advantages:
1. Uniform wall thickness -- base material is formed strip, with good consistency of wall thickness and high surface finish, reaching industrial clean surface level 2B.
2. Low residual stress -- the steel pipe after forming uses a high temperature of more than 1040 degrees to carry out bright annealing on the stainless steel pipe to eliminate stress.
3. High weld strength -- welding adopts fusion welding, the material composition remains unchanged, after high temperature heat treatment, the weld and the base metal have the same intergranular structure, the weld is flattened, reverse bending, flaring and other damage experiments, there will be no cracking or cracks, burring and other problems. In addition to eddy current testing and water pressure or gas test to ensure the quality of the pipe.
4. Good consistency -- good consistency of the outside diameter, wall thickness, length and straightness of the pipe, high processing accuracy.
