The differences between stainless steel welded pipe and stainless steel seamless steel pipe for fluid transportation

Stainless steel welded steel pipes are referred to as welded pipes, commonly used steel pipes or steel strips are welded after being crimped by a unit and a mold. Welded steel pipes have simple production process, high production efficiency, many varieties and specifications, and less equipment, but their general strength is lower than seamless steel pipes. Since the 1930s, with the rapid development of high-quality strip continuous rolling production and the advancement of welding and inspection technology, the quality of welds has been continuously improved, and the variety and specifications of welded steel pipes have increased, and in more and more fields, especially in Heat exchange spare tubes, decorative tubes, medium and low pressure fluid tubes, etc. have replaced seamless steel tubes.

Features of welded pipe

   1. Small-caliber stainless steel welded pipes are continuously produced online. The thicker the wall, the greater the investment in the unit and welding equipment, and the less economical and practical it is. The thinner the wall thickness, the lower its input-output ratio; secondly, the process of the product determines its advantages and disadvantages. Generally, the welded steel pipe has high precision, uniform wall thickness, and high brightness inside and outside the pipe (the steel pipe is determined by the surface grade of the steel plate). Surface brightness), can be arbitrarily fixed length. Therefore, it embodies its economy and beauty in high-precision, medium and low-pressure fluid applications.

   Stainless steel seamless steel pipe is a long strip of steel with a hollow section and no joints on the periphery. It is a steel pipe that is resistant to weak corrosive media such as air, steam, water, and chemical corrosive media such as acid, alkali, and salt. Also known as stainless acid-resistant steel pipe.

Features of seamless pipe:

   1. The thicker the wall thickness of the product, the more economical and practical it is, and the thinner the wall thickness, its processing cost will rise significantly;

   2. The process of this product determines its limited performance. Generally, seamless steel pipes have low precision: uneven wall thickness, low brightness on the inside and outside of the pipe, high cost of sizing, and there are pitting and black spots on the inside and outside that are not easy to remove;

   3. Its detection and shaping must be processed offline. Therefore, it embodies its superiority in terms of high pressure, high strength, and mechanical structural materials.

   There are two types of stainless nickel alloy steel pipes-seamless pipes and seamed pipes. The degree of universality of seamless pipes and seamed pipes varies around the world: in the United States, the use of seamed pipes accounts for 95% of stainless steel pipes, in Europe, seamless pipes account for 60%, and Japan’s 75%. At present, sales of seam pipes in Europe are rising sharply, while seamless pipes are falling sharply. In Asia, Taiwan and Indonesia have substantially increased the production of seamed pipes, resulting in a decrease in sales of seamless pipes in Japan.

   The development of seamed pipes began in the United States during World War II. Since almost all the seamless pipe equipment was from Germany at that time, new equipment could not be obtained with the outbreak of the war, so the United States used its own technology to produce pipes. The two companies Trent and Carpenter were pioneers in the development of seam pipe production technology, thus forming a new industry. After the war, Germany and Japan rebuilt their seamless tube production technology industry, but it was decades later than the United States’ seam tube technology. In the past two decades, seam pipe production technology has been spread to Italy, France, Holland, England, Sweden, Taiwan, Indonesia, India and other small countries and regions. All over the world, the application of seamed pipes has been rapidly developed.

The difference between seamed pipe and seamless pipe:

Concentricity: The manufacturing process of seamless pipe is to punch a hole in a stainless steel billet at a temperature of 2200°F. At this high temperature, the tool steel becomes soft and spirally formed from the hole after punching and drawing. come out. In this way, the wall thickness of the pipeline is uneven and the eccentricity is high. Therefore, ASTM allows the wall thickness difference of seamless pipes to be greater than that of seamed pipes. The slotted pipe is made of precise cold-rolled sheet (with a width of 4-5 feet per coil). These cold rolled sheets usually have a maximum wall thickness difference of 0.002 inches. The steel plate is cut to a width of πd, where d is the outer diameter of the pipe. The tolerance of the wall thickness of the slit pipe is very small, and the wall thickness is very uniform throughout the circumference.

Welding performance:

   Generally there is a certain difference in chemical composition between seamed pipe and seamless pipe. The steel composition used to produce seamless pipes is only the basic requirement of ASTM. The steel used to produce seamed pipes contains chemical components suitable for welding. For example, the mixing of silicon, sulfur, manganese, oxygen, and triangular ferrite in a certain proportion can produce a weld melt that is easy to transfer heat during the welding process, so that the entire weld can be penetrated. Steel pipes lacking the above chemical composition, such as seamless pipes, will produce various unstable factors during the welding process, and are not easy to weld firmly and incompletely.

Grain size:

   Generally, the grain size of a metal is related to the heat treatment temperature and the time to maintain the same temperature. Therefore, the grain size of the annealed slit pipe and seamless pipe are the same. If the seam pipe adopts the minimum cold treatment, the grain size of the weld is smaller than the grain size of the welded metal, otherwise, the grain size is the same.


  The strength of the pipe depends on the alloy composition, so seamless pipes and seamed pipes containing the same alloy and the same heat treatment have essentially the same strength. After the tensile test and the three-dimensional vibration test, the tearing of the seamed pipe almost all occurred far away from the welding point or the heated area. This is because the weld has less impurities and slightly higher nitrogen content, so the strength of the weld is better than other parts. However, ASME’s Boiler and Pressure Vessel Association believes that seamed pipes can only withstand 85% of the allowable pressure. This is mainly due to the fact that data collection is earlier than the improved welding equipment used today. ASME stipulates that 100% of seamed pipes that have passed the ultrasonic test can fully withstand the allowable pressure. Similarly, Europe and Asia also stipulate that seamed pipes that pass the eddy current test can guarantee the high quality of their welding performance, and the eddy current test needs to go through a legal procedure and be carried out by a licensed organization. Trent’s eddy current test was approved by the Swedish Ministry of Power. ASME believes that the small current loss is based on the high-quality performance of the slit pipe.

Corrosion resistance:

   The corrosion resistance also depends on the composition of the alloy. The corrosion resistance of seamless pipes with the same chemical composition and fully heat-treated seamed pipes is the same. The supplementary test provided by ASTM proves that the corrosion resistance of the weld is equal to or better than the metal being welded. In an acid chloride environment, the corrosion of the welded joints of incompletely heat-treated seamed pipes will accelerate, but this is only required for corrosion testing. In fact, the environment is not so harsh. Flexibility and extensibility: The extensibility of the weld can be verified by the following tests specified by ASTM: bend 45°, then bend to 90°, and then flatten along the weld; then turn the pipe with the seam and repeat the above steps to make the weld The bending inner diameter reaches 180°. The standard for welding seam quality is that no tearing or intergranular separation is allowed under the condition of 40 times magnification. The bending radius of the pipe is controlled by the composition of the alloy, and the minimum bending radius is generally 2D. The ideal welding condition is that the weld is in a neutral or compressed state. Moreover, the pipeline should be annealed to reduce its hardness, thereby improving bending performance.

Wall thickness/diameter:

   Thin-walled tubes with small thickness/diameter values ​​are best produced by welding. Thick-walled pipes with large thickness/diameter values ​​are best made by stamping.

Comprehensive quality:

   Generally, the quality of seam pipes is better than seamless pipes, because the seam pipes are made of accurate cold-rolled plates that have passed inspection, so any defects are limited to the weld. Seamless pipes are stamped from stainless steel blanks by punches, resulting in many tearing phenomena in the pipe wall formed by extrusion. The eddy current test shows that the defective rate of seamed pipes is usually lower than that of seamless pipes. Using ultrasonic testing, the background noise of seamless pipes is so great that it is difficult to find defects. The background noise of the slit pipe is very low, and the defect is easy to find.

   As long as the pressure is not applied, the use of welded pipe and seamless pipe for pure transportation is the same, and the cost of welded pipe is lower! ! !

   is mainly different in pressure resistance. The fluid is transmitted by pressure flow. The pressure capacity of welded pipe is worse than that of seamless pipe, and the cost is lower

  The difference lies in one, the flaw detection process. Stainless steel pipe includes: high pressure, fluid, structure, it has a wider range. Stainless steel pipes for fluids have more uses. Fluid stainless steel pipes can be used for structures, while structural stainless steel pipes cannot be used for fluid steel pipes.

Post time: Oct-25-2020