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Welded stainless steel pipe and seamless stainless steel tube/pipe difference
- Apr 16, 2018 -

Stainless steel pipe is a kind of hollow circular steel, which is widely used in oil, chemical, medical, food, light industry, mechanical instruments and other industrial pipelines, as well as mechanical structural components. In addition, when bending and torsional strength are the same, the weight is lighter, so it is also widely used in manufacturing mechanical parts and engineering structures. They are also used to produce various conventional weapons, barrels, shells and so on.


1, concentricity

The manufacturing process of the seamless tube is to press a hole in the stainless steel billet at the temperature of 2200 f. At this high temperature, the tool steel becomes soft and spirally formed from the hole through the punching and drawing. The wall thickness of the pipe is not uniform and the eccentricity is high. Thus ASTM allows seamless pipe wall thickness ratio of the wall thickness difference of seamed tube. Through the seamed tube cold rolling plate precision (per roll width of 4-5 feet). These cold rolled plates usually have a maximum wall thickness difference of 0.002 inches. The steel plate is cut into the width of PI D, where D is the outer diameter of the pipe. Tube wall thickness tolerance is very small, and the entire circumference of the wall thickness is very uniform.


2, welding performance

Chemical composition of seamed pipe and seamless pipe are different. The steel composition of seamless pipe is only the basic requirement of ASTM. The production of seamed tube steel containing suitable for chemical composition of welding. For example, a mixture of silicon, sulfur, manganese, oxygen and triangulonites and other elements in a certain proportion can produce a heat transfer welding weld in the welding process, so that the whole weld is penetrated. The steel pipes which lack the above chemical components, such as seamless pipes, will produce various unstable factors during welding process, so it is not easy to weld and weld.


3. Grain size

The grain size of metal is related to the time of heat treatment and the same temperature. The welded stainless steel pipe and seamless stainless steel tube as the annealed grain size. If the seam pipe is treated with 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.


4, pipeline strength

The strength of the pipe depends on the composition of the alloy. Therefore, the seamless stainless steel tubes and the seamless stainless steel tubes with the same alloy and the same heat treatment are essentially the same strength.

After tensile test and three dimensional vibration test, the tear of the stainless steel pipe seams almost occurs in the place far away from the welding point or heating area. This is because the weld line has less impurities and a higher nitrogen content, so the strength of welding is better than that of other parts. However, the ASME (American Association of Mechanical Engineers) believes that the seamed stainless steel pipes can only bear 85% of the license pressure, mainly because the data collection is earlier than the improved welding equipment used today.

The provisions of ASME 100% completely under permit pressure through ultrasonic testing of welded stainless steel pipe. Similarly, Europe and Asia also stipulate that sewn stainless steel pipes through eddy current testing can ensure the quality of their welding performance. Eddy current testing is carried out by legal procedures and licensed institutions. TRENT's eddy current test has been approved by the Swedish power department. ASME believes the current loss is relatively small, high-quality performance based on the seamed tube.


5, corrosion resistance

The corrosion resistance is also determined by the composition of the alloy. Seamless stainless steel pipe of the same chemical composition and heat treatment of fully welded stainless steel pipes for corrosion resistance is consistent. The supplementary test provided by ASTM proves that the corrosion resistance of the welded joint is equal or better than that of the welded metal. In the environment of acid chloride, the corrosion of the welded joints of the welded steel pipes with incomplete heat treatment will accelerate, but this is only the need of corrosion testing, and the environment is not so bad in fact.


6, flexural and extensibility

The extensibility of the welding place can be verified by the following tests specified by ASTM: bending 45 degrees, bending to 90 degrees, leveling along the weld seam, and then turning the stainless steel seam pipe to repeat the above steps to make the weld bending inner diameter to 180 degrees. The standard of weld quality is not allowed to appear tearing or intergranular separation when magnified 40 times. The bending radius of pipe is controlled by the composition of alloy, and the minimum bending radius is 2D. The ideal welding condition is whether the weld is neutral or compressed. Moreover, the pipeline should be annealed to reduce its hardness and improve its bending performance.


7. Price

Welded stainless steel pipe price is only half of the stainless steel seamless pipe.


8, wall thickness / diameter

Thin wall stainless steel pipe with small thickness / diameter is best produced by welding. Thick wall pipe with large thickness / diameter is best made by stamping.


9. Comprehensive quality

Generally, the quality of the welded stainless steel pipe is better than the seamless stainless steel tube, because the stitches are made of the exact cold rolled plate through the inspection, so any defect is restricted to the welding place. Seamless pipe is punched by stainless steel billet and punch, resulting in many tearing phenomena in the tube wall formed by extrusion. The eddy current test shows that the welded stainless steel pipes of the defective rate is usually lower than the rate of defective seamless stainless steel tube. With ultrasonic testing, the background noise of seamless stainless steel tube is very large, so it is hard to detect its defects. The background noise of welded stainless steel tube is very low, easy to find defects.