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  • E-mali: sxthsteel@sxth-group.com
  • Tel: +86-29-89636557
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  • The difference between seamless stainless steel pipe and welded stainl ess steel pipe
    Apr 08, 2018

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    Stainless steel tube is a kind of hollow long round steel, which is widely used in petroleum, chemical, medical, food, light industry, mechanical instrumentation and other industrial transmission pipelines and mechanical structural components. In addition, when the bending and the torsional strength are the same, the weight is light, so it is also widely used for manufacturing mechanical parts and engineering structures. Also used to produce a variety of conventional weapons, barrels, shells and so on.

    1. Concentricity

    The seamless tube is manufactured by punching a hole in a stainless steel billet at a temperature of 2200°F. At this high temperature, the tool steel is punched and drawn into a soft and helical shape from the hole. In this way, the wall thickness of the pipe is uneven and the eccentricity is high. Thus the astm allows the difference in wall thickness of the seamless pipe to be greater than the wall thickness difference of the seamed pipe. Slotted tubes are made from precision cold-rolled plates (4-5 ft. per roll). These cold rolled sheets typically have a maximum wall thickness difference of 0.002 inches. The steel plate is cut to the width of πd, where d is the outer diameter of the pipe. The tolerance of wall thickness of the seam pipe is very small, and the wall thickness of the entire circumference is very uniform.

    2. Welding performance

    Generally there are some differences in the chemical composition of the seam pipe and seamless pipe. The production of seamless steel components is only a basic requirement to achieve astm. The production of steel tubes with seams contains chemical components suitable for welding. For example, silicon, sulfur, manganese, oxygen, and elements such as trigonal ferrite may be mixed in a certain ratio to produce a weld fused with heat during the welding process, so that the entire weld seam is thoroughly penetrated. Pipes lacking the above chemical composition, such as seamless pipes, can cause various unstable factors in the welding process, and are not easily soldered and not penetrated.

    3. Grain size

    The grain size of the metal is related to the heat treatment temperature and the time of maintaining the same temperature. The annealed seamed stainless steel tubes and seamless stainless steel tubes have the same grain size. If there is a minimum cold treatment on the seam, the grain size of the weld is smaller than the grain size of the weld metal. Otherwise, the grain size is the same.

    4. Pipe strength

    The strength of the pipe depends on the alloy composition, so the seamless stainless steel pipe and the seamed stainless steel pipe containing the same alloy and the same heat treatment are intrinsically strong in strength.

    After the tensile test and the three-dimensional vibration test, the tearing of the seamed stainless steel pipe almost took place away from the welding point or the heated area. This is because there are fewer impurities at the weld and a slightly higher nitrogen content, so the strength at the weld is better than at other sites. However, ASME (American Society of Mechanical Engineers) believes that the seamed stainless steel pipe can only withstand 85% of the allowable pressure, mainly due to the collection of data earlier than the improved welding equipment used today.

    ASME stipulates that 100% ultrasonically tested seamed stainless steel tubes can fully withstand the allowable pressure. Similarly, Europe and Asia have also stipulated that the eddy current test of the seamed stainless steel pipe can guarantee the high quality of its welding performance, and the eddy current test needs to be carried out by a lawful and licensed institution. TRENT's eddy current test passed the permission of the Swedish Ministry of Power. ASME believes that the small current loss is based on the superior performance of a seamed pipe.

    5. Corrosion resistance

    The corrosion resistance also depends on the composition of the alloy. The corrosion resistance of the seamless stainless steel pipe and the fully heat-treated seamed stainless steel pipe with the same chemical composition are the same. Supplemental tests provided by ASTM demonstrate that the corrosion resistance of the weld is equivalent to or better than the metal being welded. In an acid chloride environment, the corrosion of joints in incompletely heat-treated seamed steel tubes will accelerate, but this is only a requirement for corrosion testing, and in fact the environment is not so bad.

    6. Flexibility and Extensibility

    The extensibility of the weld can be verified by the following tests specified by the ASTM: bending 45°, bending to 90°, and flattening along the weld; then turning off the stainless steel seamed tube Repeat the above steps to make the weld bend inside diameter 180° . The criterion for the quality of the weld pass is that no tear or intergranular separation is allowed at a magnification of 40 times. The bend radius of the pipe is controlled by the composition of the alloy, and the minimum bend radius is generally 2d. The ideal welding condition is that the weld is in neutral or under pressure. Moreover, the pipe should be annealed to reduce its hardness, thereby improving the bending performance.

    7. Price

    Seamless stainless steel tubes are usually only half the price of stainless steel seamless tubes.

    8. Thickness/ diameter

    Thin-walled stainless steel tubes having a small thickness/diameter value are preferably produced by welding; thick-walled tubes having a large thickness/diameter value are preferably formed by a stamping method.

    9. Comprehensive quality

    In general, the quality of seamed stainless steel pipes is better than that of seamless stainless steel pipes because the seamed stainless steel pipes are made of precision cold-rolled plates that have passed the inspection, and any defects are limited to the welded joints. The seamless tube is punched from the stainless steel blank by punches, resulting in many tearing phenomena in the extruded tube wall. The eddy current test showed that the defective rate of the seamed stainless steel pipe was usually lower than that of the seamless stainless steel pipe. With ultrasonic testing, the background noise of the seamless stainless steel tube is so large that it is difficult to find defects. The background noise of the seamed stainless steel pipe is very low, and it is easy to find the defect.