The difference between a stainless steel tube and a seamless stainless steel tube

Stainless steel pipe is a kind of hollow long round steel, which is widely used in industrial transportation pipelines such as petroleum, chemical, medical, food, light industry, mechanical instrumentation and mechanical structural components. In addition, when the bending and torsional strength are the same, the weight is light, so it is also widely used in the manufacture of 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 which the tool steel is softened and spirally formed from the hole by punching and drawing. Thus, the wall thickness of the pipe is uneven and the eccentricity is high. Thus astm allows the wall thickness difference of the seamless pipe to be larger than the wall thickness difference of the seamed pipe. The seamed tubes are made from precision cold rolled sheets (4-5 feet per roll width). These cold rolled sheets typically 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 seamed tube is small and the wall thickness of the entire circumference is very uniform.

2, welding performance

There are generally differences in the chemical composition of the seamed tube and the seamless tube. The steel components that produce seamless pipes are only the basic requirements for reaching astm. Steels with seams contain chemical components suitable for welding. For example, silicon, sulfur, manganese, oxygen, and a certain ratio of elements such as triangular ferrite can produce a weld melt that is easy to transfer heat during the welding process to weld the entire weld. Steel pipes lacking the above chemical components, such as seamless pipes, have various unstable factors during the welding process, and are not easily welded and cannot be welded.

3, grain size

The grain size of the metal is related to the heat treatment temperature and the time to maintain the same temperature. The annealed stainless steel pipe and the seamless stainless steel pipe have the same grain size. If the seam is treated with minimal cold treatment, the grain size of the weld is smaller than the grain size of the metal being welded. Otherwise, the grain size is the same.

4, pipeline 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 have the same strength in nature.

After the tensile test and the three-dimensional vibration test, the tear of the seamed stainless steel tube occurs almost everywhere away from the welded or heated area. This is because the weld has less impurities and a slightly higher nitrogen content, so the weld is stronger than other parts. However, ASME (American Society of Mechanical Engineers) believes that seamed stainless steel pipes can only withstand 85% of the permitted pressure, mainly because the data is collected earlier than the improved welding equipment used today.

ASME stipulates that 100% of the slotted stainless steel tubes that pass the ultrasonic test can be fully subjected to the permissible pressure. Similarly, Europe and Asia also require slitted stainless steel tubing that is eddy current tested to ensure high quality weldability, and eddy current testing is performed by legally licensed and licensed agencies. TRENT's eddy current test was approved by the Swedish Ministry of Power. ASME believes that the small current loss is based on the superior performance of a seamed tube.

5, corrosion resistance

Corrosion resistance also depends on the composition of the alloy. The corrosion resistance of the seamless stainless steel tube with the same chemical composition and the completely heat treated slotted stainless steel tube is consistent. Supplemental testing provided by ASTM demonstrates that the corrosion resistance of the weld is equivalent or better than the metal being welded. In the case of acid chloride, the corrosion of the welded joint of the incompletely heat treated seamed steel pipe will accelerate, but this is only the need of corrosion test, and the actual environment is not so bad.

6, bending and extensibility

The extensibility of the weld can be verified by the following test specified by ASTM: bending 45°, then bending to 90°, and then flattening along the weld; then turning the stainless steel with the seam pipe to repeat the above steps to make the weld bend inner diameter reach 180° . The standard for the pass quality of welds is that tearing or intergranular separation is not allowed at 40x 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 pressurized state. Moreover, the pipe should be annealed to reduce its hardness, thereby improving the bending performance.

7, the price

The price of a slotted stainless steel tube is usually only half that of a stainless steel seamless tube.

8, wall thickness / diameter

Thin-walled stainless steel pipes of small thickness/diameter value are preferably produced by welding; thick-walled pipes of large thickness/diameter value are preferably formed by stamping.

9, comprehensive quality

In general, the quality of a slotted stainless steel tube is superior to that of a seamless stainless steel tube because the slotted stainless steel tube is made of a precision cold rolled sheet that has been inspected and passed, so that any defects are limited to the weld. The seamless tube is stamped from a stainless steel billet, resulting in many tearing in the extruded tube wall. The eddy current test shows that the defective rate of the slotted stainless steel tube is usually lower than the defective rate of the seamless stainless steel tube. With ultrasonic testing, the background noise of seamless stainless steel tubes is so large that it is difficult to find defects. The background noise of the slotted stainless steel tube is very low, and it is easy to find the defect.