The Difference Between Seam Stainless Steel Pipe and Seamless Stainless Steel Pipe

Stainless steel pipes are hollow long cylindrical steel materials, mainly used in industries such as petroleum, chemical, medical, food, light industry, machinery, and instrumentation for industrial pipeline transportation and mechanical structural components. Additionally, they are widely used in the manufacture of machinery parts and engineering structures due to their lighter weight when bending and withstanding torsion strength. They are also commonly used in the production of various conventional weapons, gun barrels, shells, etc.

Concentricity:

The manufacturing process of seamless pipes involves punching a hole in a stainless steel square billet at a temperature of 2200°F, where the tool steel becomes soft and spirally forms from the hole through punching and drawing. This results in uneven wall thickness and high eccentricity of the pipe. Therefore, ASTM allows for a greater wall thickness difference in seamless pipes compared to seam pipes. Seam pipes are made from precise cold-rolled plates (each coil width is 4-5 feet). These cold-rolled plates typically have a maximum wall thickness difference of 0.002 inches. The steel plate is sheared to a width of πd, where d is the outer diameter of the pipe. The tolerance of the wall thickness of seam pipes is small, and the wall thickness around the entire circumference is very uniform.

Welding Performance:

Generally, there is a certain difference in the chemical composition between seam pipes and seamless pipes. The steel used to produce seamless pipes only meets the basic requirements of ASTM, while the steel used to produce seam pipes contains chemical compositions suitable for welding. For example, a mixture of elements such as silicon, sulfur, manganese, oxygen, and ferrite in certain proportions can produce a weld metal that is easy to transfer heat during the welding process, thereby ensuring complete fusion of the entire weld seam. Steel pipes lacking these chemical compositions, such as seamless pipes, will encounter various unstable factors during the welding process, making them difficult to weld securely and completely.

Grain Size:

The grain size of metal is related to the temperature of heat treatment and the time it is held at that temperature. The grain size of annealed seam stainless steel pipes and seamless stainless steel pipes is the same. If the seam pipe undergoes minimal cold treatment, the grain size of the weld metal will be smaller than that of the base metal; otherwise, the grain size will be the same.

Pipe Strength:

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

After tensile testing and three-dimensional vibration testing, the tearing of seam stainless steel pipes mostly occurs far from the weld or in the heat-affected zone. This is because there are fewer impurities and slightly higher nitrogen content at the weld, resulting in better strength at the weld than in other areas. However, ASME (American Society of Mechanical Engineers) stipulates that seam stainless steel pipes can only withstand 85% of the allowable pressure, mainly because the data collection occurred before the use of improved welding equipment currently used.

ASME specifies that seam stainless steel pipes that pass 100% ultrasonic testing can fully withstand the allowable pressure. Similarly, Europe and Asia also specify that seam stainless steel pipes tested by eddy current testing can guarantee the high-quality welding performance, which requires legitimate procedures and testing by licensed institutions.

Corrosion Resistance:

The corrosion resistance depends on the alloy composition. Seam stainless steel pipes with the same chemical composition as seamless stainless steel pipes and those subjected to complete heat treatment have the same corrosion resistance. Supplementary tests provided by ASTM demonstrate that the corrosion resistance at the weld is equivalent to or better than that of the base metal. In acidic chloride environments, the corrosion at the weld of incompletely heat-treated seam steel pipes may accelerate, but this is only required for corrosion testing, and the actual environment is not so severe.

Bendability and Ductility:

The ductility of the weld can be verified by the following tests specified by ASTM: bend at 45°, then bend to 90°, and flatten along the weld; then reverse the seam stainless steel pipe and repeat the above steps to bend the weld to an inner diameter of 180°. The standard for the quality of the weld is that no tearing or intergranular separation is allowed under a 40x magnification. The minimum bend radius of the pipe is controlled by the alloy composition, generally with a minimum bend radius of 2d. The ideal welding condition is that the weld is in a neutral or compressive state. Additionally, the pipe should be annealed to reduce its hardness and improve its bendability.

Price:

The price of seam stainless steel pipes is usually only half that of seamless stainless steel pipes.

Wall Thickness/Diameter:

Thin-walled stainless steel pipes with a small wall thickness/diameter ratio are best produced by welding methods, while thick-walled pipes with a large wall thickness/diameter ratio are best made by punching methods.

Overall Quality:

The quality of seam stainless steel pipes is superior to that of seamless stainless steel pipes because seam stainless steel pipes are made from inspected cold-rolled plates, limiting any defects to the weld. Seamless pipes, on the other hand, are formed from stainless steel billets through head punching, resulting in many tearing phenomena in the pipe wall formed by extrusion. Eddy current testing shows that the defect rate of seam stainless steel pipes is usually lower than that of seamless stainless steel pipes. With ultrasonic testing, the background noise of seamless stainless steel pipes is so high that it is difficult to detect defects, whereas the background noise of seam stainless steel pipes is low, making it easy to find defects.