What is the difference between welded and seamless stainless steel pipes

01. Coaxiality 

The manufacturing process of seamless pipes involves punching a hole in a stainless steel billet at a temperature of 2200°F. At this high temperature, the tool steel is deformed through punching and stretching, becoming soft and spirally shaped and emerging from the hole. As a result, the wall thickness of the pipes is uneven and the eccentricity is high. Therefore, ASTM allows the wall thickness difference of seamless pipes to be larger than that of welded pipes. Welded pipes are made from precisely rolled cold-rolled sheets (each roll is 4-5 feet wide). These cold-rolled sheets usually have a maximum wall thickness difference of 0.05mm. The steel plates are cut to a width of πd, where d is the outer diameter of the pipe. The tolerance of the wall thickness of welded pipes is very small, and the wall thickness throughout the circumference is very uniform. 

02. Welding Performance 

Generally, there are certain differences in the chemical composition between welded pipes and seamless pipes. The steel used to produce seamless pipes merely meets the basic requirements of ASTM. However, the steel used to produce welded pipes contains chemical components suitable for welding. For example, silicon, sulfur, manganese, oxygen, and elements such as ferrite can be mixed in certain proportions. This mixture can produce a weld slag that is easy to conduct heat during the welding process, allowing the entire weld to penetrate. Pipes without these chemical components, such as seamless pipes, will generate various unstable factors during the welding process, making it difficult to weld firmly and not penetrating the weld. 

03. Grain Size 

The size of the metal grains is related to the heat treatment temperature and the duration of maintaining the same temperature. The grain size of both the annealed welded stainless steel tubes and the seamless stainless steel tubes is the same. If the welded tubes undergo the minimum cold treatment, the grain size of the weld seam is smaller than that of the welded metal, otherwise, the grain size is the same. 

04. Pipeline Strength 

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

After conducting tensile tests and three-dimensional vibration tests, the tearing of the seamless stainless steel pipe almost always occurred in areas far from the welding point or the heated zone. This is because the weld seam has less impurities and a slightly higher nitrogen content, so the strength at the weld is better than at other parts. However, ASME (American Society of Mechanical Engineers) believes that seamless stainless steel pipes can only withstand 85% of the permitted pressure. This is mainly due to the fact that the data was collected earlier than the improved welding equipment currently in use. 

According to ASME regulations, seamless stainless steel pipes that pass 100% ultrasonic testing can fully withstand the permitted pressure. Similarly, both Europe and Asia stipulate that seamless stainless steel pipes that pass eddy current testing can ensure their excellent welding performance. The eddy current testing must be conducted through legal procedures and by institutions holding licenses. 

05. Corrosion Resistance 

The corrosion resistance is also dependent on the composition of the alloy. Seamless stainless steel pipes with the same chemical composition and welded stainless steel pipes that have undergone complete heat treatment have the same corrosion resistance. Supplementary tests provided by ASTM prove that the corrosion resistance at the welding joint is equivalent or better than the welded metal. In an acidic chloride environment, the corrosion at the welded joints of the unheated welded steel pipes will accelerate, but this is only for the purpose of the corrosion test. In fact, the actual environment is not so harsh. 

06. Flexibility and Extensibility 

The extensibility of the weld can be verified through the following tests specified by ASTM: bend at 45°, then bend to 90°, and then flatten along the weld seam; then turn the stainless steel seamless pipe around and repeat the above steps to make the inner diameter of the weld bend reach 180°. The weld quality is in accordance with the standard that no tearing or intergranular separation is allowed when magnified 40 times. 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 conditions are that the weld is in a neutral or compressed state. Moreover, the pipe should undergo annealing treatment to reduce its hardness, thereby improving the bending performance. 

07. Wall thickness/diameter 

For thin-walled stainless steel pipes with small thickness/diameter values, the welding method is the best choice for production; while for thick-walled pipes with large thickness/diameter values, the stamping method is the most suitable. 

08. Comprehensive Quality 

Generally, welded stainless steel pipes have better quality than seamless stainless steel pipes. This is because welded stainless steel pipes are made from precisely cold-rolled sheets that have been inspected and approved, so any defects are confined to the welding points. Seamless pipes, on the other hand, are formed by stamping stainless steel blanks with punches, resulting in many tear-like phenomena in the tube walls formed by the extrusion process. Tests using eddy current technology show that the defect rate of welded stainless steel pipes is usually lower than that of seamless stainless steel pipes. When using ultrasonic testing, the background noise of seamless stainless steel pipes is very high, making it difficult to detect their defects. In contrast, the background noise of welded stainless steel pipes is very low, making it easier to locate the defect areas.