Methods for correctly distinguishing welded steel pipe and seamless steel pipe

Common steel pipes mainly include welded steel pipes and seamless steel pipes. During the project construction, appropriate steel pipes will be selected according to the specific construction conditions, and cost will be the primary factor. Therefore, this paper distinguishes welded steel pipe and seamless steel pipe from the performance, process and identification method of steel pipe, hoping to provide reference for the selection of steel pipe in the process of engineering construction.

With the continuous progress of science and technology, China’s steel pipe production process has also been significantly improved. At present, there are two main types of steel pipes, namely, welded steel pipes and seamless steel pipes. Because the cost of welded steel pipes is obviously lower than that of seamless steel pipes, some businesses, in order to seek more economic benefits, process welded steel pipes, and then pretend to be seamless steel pipes, and then sell them into the market, causing market confusion. The welded steel pipes processed can not be effectively identified from the appearance. Once welded steel pipes are used when seamless steel pipes should be used in the construction process, gas or liquid leakage will occur, and even explosion may occur in serious cases. Therefore, welded steel pipes and seamless steel pipes must be identified to ensure project quality.

1. Meaning of welded steel pipe and seamless steel pipe

1.1 Welded steel pipe

Welded steel pipes are generally made of steel strip or steel plate into a round or square shape under the action of external force, and then the joints on the surface are welded to form welded steel pipes. The commonly used processes are electric arc welding, high frequency or low frequency, cold rolling, etc., but welding with these processes will lead to welded pipes. At present, according to the weld shape, it can be divided into straight seam and spiral welded steel pipe, but the processes adopted by the two are different. Spiral steel pipe is welded by submerged arc welding, which has the advantages of high welding efficiency and less dust. For high-frequency or low-frequency welding, it is mainly based on the principle of electromagnetic induction. Because the AC charge will have proximity and turbine thermal effects in the conductor, it can achieve the fusion of the steel at the weld edge and finally complete the welding.

1.2 Seamless steel pipe

Seamless steel pipes are mainly formed by piercing solid billets or ingots into tubes, and then by hot rolling and cold rolling. The seamless steel pipe contains a mesopore section, so the hollow section can be used as a pipe for conveying fluid.

2. Identification method of welded steel pipe and seamless steel pipe

2.1 Metallographic method

Metallographic method is one of the main methods to distinguish welded steel pipe and seamless steel pipe. There is no welding material added to the high-frequency resistance welded steel pipe, so the weld in the welded steel pipe is very narrow. If the method of rough grinding and re corrosion is adopted, the weld cannot be clearly seen. Once the high-frequency resistance welded steel pipe is welded without heat treatment, the weld structure will be essentially different from the steel pipe parent material. At this time, the metallographic method can be used to distinguish the welded steel pipe from the seamless steel pipe. In the process of distinguishing the two steel pipes, it is necessary to cut a small sample with a length and width of 40mm at the welding point, carry out rough grinding, fine grinding and polishing, and then put it under the metallographic microscope to observe the structure. When ferrite and widmanstatten, parent metal and weld zone are observed, welded steel pipes and seamless steel pipes can be accurately distinguished [1].

2.2 Corrosion method

In the process of distinguishing welded steel pipe and seamless steel pipe by corrosion method, the weld seam of the processed welded steel pipe shall be polished. After polishing, the polished trace shall be visible. Then, the end face shall be polished with sandpaper at the weld seam, and the end face shall be treated with 5% nitric acid alcohol solution. If there is an obvious weld, it can be proved that the steel pipe is a welded steel pipe. The end faces of seamless steel pipes have no obvious difference after corrosion.

3. Performance of welded steel pipe and seamless steel pipe

3.1 Properties of welded steel pipes

The welded steel pipe has the following properties because it is processed by high-frequency welding, cold rolling and other processes. First, the thermal insulation function is good. The heat loss of welded steel pipe is relatively small, only 25%, which is not only conducive to transportation, but also reduces the cost. Second, it is waterproof and corrosion-resistant. In the process of project construction, it is not necessary to set pipe ditches separately, but to directly bury steel pipes underground or underwater, thus reducing the construction difficulty of the project. Third, it has impact resistance. Even in the low temperature environment, the steel pipe will not be damaged, so its performance has certain advantages.

3.2 Seamless steel pipe

Because the tensile strength of the metal material of seamless steel pipe is relatively high, its anti destruction ability is stronger, and it has a hollow channel, so it can effectively transport fluid. It is precisely because of its strong transmission capacity. Therefore, the corrosion resistance of seamless steel pipe is higher than that of welded steel pipe, and its stiffness is relatively high [2]. Therefore, seamless steel pipes can be widely used in projects with high construction requirements due to the more loads they carry.

4. Distinguish welded steel pipe and seamless steel pipe according to process

In the process of distinguishing welded steel pipes and seamless steel pipes according to the process, welded steel pipes are welded according to cold rolling, extrusion and other processes. In addition, spiral pipe welding and straight seam pipe welding will be formed when steel pipes are welded by high-frequency, low-frequency arc welding and resistance welding processes, and circular steel pipes, square steel pipes, elliptical steel pipes, triangular steel pipes, hexagonal steel pipes, rhombic steel pipes, octagonal steel pipes will be formed, Even more complex steel pipes. In a word, different processes will form different shapes of steel pipes, so that welded steel pipes and seamless steel pipes can be clearly distinguished. However, in the process of identifying seamless steel pipes according to the process, it is mainly based on the hot rolling and cold rolling treatment methods. There are also two main forms of seamless steel pipes, namely, hot rolling seamless steel pipes and cold rolling seamless steel pipes [3]. Hot rolled seamless steel tubes are formed by piercing, rolling and other processes, especially for large diameter and thick seamless steel tubes; Cold drawn tubes are formed by cold drawing with tube blanks. The strength of the material is lower, but the surface and internal control surface are smooth.

5. Distinguish between welded steel pipes and seamless steel pipes according to their uses

Welded steel pipes have higher bending and torsional strength and more sufficient bearing capacity, so they are generally widely used in the manufacturing of mechanical parts. For example, oil drill pipe, automobile transmission shaft, bicycle frame and steel scaffolding used in construction are all made of welded steel pipes. However, the seamless steel pipe can be used as a pipeline to transport fluid because of its hollow section and long steel bars without joints around it [4]. For example, it can be used as a pipeline to transport oil, natural gas, gas, water, etc. In addition, the bending strength of seamless steel pipes is relatively small, so they are generally used for superheated steam pipes of low and medium pressure boilers, boiling water pipes and superheated steam pipes for locomotive boilers. In a word, welded steel pipes and seamless steel pipes can be clearly distinguished by the classification of their uses.

6. Conclusion

In order to ensure higher engineering construction quality in China, it is necessary to distinguish between welded steel pipes and seamless steel pipes, and then use appropriate steel pipes for construction. In the process of identifying welded steel pipe and seamless steel pipe, metallographic method and corrosion method can be used to inspect the weld by observing the structure and weld, and finally identify them accurately. At the same time, the two types of steel pipes can also be distinguished according to their properties and uses, and then reasonable steel pipes can be selected for construction.

Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, and Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels, and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]