Common steel pipes mainly include welded steel pipes and seamless steel pipes. During the construction process, suitable steel pipes will be selected according to the specific construction conditions, and the cost will be the primary factor. Therefore, this article gives you an idea of distinguishing between welded steel pipes and seamless steel pipes from the aspects of performance and technology of steel pipes.

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 common steel pipes, namely welded steel pipes and seamless steel pipes. Because the cost of welded steel pipes is significantly lower than that of seamless steel pipes, some factories process welded steel pipes to replace seamless steel pipes in order to obtain more economic benefits. The welded steel pipe is sold as the seamless steel pipe on the market, making the market being in disorder. We can’t distinguish the processed welded steel pipe from the outside. Once the welded steel pipe is used as the seamless steel pipe in engineering construction, it will cause gas or liquid leakage. In severe cases, it is easy to cause explosions. Therefore, the two pipes must be distinguished from each other to ensure engineering quality.

A steel strip or steel plate is deformed into a circle or a square under the action of external force, and then the seams on the surface are welded to form a welded steel pipe. Commonly used processes include arc welding, high frequency or low frequency, cold rolling, etc. However, using these processes for welding will cause welding seams. At present, welded steel pipes can be divided into spiral welded steel pipes and straight seam welded steel pipes based on the welding shape, but their processes are different. The spiral welded 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. Due to the proximity and turbine heating effects of AC charges in the conductor, the steel at the edge of the welded seam can be melted and the welding is finally completed.

Solid tube billets or steel ingots are perforated, making hollow billets. Hollow billets become seamless steel pipes through processing such as hot rolling, cold rolling and cold-drawing. The seamless steel pipe contains a hollow section, which can be used for conveying fluid.

There are three main methods for distinguishing between welded steel pipes and seamless steel pipes.
The metallographic method
The metallographic method is one of the main methods to distinguish between welded steel pipes and seamless steel pipes. High-frequency resistance welded steel pipes do not add welding materials, so the welded seam in the welded steel pipes is very narrow. The welded seam cannot be clearly seen if the method of rough grinding and corrosion is used. Once the steel pipe is welded by high-frequency resistance welding without heat treatment, the welded structure will be essentially different from the steel pipe parent material. At this time, the metallographic method can be used to distinguish between welded steel pipes and seamless steel pipes. In the process of distinguishing between the two steel pipes, it is necessary to cut a sample with a length and width of 40mm at the welding part, and rough grind, fine grind and polish the sample. Then place the sample under a metallurgical microscope to observe the structure. When the structure of ferrite, the base metal or structure of the weld zone is observed, the welded steel pipe can be distinguished from the seamless steel pipe.

The corrosion method
In the process of distinguishing between welded steel pipes and seamless steel pipes by the corrosion method, the welded seam of the welded steel pipe should be polished. The polished traces should be visible after the polishing is done. Use a sandpaper to grind the end face and then use 5% nitric acid alcohol solution to treat the end face. If there is an obvious welded seam, it can be proved that the steel pipe is a welded steel pipe. There is no obvious difference at the end faces of seamless steel pipes after corrosion.