Distinguishing method of welded steel pipe and seamless steel pipe

1. Metallographic method: Metallographic method is the main method to distinguish welded steel pipes and seamless steel pipes. There is no welding material added to the high-frequency resistance welded steel pipe, so the weld seam in the welded steel pipe is very narrow, and the weld seam cannot be seen if the rough grinding and corrosion method is used. Once the high-frequency resistance welded steel pipe is welded without heat treatment, the structure of the weld seam will be essentially different from the parent material of the steel pipe. 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 weld, conduct rough grinding, fine grinding, and polishing on it, and then put it under a metallographic microscope to observe the structure. Welded steel pipes and seamless steel pipes can be accurately distinguished when ferrite and widmansite, the structure of the base metal, and the weld zone are observed.

2. Corrosion method: In the process of using the corrosion method to distinguish welded steel pipes from seamless steel pipes, grinding should be carried out at the weld seam of the processed welded steel pipe. Polish the end face with sandpaper, and treat the end face with a 5% nitric acid alcohol solution. If there is an obvious weld seam, it can prove that the steel pipe is a welded steel pipe. However, the end face of the seamless steel pipe has no obvious difference after being corroded.
Performance of welded steel pipes: Welded steel pipes have the following properties due to high-frequency welding, cold rolling, and other processes.
First, the heat preservation function is good. The heat loss of welded steel pipes is relatively small, only 25%, which is not only conducive to transportation but also reduces costs.
Second, it has waterproof and corrosion-resistant. In the process of engineering construction, it is not necessary to set up pipe trenches separately. It is only necessary to bury the steel pipes directly into the ground or underwater, thereby reducing the construction difficulty of the project.
Third, it has impact resistance. Even in a low-temperature environment, the steel pipe will not be damaged, so its performance has certain advantages.
The performance of seamless steel pipe: due to the high tensile strength of the metal material of the seamless steel pipe, its ability to resist damage is stronger, and it has a hollow channel, so it can effectively transport fluids. The corrosion resistance is higher than that of welded steel pipe, and its rigidity is relatively large. Therefore, the more load the seamless steel pipe can carry, it can be widely used in projects with higher construction requirements.

3. Identify welded steel pipes and seamless steel pipes according to the process: In the process of identifying 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, high-frequency and low-frequency arcs are used When welding steel pipes by welded pipe and electric resistance welded pipe technology, spiral pipe welding and straight seam pipe welding will be formed, and round steel pipes, square steel pipes, oval steel pipes, triangular steel pipes, hexagonal steel pipes, rhombus steel pipes, octagonal steel pipes, and even steel pipes will be formed. It is a more complicated steel pipe. In short, different processes will form steel pipes of different shapes, so that welded steel pipes and seamless steel pipes can be clearly distinguished. However, the process of identifying seamless steel pipes according to the process is mainly based on hot-rolled and cold-rolled processing methods, and there are mainly two forms of seamless steel pipes, namely hot-rolled seamless steel pipes and cold-rolled seamless steel pipes. Hot-rolled seamless steel pipes are formed by piercing, rolling, and other processes, especially large-diameter and thick seamless steel pipes are welded by this process; cold-drawn pipes are formed by cold-drawing tube blanks, and the strength of the material is lower, but its exterior and internal control surfaces are smooth.

4. Distinguish welded steel pipes and seamless steel pipes according to their usage: Welded steel pipes have higher bending and torsional strength and more load-bearing capacity, so they are generally widely used in the manufacture of mechanical parts. For example, oil drill pipes, automobile drive shafts, bicycle frames, and steel scaffolding used in building construction are all made of welded steel pipes. However, seamless steel pipes can be used as pipes for conveying fluids because they have hollow sections and long strips of steel without seams around them. For example, it can be used as a pipeline for transporting oil, natural gas, gas, water, etc. In addition, the bending strength of seamless steel pipe is relatively small, so it is generally widely used in superheated steam pipes for low and medium-pressure boilers, boiling water pipes, and superheated steam pipes for locomotive boilers. In short, through the classification of uses, welded steel pipes and seamless steel pipes can be clearly distinguished.

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