Common steel pipes mainly include welded pipes and seamless pipes (SMLS). In the process of engineering construction, appropriate steel pipes will be selected in combination with specific construction conditions, and cost will be the primary factor. Therefore, this paper distinguishes welded steel pipes and seamless steel pipes from the aspects of steel pipe performance, technology and identification methods, hoping to provide reference for the selection of steel pipes in the process of engineering construction.
With the continuous advancement of science and technology, my country's steel pipe production process has also been significantly improved. At present, there are two main forms of common 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, in order to seek more economic benefits, some merchants process welded steel pipes, and then pretend to be seamless steel pipes, and then sell them into the market, causing market confusion. The processed welded steel pipe cannot be effectively distinguished from the appearance. Once in the construction process, the seamless steel pipe is used but the welded steel pipe is used, which will cause gas or liquid leakage. In severe cases, it is easy to cause an explosion. Therefore, it is necessary to identify the welded steel pipe. Steel pipe and seamless steel pipe, so as to ensure the quality of the project.
Welded steel pipes are generally formed by deforming steel strips or steel plates into round or square shapes under the action of external force, and then welding the seams on the surface to form welded steel pipes. The commonly used processes include arc welding, high frequency or low frequency, cold rolling, etc., but welding with these processes will lead to weld seams in steel pipes. At present, according to the shape of the weld, it can be divided into straight seam and spiral welded steel pipe, but the processes used by the two are different. The 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. Since the AC charge will cause proximity and turbine heating effects in the conductor, it can achieve the melting of the steel at the edge of the weld and finally complete the welding.
The seamless steel pipe is mainly to perforate a solid tube billet or steel ingot to make a capillary pipe, and then process it through hot rolling, cold rolling, cold drawing, etc., and then form a seamless steel pipe. Since the seamless steel pipe contains a hollow section, this hollow section can be used as a pipe for conveying fluid.
There are three main ways to identify welded steel pipes and seamless steel pipes:
1. Metallographic method
Metallographic method is one of the main methods to distinguish welded steel pipes and seamless steel pipes. High-frequency resistance welded pipe (ERW welded pipe) does not add welding materials, so the weld seam in the welded steel pipe is very narrow, and the weld seam cannot be clearly seen if the method of rough grinding and corrosion 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 identifying the two steel pipes, it is necessary to cut a small sample with a length and width of 40 mm at the welding point, conduct rough grinding, fine grinding and polishing on it, and then observe the structure under a metallographic microscope. Welded steel pipes and seamless steel pipes can be accurately distinguished when ferrite and widmansite, base metal and weld zone microstructures are observed.
2. Corrosion method
In the process of using the corrosion method to identify welded steel pipes and seamless steel pipes, the welded seam of the processed welded steel pipe should be polished. After the grinding is completed, the traces of grinding should be visible, and then the end face of the welded seam should be polished with sandpaper. And use 5% nitric acid alcohol solution to treat the end face. If there is an obvious weld, 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.
The performance of welded steel pipe
Welded steel pipe has 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 resistance. In the process of engineering construction, it is not necessary to set up pipe trenches separately.
It can be directly buried in 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 fluid. 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. Distinguish 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. When the steel pipe is welded, it will form a spiral welded pipe and a straight seam welded pipe, and will form a round steel pipe, a square steel pipe, an oval steel pipe, a triangular steel pipe, a hexagonal steel pipe, a rhombus steel pipe, an octagonal steel pipe, and even a more complex 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, 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 mainly two types of seamless steel pipes, which are divided into 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. Classify by use
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, we can clearly distinguish welded steel pipes and seamless steel pipes.
In order to ensure the higher quality of engineering construction in our country, 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 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 identify them. At the same time, the two kinds of steel pipes can also be distinguished according to their performance and use, and then a reasonable steel pipe can be selected for construction.
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