Common steel pipes mainly include welded steel pipes and seamless steel pipes. In the process of engineering construction, appropriate steel pipes will be selected according to the specific construction conditions, and the cost will be the primary factor. Therefore, this paper distinguishes welded steel pipes and seamless steel pipes from the aspects of performance, technology and identification methods of steel pipes, hoping to provide reference for the selection of steel pipes in the process of engineering construction.
With the continuous progress of science and technology, there are two main forms of common steel pipes, namely welded steel pipes and seamless steel pipes. Since the cost of welded steel pipes is significantly 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 flow them into the market for sales, causing market confusion. Welded steel pipes that are processed and processed from the outside cannot be effectively distinguished. Once seamless steel pipes are used in the construction process, welded steel pipes are used, which will cause gas or liquid leakage, and in severe cases, it is easy to cause explosions. Therefore, it is necessary to distinguish welding steel pipes. Steel pipes and seamless steel pipes, thus ensuring the quality of the project.
Welded steel pipes are generally formed by deforming steel strips or steel plates into a circle or square under the action of external force, and then welding the seams on the surface to form welded steel pipes. The commonly used processes are arc welding, high frequency or low frequency, cold rolling, etc., but welding by these processes will result in welds in the steel pipe. At present, according to the shape of the weld, it can be divided into straight seam and spiral welded steel pipes, but the processes used for 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. Due to the proximity and turbine thermal effects of AC charges in the conductor, the steel at the edge of the weld can be melted, and the welding is finally completed.
The seamless steel pipe is mainly to perforate the solid tube blank or steel ingot to make a capillary tube, and then process it through hot rolling, cold rolling, and cold drawing to form a seamless steel pipe. Since the seamless steel pipe contains a hollow section, this hollow section can be used as a pipeline for conveying fluids.
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 welded seam in the welded steel pipe is very narrow. If the method of rough grinding and re-corrosion is used, the weld seam cannot be clearly seen. Once the high-frequency resistance welded steel pipe is welded and has not undergone 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 and 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 40mm at the welding place, and perform rough grinding, fine grinding and polishing on it, and then put it under a metallographic microscope to observe the structure. Welded and seamless steel pipes can be accurately distinguished when ferrite and Widmanrite, base metal and weld zone structures are observed.
In the process of using the corrosion method to distinguish welded steel pipes and seamless steel pipes, grinding should be carried out on the welded seams of the processed welded steel pipes. After grinding, the grinding marks should be visible, and then the end faces should be polished with sandpaper at the welding seams. And treat the end face with 5% nitric acid alcohol solution, if there is an obvious weld, it can prove that the steel pipe is a welded steel pipe. However, there is no significant difference in the end face of the seamless steel pipe after corrosion.
1)Properties of welded steel pipes
Because the welded steel pipe is processed by high frequency welding, cold rolling and other processes, it has the following properties.
First, the thermal insulation function is good. The degree of heat loss of welded steel pipe 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 project construction, there is no need to set up a separate pipe trench, but the steel pipe can be directly buried underground or underwater, thus 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.
2)Properties of seamless steel pipes
Due to the high tensile strength of the metal material of the seamless steel pipe, its anti-destructive ability is stronger, and it has a hollow channel, so it can effectively transport fluid. It is its strong transport ability, so the corrosion resistance of seamless steel pipe is higher than that of welding. Steel pipe, and its rigidity is relatively large. Therefore, the more loads the seamless steel pipe can carry, it can be widely used in projects with higher construction requirements.
In the process of identifying welded steel pipes and seamless steel pipes according to the process, all welded steel pipes are welded according to cold rolling, extrusion and other processes. Spiral pipe welding and straight seam pipe welding will form round steel pipes, square steel pipes, oval steel pipes, triangular steel pipes, hexagonal steel pipes, diamond steel pipes, octagonal steel pipes, and even more complex steel pipes. In short, the use of 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 distinguished according to the hot-rolled and cold-rolled processing methods, and there are two main 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 perforating, rolling and other processes, especially large-diameter and thick seamless steel pipes are welded by this process; cold-drawn pipes are cold-drawn with tube blanks, and the strength of the material is lower. , but its exterior and interior control surfaces are smooth.
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 construction are all made of welded steel pipes. However, seamless steel pipes can be used as pipes for conveying fluids because they have a hollow cross-section and are long strips of steel without joints 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.
In order to ensure a higher quality of construction, it is necessary to distinguish between welded steel pipes and seamless steel pipes, and then use suitable steel pipes for construction. In the process of identifying 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 identify them accurately. At the same time, two kinds of steel pipes can be distinguished according to their properties and uses, and then a reasonable steel pipe can be selected for construction.