There are four main methods to distinguish welded steel pipe from seamless steel pipe:
1. Metallographic method
Metallographic method is one of the main methods to distinguish welded steel pipe from seamless steel pipe. High frequency resistance welded steel pipe does not add welding material, so the weld of welded steel pipe is very narrow, if the method of rough grinding and corrosion is adopted, the weld can not be clearly seen. Once the high frequency resistance welded steel pipe is welded and not heat treated, the weld structure will be fundamentally different from the steel pipe parent material. At this time, metallographic method can be used to distinguish welded steel pipe and seamless steel pipe. In the process of distinguishing the two kinds of steel pipes, it is necessary to cut small samples with length and width of 40mm at the weld, roughly grind, fine grind and polish them, and then put them under metallographic microscope to observe the structure. When ferrite and widmanstatten, base metal and weld zone structure are observed, welded steel pipe and seamless steel pipe can be distinguished accurately.
2. Corrosion method
In the process of distinguishing welded steel pipe from seamless steel pipe by corrosion method, the weld seam of welded steel pipe should be polished. After polishing, the polished trace should be seen. Then the end face should be polished with sandpaper at the weld seam, and the end face should be treated with 5% nitric acid alcohol solution. If there is an obvious weld seam, the end face should be polished with sandpaper at the weld seam. It can be proved that this steel pipe is welded steel pipe. There is no significant difference in the end face of seamless steel tube after corrosion.
Performance of Welded Steel Pipe
Welded steel pipe has the following properties because it is processed by high frequency welding and cold rolling.
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.
Secondly, it is waterproof and corrosion-resistant. In the process of Engineering construction, it is not necessary to install pipe trenches alone, but to bury the pipe directly underground or underwater, which reduces the construction difficulty of the project.
Thirdly, it has impact resistance. Even at low temperature, the steel pipe will not be damaged, so its performance has certain advantages.
Performance of Seamless Steel Pipe
Because of the high tensile strength of the seamless steel tube metal material, its anti-destruction ability is stronger, and it has hollow passage, so it can effectively transport fluid, it is its strong transport ability, so seamless steel tube has higher corrosion resistance than welded steel tube, and its stiffness is relatively large. Therefore, the more load the seamless steel tube bears, so it can be widely used in projects with higher construction requirements.
3. Identify welded and seamless steel tubes according to process
In the process of distinguishing welded steel pipe and seamless steel pipe according to technology, all welded steel pipe is welded according to cold rolling and extrusion. In addition, spiral pipe welding and straight seam pipe welding will be formed when high frequency and low frequency arc welding pipe and resistance welding pipe are used to weld steel pipe, and circular and square pipe will be formed. Elliptical steel pipe, triangular steel pipe, hexagonal steel pipe, rhombic steel pipe, octagonal steel pipe, and even more complex steel pipe. In a word, different shapes of steel pipes will be formed by different processes, so that welded steel pipes and seamless steel pipes can be distinguished clearly. However, in the process of identifying seamless steel pipes according to technology, it is mainly based on hot rolling and cold rolling treatment, and seamless steel pipes also have two main forms, namely hot-rolled seamless steel pipes and cold-rolled seamless steel pipes. Hot-rolled seamless steel pipes are formed by piercing and rolling, especially large diameter and thickness seamless steel pipes are welded by this process; cold-drawn pipes are cold-drawn by tube blanks, which have lower strength, but smooth external and internal control surfaces.
4. Distinguishing welded and seamless steel pipes by use
Welded steel pipe has higher flexural and torsional strength and more load-bearing capacity, so it is widely used in the manufacture 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 pipe. However, seamless steel pipe can be used as a pipeline for conveying fluids because of its hollow section and long steel without joints around it. For example, it can be used as a pipeline to transport oil, natural gas, gas, water and so on. In addition, the bending strength of seamless steel tube is relatively small, so it is widely used in superheated steam pipes of low and medium pressure boilers, boiling pipes and superheated steam pipes for locomotive boilers. In a word, welded steel pipe and seamless steel pipe can be clearly distinguished by the classification of their uses.