1. Welded pipes are round pipes welded by steel plates, which are divided into high frequency resistance welded pipes (ERW welded pipes), straight seam arc welded pipes (LSAW), and spiral welded pipes.
It is indicated by "SC" in electrical engineering. It can be used as a water gas pipe or as a threading pipe, which is relatively thick.
2. The line pipe is also the wire pipe, which is relatively thin, indicated by "T" and can only be used for threading pipe.
3. The ERW pipe is a "high frequency resistance welded steel pipe", which is different from the ordinary welded pipe welding process. The weld is formed by melting the base material of the steel strip body, and the mechanical strength is better than that of the general welded pipe.
ERW longitudinally welded pipes are generally used as wire casings in the power industry. Performance characteristics: 100% ultrasonic testing of the base material ensures the internal quality of the pipe body; no unwinding-disc shearing process, base material pressure pit , Less scratches; After the stress is eliminated, the finished pipe basically has no residual stress; the weld seam is short, and the probability of defects is small; it can conditionally transport moist sour natural gas; after the diameter is expanded, the geometrical size of the steel pipe is accurate; welding It is carried out in a straight line in the horizontal position after the forming is completed. Therefore, the control of staggered edges, slits, pipe diameter and circumference is better, and the welding quality is excellent.
The biggest difference between ERW steel pipe and seamless steel pipe is that ERW has a weld, which is also the key to the quality of ERW steel pipe. Modern ERW steel pipe production technology and equipment, due to years of unremitting efforts internationally, especially in the United States, has made the seamlessness of ERW steel pipes a relatively satisfactory solution.
Some people divide the seamlessness of ERW steel pipes into geometric seamlessness and physical seamlessness. Geometric seamlessness is to remove the internal and external burrs of the ERW steel pipe. Due to the continuous improvement and perfection of the structure of the internal burr removal system and the cutting tools, the removal of burrs in steel pipes with large and medium diameters has been well handled. The internal burr can be controlled within -0.2mm～0.5mm.
Physical seamlessness refers to the difference between the metallographic structure and the base metal inside the weld, which causes the mechanical properties of the weld area to decrease, and measures need to be taken to make it uniform and consistent. The high-frequency welding thermal process of ERW steel pipe creates a temperature distribution gradient near the edge of the tube blank, and forms characteristic areas such as melting zone, semi-melting zone, overheated structure, normalizing zone, incomplete normalizing zone, and tempering zone. Among them, the superheated zone structure due to the welding temperature above 1000 ℃, the austenite grains grow rapidly, and the hard and brittle coarse crystal phase will be formed under cooling conditions. In addition, the existence of temperature gradient will cause welding stress. In this way, the mechanical properties of the weld area are lower than that of the base metal. The physical seamlessness is to heat the weld area to AC3 (927°C) with an intermediate frequency induction heating device through the local conventional heat treatment process of the weld, and then perform 60m The length and speed of the air cooling process are 20m/min, and then water cooling is needed when necessary. The use of this method achieves the purpose of eliminating stress, softening and refining the structure, and improving the comprehensive mechanical properties of the welding heat affected zone. So far, the world's advanced ERW units have generally adopted this method to process welds and have achieved better results. The high-quality ERW steel pipe not only cannot distinguish the weld, but also the weld coefficient reaches 1, realizing the match between the weld area and the base material.