Straight seam electric resistance welded steel pipe (ERW) and spiral submerged arc welded steel pipe (SSAW) can better meet the requirements of oil and natural gas transmission pipelines. For steel pipes of medium caliber (Ф630mm), ERW steel pipes are mostly used abroad. Spiral submerged arc welded pipes are generally only used for low-pressure fluid transmission, civil buildings, structures and pile-driving pipes. At present, China's current long-distance pipelines mainly use SSAW steel pipes. This is mainly because there is no ERW steel pipe in China, so the design of steel pipes is limited. Secondly, it is related to the understanding of ERW steel pipes. However, the widespread use of medium-caliber ERW steel pipes in the world, especially on the submarine pipelines, has fully demonstrated the advantages of ERW steel pipes. The promotion of the use of ERW steel pipes in medium-caliber oil and gas pipelines in China is a trend in the future.
Electric-Resistance-Welded steel tubes are referred to as ERW steel tubes, which have many advantages such as high productivity, low cost, high dimensional accuracy and beautiful appearance. In recent years, countries have been actively studying the use of ERW steel pipes instead of seamless steel pipes and spiral seam welded steel pipes. In industrialized countries, the diameter is below 610mm, and the proportion of ERW has reached about 70%. In the past, due to the poor reliability of the weld, there was a great limitation in the scope of use. In the past 10 years, with the improvement of production technology and the continuous improvement of product quality, the weld quality of ERW steel pipes can be compared with UOE steel pipes. Therefore, it is widely used in oil and gas pipelines under different geographical conditions and harsh conditions.
1. ERW steel pipe features
(1) The research on the welding mechanism of ERW steel pipe has made gratifying achievements, and the optimization of the welding process is guided by theory. The computer control of all the optimization parameters is realized, and the weld quality is improved.
(2) Weld line annealing adopts the fluorescent line positioning technology to make the annealing strictly in the specified part, so that the weld quality is further improved.
(3) The FATT value of the ductile and brittle turning temperature of the weld zone of the early ERW steel pipe is much higher than that of the parent metal, and the highest difference is 40-50 degrees. Due to the advancement of metallurgical technology, the FATT value of the welded pipe base material can be as low as about -70-80 degrees, so that even if there is a certain difference between the two, the user's requirements can be satisfied.
(4) Due to the application of automation and ultrasonic testing technology, the ultrasonic automatic tracking online detection can be realized, so that the probability of the weld seam forming a defect due to missed detection is further reduced.
2. Comparison of characteristics between ERW steel pipe and spiral submerged arc welded steel pipe
Because the forming, welding and process of ERW steel pipe and spiral submerged arc welded pipe are different in the pipe making process, there are certain differences in the performance of the two steel pipes.
(1) The surface quality of ERW steel pipe is better than that of spiral submerged arc welded pipe. The surface of ERW steel pipe is smooth, flat and beautiful. The spiral submerged arc welded pipe has the inner and outer weld seam height.
(2) The weld of ERW steel pipe is shorter than that of spiral submerged arc welded pipe. The weld of ERW steel pipe is as long as the length of steel pipe, and the length of weld of spiral submerged arc welded pipe is generally 1.4-1.7 times of the length of steel pipe. The longer the weld, the higher the chance of weld defects.
(3) The geometrical requirements for the ERW steel pipe and the spiral submerged arc welded pipe are basically the same in the standard, but in terms of the physical object, the ERW steel pipe has a sizing and straightening process, and therefore, the geometrical dimension is higher than that of the spiral submerged arc welded pipe.
(4) ERW steel pipe weld defect type is less than spiral submerged arc welded pipe. The spiral submerged arc welded pipe is coated and welded, and there are many types of defects. In addition to the volume defects, there are line defects. ERW steel pipe welds only have line defects, and the welds are straight, easy to track during the inspection process, and the defects are at the maximum reflection. Once the defect occurs, the ultrasonic reflection is strong and easy to detect.
(5) The residual stress of ERW steel pipe is lower than that of spiral submerged arc welded pipe. Since the deformation of the ERW steel pipe during the deformation process is relatively sufficient, and the subsequent sizing and ellipsoidal changes and the diameter reduction, the residual stress of the ERW steel pipe becomes small. The physical inspection shows that the axial and lateral incision displacements of the ERW steel tubes of the same specification and the same material are smaller than those of the spiral submerged arc welded tubes.
(6) The overall performance of the ERW steel pipe is consistent. When the steel pipe is defective, the steel pipe is rejected, and the repair welding is not allowed. The spiral submerged arc welded pipe allows for repair welding, and the performance of the repair welding is inconsistent with the overall performance of the steel pipe, and the stress corrosion cracking is prone to occur in the repair welding. The experiment proves that the experimental life of stress corrosion fatigue of ERW steel pipe is longer than that of spiral submerged arc welded pipe.
(7) The quality of ERW steel pipe coating is higher than that of spiral submerged arc welded pipe.
ERW production units have large investment and many processes, and the processing cost of steel pipes is high. Since the ERW steel pipe is not repaired, the head pipe is not allowed, and the relative yield is low. Therefore, the direct price of the ERW steel pipe is about 300 yuan / T higher than the spiral submerged arc welded steel pipe. The overall cost of the board from the pipeline construction does not increase. First, the ERW steel pipe has good quality and long service life. Secondly, the ERW steel pipe has good surface quality, uniform coating, saving a large amount of coating materials, and low cost of steel pipe anti-corrosion. Third, the ERW steel pipe has high geometric dimensional accuracy, fast butt welding speed, and on-site construction. High efficiency, in addition to low spinning costs.