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. With the modern ERW steel pipe production process and equipment, due to years of unremitting efforts in the world, especially in the United States, the seamless ERW steel pipe has been satisfactorily solved. Some people divide the seamless ERW steel pipe into geometric seamless and physical seamless. Geometric seamless is to remove internal and external burrs of ERW steel pipe.
Due to the continuous improvement and perfection of the structure of the internal burr removal system and the cutting tools, the internal burr removal of large and medium-sized steel pipes has been well handled. The internal burr can be controlled within -0.2mm ~ o.5mm. Physical seamless refers to the difference between the metallographic structure in the weld and the base metal, which leads to the decline of the mechanical properties in the weld area. Measures should be taken to make it uniform and uniform.
The high-frequency welding thermal process of ERW steel tube causes the temperature distribution gradient near the edge of the tube blank, and forms the characteristic areas such as melting zone, semi melting zone, overheating structure, normalizing zone, incomplete normalizing zone, tempering zone, etc. Because the welding temperature is above 1000 ℃, the austenite grain grows rapidly, and hard and brittle coarse-grained phase will be formed under cooling conditions. In addition, the existence of temperature gradient will produce welding stress. In this way, the mechanical properties of the weld area are lower than that of the base metal. Physical seamless is to heat the weld area to AC3 (927 ℃) by using the medium frequency induction heating device through the local conventional heat treatment process of the weld, and then conduct an air cooling process with a length of 60m and a speed of 20m/ min, and then water cooling when necessary. This method can eliminate stress, soften and refine microstructure, and improve the comprehensive mechanical properties of HAZ. So far, the advanced ERW units in the world have generally adopted this method to treat the welds, and have achieved good results.
The high-quality ERW steel pipe can not distinguish the weld, and the weld coefficient reaches 1, which realizes the matching between the weld zone structure and the base metal.
ERW steel pipe has the advantages of using hot rolled coil as raw material, uniform wall thickness can be controlled at about ± 0.2mm, two ends of steel pipe shall be repaired and beveled according to American APL standard or gb/t9711.1 standard, and delivery in fixed length. In recent years, various natural gas pipeline network projects and gas companies have widely adopted ERW steel pipe as the main steel pipe of urban pipeline network.