The welded pipe is divided into two types: seamless welded pipe and welded welded pipe according to its manufacturing method. Seamless welded pipes are made of high-quality carbon steel or alloy steel, which are hot rolled and cold rolled (drawn). The welded welded pipe is formed by welding a steel plate rolled into a tubular shape by a seam or a spiral seam. In terms of the manufacturing method, it is further divided into a welded pipe for low-pressure fluid transportation, a spiral-welded welded pipe, a direct-welded welded pipe, an electric welded pipe, and the like. Seamless welded pipe can be used for various liquid and ceramic welded pipes,
Gas pipelines, etc. Welded pipes can be used for water pipes, gas pipes, heating pipes, etc.
The whole welded pipe is heated to the normalizing temperature, so that all the tissues are completely austenitized, and then air-cooled. This can basically eliminate the metallographic difference between the pipe body and the weld, and completely eliminate the internal stress caused by the forming and welding of the welded pipe, but the grain state of the weld and the pipe still differs, which will bring the mechanics. Deficiencies in performance and corrosion resistance.
The process is mostly off-line, using a protective gas heat treatment furnace to heat the welded pipe as a whole, and air cooling will take a long time, so the production efficiency is low.
Among the three seamless technologies, the current HFW welded pipe seamlessly adopts the weld online heat treatment technology, but comprehensively, the overall heating tension reduction of the welded pipe will be an ideal seamless pipe welding technology, and the overall heat treatment technology of the welded pipe Very few applications.
The thermal tension reduction process is actually a normalizing + deformation heat treatment of the welded pipe. After the whole base material is heated to 920-950 °C, the weld metal structure of the weld seam and the pipe body is completely austenitized, and then the multi-station large-deformation rolling is performed by the tension reducer, and the general finish rolling temperature is not low. At 850 ° C, after the end of the welded pipe, the fine and uniform grain structure is formed inside.
After the phase change of the weld, the deformation heat treatment is carried out simultaneously with the base metal, and the recrystallization process is completed under the same conditions. The metallographic structure and grain state will be consistent with the base metal, and the weld tube forming is completely eliminated. The internal stress brought about. This process has now received attention and is being gradually applied for the production of advanced HFW welded pipes.
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