TPT September 2007

T ube M ills: R olling & F orming M ethods

Progressive technology for seamless tubes by piece-by-piece reduction

There are essential drawbacks to the existing technology of hot and cold piece- by-piece tube reduction, which has been established for over 50 years. These drawbacks include a limited wall reduction that equals 8-12 per cent (maximum 35 per cent in the reducing mills with 25-28 working stands). Other drawbacks include a formation of thickened sections at both tube ends that have to be cropped to waste (resulting in a reduced output), and a significant cross- sectional wall thickness variation in tubes (up to ±12.5 per cent). Cold tube reducing also makes it impossible to obtain tubes with a reduced wall. This technology is based on the use of tensioning tubes between the reducing mill stands. Since 1955, in the calculation of the reducing parameters with wall reduction, plastic tension factors have come into use instead of kinematical tension factors. With the aim of eliminating such drawbacks of hot and cold tube reducing, Ukrtruboprom Association has developed a new technical approach. This technology is based on the cubic strain solution when undertaking tube rolling in a closed pass of any shape. The general and partial solutions of this problem have made it possible to combine the three basic deformations taking place in the rolling process with other parameters including the starting and final tube dimensions. Based on the solution of the cubic strain problem, a procedure for the calculation of the new technology parameters has been extended. Following this development, three experiments were carried out in the production conditions at an operating TPA 140 tube reducing mill. The objective of these experiments was to verify the new technology and determine its potential in comparison with the conventional technology. In the course of all three experiments, mother tubes (130mm x 6mm) were rolled to obtain tubing (73mm x 5.5mm). Analysis of the results from the first experiment showed that the new technology obtained the usual tubing at the above dimensions. Experiment number two had the objective to check the possibility of obtaining tubes with a wall reduction higher than 0.5mm. For this reason, the tube wall thickness was increased by 2.38mm in the first

seven stands of the 15-stand group and decreased to 5.32mm in the remaining eight stands. The resulting tube wall thinning was 0.32mm per stand. When the operational conditions were implemented for reducing the tubes at the above size, an average thinning of 0.033mm per stand was obtained. Thus, the new technology demonstrates its ability to obtain the wall thinning 9 times higher than in the case of conventional technology. In experiment number two, the plastic tension factor was of essential interest. Its value was conditionally determined by the use of a known formula and was equal to 1.12 (its allowable value was 0.5 to 0.65). Rupture of the tube usually takes place under these conditions. However, this emergency situation did not occur using the new technology, which is due to the absence of tension between the stands. The objective of experiment number three was to obtain a tube with shorter thickened end sections. Measuring of the final tube has shown that the thickened end section length was 2 to 2.5 times shorter. Later, the procedure was developed to accomplish tubes with a complete absence of thickened ends. The experiment results reveal advantages of the new technology over the conventional method. Results showed the possibility of an extension of the size range of thin-walled seamless tubes that cannot be presently produced by conventional technology. For example, by using a 130mm x 7mm mother tube with just one wall thickness, tubes with a wall thickness of 6.5mm, 6mm, 5.5mm, 5mm, 4.5mm, 4mm and 3.5mm can be obtained. by line 8KHH varying the speed regime of the tube reducing mill. Other conclusions show the possibility of obtaining tubes free of thickened ends without using electronic control devices. There was also the possibility of an essential increase in tube wall thickness accuracy and tubes with no cross-sectional wall thickness variation. Finally, it was noted that there was the potential for saving fuel in preheating tubes by 25 to 30 per cent and even higher. All these advantages can be achieved without any capital outlays and production costs but just by calculating adjustment parameters. Because the calculation procedure for the

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S eptember /O ctober 2007