INTERNAL MAGNETIC ABRASIVE FINISHING FOR AUSTENITIC STAINLESS STEEL TUBES BENT USING HIGH FREQUENCY INDUCTION BENDING H. Yamaguchi*1, T. Shinmura*2, H. Yokoo*2, T. Aizawa*1 * Faculty of Engineering, Utsunomiya University, Tochigi, Japan *2 Graduate School of Engineering, Utsunomiya University, Tochigi, Japan 1
Abstract This paper examines the applicability of magnetic abrasive finishing to the internal finishing of large–sized bent tubes produced by high frequency induction bending. Finishing equipment was developed for large–sized bent tubes. The finishing experiments using austenitic stainless steel tubes bent by both cold–drawing and high frequency induction bending demonstrate the finishing characteristics and reveal the differences in the finishing mechanism between the two types of elbows. The elbow made by cold–drawing required only one finishing process to achieve a 0.03 µm Ra surface. In contrast, high frequency bent elbow, which is covered with an oxide film, required two–stage finishing using two different sizes of ferrous particles for a smoothly finished surface, 0.05~0.13 µm Ra. Keywords: Internal finishing, Magnetic abrasive finishing, Austenitic stainless steel bent tube, Finishing characteristics, High frequency induction bending, Two stage finishing 1. Introduction High frequency induction bending is a hot bending process for pipes and shaped steels using high frequency induction heating. The process has the advantage of three–dimensional flexibility in bend radius and angle within the limits of the bending machine. Complex–shaped tubes, which consist of both bent and straight sections, produced by high frequency induction bending, are used for piping systems to minimize the number of welded joints, thereby reducing the production and installation costs and improving the quality of the piping system. The tubes are required to have smoothly finished inner surfaces to prevent the contamination of gas and liquid. However, the complexity of the tube shapes and the oxide film generated on the tube surface during high frequency bending considerably increase the difficulty of internal finishing of the tubes with conventional technologies. An internal magnetic abrasive finishing process was proposed for producing precisely finished inner surfaces of nonferromagnetic tubes used for 1250 piping systems . This paper examines the Timing belt applicability of the magnetic abrasive finishing to the internal finishing of large–sized bent tubes produced by high frequency induction bending. Pole Finishing equipment was developed for large–sized bent tubes, and the performance of the equipment was examined through finishing experiments using SUS316 and SUS304 austenitic stainless steel elbows made by both cold–drawing and high Wokpiece frequency induction bending. They demonstrate Yoke Robot 255 the finishing characteristics, including surface roughness and material removal, and reveal the Finishing unit Flexible shaft differences in the finishing mechanism between the Motor for pole rotation two types of elbows. 1735 Fig. 1 External photograph of experimental setup
2. Processing principle and experimental setup The poles, which consist of small permanent magnets, placed outside the bent tube generate the magnetic field needed for attracting the magnetic abrasive to the finishing area by magnetic force. When the poles rotate around the bent tube, the magnetic abrasive, driven by magnetic force, rotates along the Table 1 Experimental conditions inner surface of the bent tube along with the poles, Workpiece SUS316 stainless steel 90° elbow and removes material from the surface. Manipulating OD: 76.3 mm, ID: 72.3 mm, the rotating poles along the tube axis causes the Radius of curvature: 400 mm magnetic abrasive to follow the poles' motion, Ferrous Electrolytic iron particles: 56 g finishing the entire inner surface of the tube . particles (330 µm in mean dia.) WA magnetic 14 g Previous research developed...
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