Elbows, which belong to butt-welded steel pipe joints, are used in piping of various plants. Elbows are mainly manufactured by hot mandrel bending. In hot mandrel bending, short straight pipes are inserted into an inner tool of mandrel and are bent at specified bending radius, while they are compressed by pusher from one end of pipes and their outer diameter are expanded. Elbows manufactured by this bending process have small deviation in wall thickness and also high accuracy in roundness, even when they are processed into short bending radius. Shape and dimension of mandrel are designed by trial and error through many experimental tests. However, there are a few research works on hot mandrel bending. We focus on expansion ratio and bending radius ratio of mandrel, which are main variables, and greatly affect deformation behavior of hot mandrel bending. And we conducted experiment and numerical analysis using mandrels which have several kinds of expansion ratio and bending radius ratio. Following items are clarified by this research work: At extrados, axial strain decreases slightly, when expansion ratio increases. As a results, neutral axis of bending moves from extrados toward intrados by increasing of expansion ratio. And at intrados, wall thickness strain at intrados decreases, when expansion ratio increases. Wall thickness strain at intrados decreases, when bending radius ratio increases. Forming load increases, when expansion ratio increases and with bending radius ratio decreases. When experimental conditions do not coincide with optimized expansion ratio and bending radius ratio, deviation of wall thickness occurs in elbow.

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