Effect of cross-section distortion on the performance of small-radius induction bends

Abstract

While a small-radius induction bend is manufactured, various cross-section distortions may occur, such as its cross section tending to be elliptic, and wall thickness reduction or increase. These defects may deteriorate the bearing capacity of pipelines. In order to study the effect of cross-section distortion on the performance of small-radius induction bends, 3D (D refers to pipe diameter) induction bends were manufactured based on different combinations of process parameters. Then, the cross-section distortion, structure properties and their correlation of 3D bends and conventional 5D bends were analyzed comprehensively by means of cross-section geometry analysis, distortion analysis, physical and chemical property test, micro-structural analysis and so on. The following results were obtained. First, the wall thickening/thinning rate of 3D bends is about 2 times that of 5D bends, and the roundness of 3D bends is about 2–4 times that of 5D bends. Second, the cross-section distortion of 3D bends is obvious, so its heating temperature varies greatly in different parts, leading to different structure morphologies and contents. As a result, the numerical distribution of mechanical properties of each part of 3D bends (e.g. intrados, extrados and neutral zone) is discrete. Third, the strength at the intrados of most bends doesn't meet the standard, and it is sorted form the higher to the lower as extrados, neutral zone and intrados while the sequence of Charpy impact toughness is right contrary to the strength. Fourth, the strength at intrados and extrados decreases with the increase of roundness. The strength at intrados decreases with the increase of thickening rates. And the strength at extrados increases with the increase of thinning rates. Keywords: Small-radius induction bend, Cross-section distortion, Induction coil, Intrados, Extrados, Neutral zone, Strength, Roundness, Thickening rate, Thinning rate