The Improvement of Low Temperature CVN Toughness In Structural Pipes

by N.Y. Yen, C.C. Huang, Y.T. Chang, C.E. Hsu

Publisher - Chung Hung Steel Corporation

Category - Engineering & IT

This paper specifies the improvement of CVN toughness of weld seam at 0℃ and fracture mode in JIS STK490 pipe for General Structure. The decarburized layer often occurs at fusion line of carbon steel pipe manufactured by ERW (Electric Resistance Welding) process. Decarburized layer with low carbon content forms continuous ferrite area and results in low strength. Even online MF (Middle Frequency) heat treatment supplies to the weld seam, the decarburized layer cannot be tempered to another phase. Therefore, the fracture appears cleavage morphology macroscopically after impact test. Increasing squeeze amount appropriately during ERW process can discharge the decarburized layer from the fusion line. The ideal structure at weld seam appears a mixture of ferrite and pearlite after online heat treatment. Observing the shear-fracture on impact specimen, it appears a tear-like and gray surface macroscopically, and ductile-dimple fracture morphology microscopically which is a favorable fracture mode to increase impact absorbed energy. The mechanism of improving weld seam can be further applied to the pipes of mechanical structure for cold drawn or the pipes acquired CVN toughness at low temperature for API line pipes. The uniform deformation of weld seam is expected and beneficial to improve its CVN

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