Mechanical Properties of the Bend for Shannxi—Beijing Gas Transmission Pipeline

DN650 bend, with the curvature radius of 5D and five times of the outer diameter, were fabricated by using X60 and the hot-bending with medium frequency induction heating. Because there was no clear requirements to the mechanical properties of bends in domestic standards, and foreign standards were not completely appropriated to the pipeline, so, the technical specification and indexes of mechanical properties for large bends were worked out based on some international standards or codes. Some specified indexes, such as σ_s≥415 MPa, σ_b≥51 MPa, yield strength ratio≤0.88, elongation to API-5L, crystal-size to 9 or more without heating treatment, conformed to MSS SP75-1993. The measuring results showed that the mechanical properties for fabricated bends completely conformed to the technical specification. Comparing with the raw material of steel pipe, the bends had lower σ_b and much lower σ_s. Therefore, the yield strength ratio reduced, but the elongation (%) increased, which showed that the plasticity of bends increased while bending. Some suggestions on working out the technical standards of bend in gas transmission pipeline were set forth.