The three kinds of dimensions that is Φ325mm×8mm, Φ426mm×8mm and Φ711mm×10.3mm X52 spiral seam induction bends were manufactured via the induction bending process. Based on that, the property and structure of spiral seam induction bends were investigated. According to the processing technic and design feature of spiral seam bend, the destructive test sampling plan of spiral seam induction bend was proposed. Through the mechanical property tests and design proof test, the spiral seam induction bend was studied and evaluated. The tension and impact tests results show that the strength-toughness of spiral seam induction bend can approach the property of mother pipe when the suitable process was adopted. The transverse guided-weld bend tests results show that the root bend samples in several fixed location of Φ325mm×8mm bend pipes crack, which indicate that the smaller diameter spiral seam pipe is not fit for the hot bending. The observational check results show that the welding joint misalignment occurs at the inner arc of Φ711mm×10.3mm spiral seam induction bend, which indicate that the configuration of spiral seam induction bend is easy to be affected during bending process with the decrease of thickness-to-radius ration of pipe. The design proof test of Φ426mm×8mm bend pipe shows that the spiral seam induction bend does not rupture at the computed proof test pressure.

Induction heated bending pipe is an important connecting piece widely used in natural gas pipeline. As the persistent development of natural gas pipeline project, the X80 induction heated bending pipe needed to be developed and studied. The local induction quenching process and the wholly induction quenching process were adopted to develop the Φ1219mm×22mm X80 bending pipe in this paper. The effects of different technologies on the bending pipe property were studied. Based on the pipe-bending machine that can not apply the continuous wholly induction quenching process, a multiple-step wholly induction quenching process was proposed and realized. The study results show that for the X80 bending pipe manufactured via the local induction quenching process, the strength of straight part is higher than the bend part. The toughness of welding seam and heat-affected zone at straight part is lower. The match of property between the straight part and the bend part is not perfect. The continuous wholly induction quenching process improve effectively the toughness of bending pipe, reduce the strength and yield ratio to a certain extent, so the match of property between the straight part and the bend part is reasonable. The multiple-step wholly induction quenching process can also improve the property of the straight part in bending pipe. The property of intermediate zone by double quenching has no clear changes. This kind of process is suitable for the pipe-bending machine that can not provide continuous wholly induction quenching process.

In the development of high grade pipeline steels, the technologies on bend pipes are key points. Most reports are mainly on the cold bending techniques which allow a small deflection angle usually less than 10°. And induction heat is then introduced for higher deflection angle bend pipes. So it is very important to know the toughness index of reheated high grade linepipes. In current used standard in China for West-East Gas project, Charpy toughness is required to be ≥120J at −20°C, and corresponding shearing area is required to be ≥85%. Based on a large number of experimental data on high grade linepipes with heat treatment, it was found in the present study that the shearing area and Charpy toughness have a linear relationship and the slope coefficient is 0.34. As a result, the when shearing area is 85%, the corresponding Charpy toughness is 250J. and This is different from that of mother pipe. To find the reasons, the fractures and microstructure of these reheated linepipes are analysed and compared with that of mother pipes.

Large amount of experimental data indicated, that the index of CVN toughness and shearing area of mother pipe of X80 are (200J, 90%). And the results showed that when charpy toughness was closing to 200J, the shearing area would reach to 90%; when the charpy toughness increased continuously, the shearing area would not increase further more. So the selection of charpy toughness value as 200J for pipeline steels is reasonable, and also its corresponding shearing area. To gain thus component index, the optimum microstructure of X80 steel should be strip-like ferrite mastered, small amount of granular bainite contained-acicular ferrite type steel, and, in which the size of MA island should be less than 1μm. This kinds of optimum microstructure of X80 pipeline steel will exert its softening role in improving toughness and simultaneity not decrease its strength.