Effects of Hardening Models on CUO Forming and Springback Simulation of High Strength Line Pipes

The UOE process is an effective approach for manufacturing the line pipes used in oil and gas transportation. During the UOE process, a steel plate is crimped along its edges, pressed into a circular pipe with an open-seam by the successively U-O forming stages. Subsequently, the open-seam is closed and welded. Finally, the welded pipe is expanded to obtain a perfectly round shape. In particular, during the O-forming stage the plate is suffered from distinct strain reversal which leads to the Bauschinger effect, i.e., a reduced yield stress at the start of reverse loading following forward strain. In the finite element simulation of plate forming, the material hardening model plays an important role in the springback prediction. In this study, the mechanical properties of API X90 grade steel are obtained by a tension-compression test. Three popular hardening models (isotropic hardening, kinematic hardening and combined hardening) are employed to simulate the CUO forming process. A deep analysis on the deformation and springback behaviors of the plate in each forming stage is implemented. The formed configurations from C-forming to U-forming are almost identical with three hardening models due to the similar forward hardening behaviors. Since the isotropic hardening model cannot represent the Bauschinger effect, it evaluates the higher reverse stress and springback in the O-forming stage which leads to a failure prediction of a zero open-seam pipe. On the contrary, the kinematic hardening model overestimates the Bauschinger effect so that predicts the larger open-seam value. Specifically, the simulation results using the combined hardening model show good agreement in geometric configurations with the practical measurements.