Investigations on external separation layer defect of nickel-based superalloy in rotary tube piercing process

A large number of thick-walled tubes of nickel-based superalloy (TWNS) are required in aviation industry and nuclear industry. The rotary tube piercing (RTP) process has great advantages in preparing the seamless tubes. However, the external separation layer defect (ESLD) hinders the application of RTP process in nickel-based superalloy. In the study, the combinations of experiment analysis and numerical simulation were adopted to explore the formation mechanism of ESLD. The experiment results reveal that the ESLD is closely related to the roll speed and diameter reduction rate, and the radial position of ESLD is determined as r/R = 0.8–0.9. The average grain size and micro-hardness on both sides of ESLD vary greatly, which reveals that the formation of ESLD is closely related to the radial strain gradient. The simulation results reveal that the positions of maximum radial strain gradient, maximum shear strain, and maximum temperature rise are all closed to the position of ESLD. Further, a novel damage model considering the effects of maximum shear strain and maximum shear stress on ESLD was proposed. By comparing the experiment results and simulation results, it is found that the proposed damage model is reliable to predict the ESLD. The formation mechanism of ESLD includes crack initiation and crack propagation. The crack initiation is related to the damage of proposed model, and the crack propagation is controlled by radial strain gradient, which provides a path for the crack propagation. The severe temperature rise intensifies the metal flow and further promotes the evolution of crack.