Evolution of inner wall wrinkle defects in the sinking zone of a thick-walled steel tube during radial forging

An axial wrinkle defect was observed in the inner wall of the sinking zone of a thick-wall steel tube processed by cold radial forging. Wrinkles can evolve into fissures. The present study focuses on the evolution of wrinkles and the effects of process parameters on them using a three-dimensional radial forging process finite element model, radial forging experiments, and surface morphology observations. The results indicated that the vertical section angle of the hammer die and the size of the tube blank significantly affect the evolution of wrinkles. The height-to-width ratio λ was introduced to describe the morphology of wrinkles. It had an approximately linear relationship with the radius reduction in the sinking zone. There was a linear correlation between the growth slope of λ and the axial to circumferential strain ratio |ε_r/ε_θ|, which can predict the λ under few process parameters. For the 30SiMn2MoVA steel, at the junction of the forging and sinking zones, the threshold of λ of the wrinkle that can evolve into a fissure is approximately 1.123.