Modelling of Mannesmann fracture initiation during cross-roll piercing

Seamless tube manufacturing utilises continuous cast cylindrical billets that, after piercing, are rolled until specified diameter, thickness and length are reached. The hollow part can be industrially obtained through cross roll piercing, being the main characteristic of this process the occurrence of a local failure at the billet centre due to the so-called Mannesmann Effect. The knowledge of those industrial parameters, which determine the start and the propagation of the axial fracture, is crucial because they determine the optimal position of the plug in order to grant both the best quality of the tube and the plug service life. The objective of the work is to develop a reliable numerical model capable to describe the industrial conditions that lead to Mannesmann fracture through the implementation into a commercial FE code of a coupled damage law based on Continuum Damage Mechanics. The fracture characterization utilises a damage model according to the Lemaitre formulation and the damage parameters identification is based on inverse analysis on hot tensile test results. In particular, a modification to the Lemaitre damage law is adopted in order to describe the different behaviour of the material in the billet section and to take into account the effect of porosity and phase distribution on the initial damage. Finally, the developed model is validated, through the comparison between numerical results and industrial trials of a non-plug piercing operation, showing that there is a good agreement for what concern the length and the initiation site of the Mannesmann cone fracture.