The selected issue of flange cracking in the radial extrusion process of hollow parts has been discussed in this paper. The researches were carried out based on numerical calculations in the Deform 3D software and experimental tests, which were carried out using pipe billets made of aluminum alloy EN AW 6063. A new methodology has been developed that allows to determine the place and approximate moment of material cohesion loss. This is determined from the results of the FEM calculation only, and does not require calibration tests. The method is based on a detailed analysis of the state of stress, state of strain in the forging, and is focused on identifying zones with an uneven distribution of the mentioned parameters. Determination of characteristic zones with a zero increase in the strain effective value makes it possible to determine, with a high approximation, the maximum (due to the phenomenon of cracking) diameter of the flange. The results of numerical calculations showed a high agreement with the results of experimental tests, in which the maximum diameter of the flange was determined.