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Tube expansion is the last forming operation of the UOE process for the production of welded pipes. This process is carried out by pressing a steel plate into a U and then O shape prior to welding. Then a radial expansion is applied in order to obtain a final circumferential shape. The expander is composed of a truncated pyramid, which acts as a bar parallel to the tube axis, covered with several expanding tools (usually 8 to 12) which looks like truncated circular sectors. They are placed such that an axial displacement of the central bar produces the tube expansion. In order to improve the expander design, in particular to optimize the contact pressure distribution between the expanding tools and the central bar, a complex three dimensional numerical model of this process was developed considering frictional contact in the interactions between the different parts of the model (central bar, expanding tools and tube). Material non-linearity is also present since the tube is plastically deformed. Model results were useful to understand the impact of the expanding tool longitudinal profile on the contact pressure distribution and on the total horizontal force required to expand the tube.