Abstract Pipes of the heat treatable steel 42CrMo4 (AISI 4140) were joined by Inertia Friction Welding (IFW) while the diameter and the wall thickness of the pipe were varied. The machine parameters axial pressure and initial rotational velocity as well as the applied energy per cross section area were examined. The resulting torsional moment and upset were continuously recorded during the process. A regression model was established and the influence of the investigated parameters on the maximum torsional moment and upset was quantified.