The aim of the research was to select a material from which a washer can be made, so that it can be connected to an E355 steel tube by Rotary Friction Welding (RFW). It was decided to choose the steel grade X6CrMo17-1. The numerical model of the RFW process was built using the finite element method (FEM) using the ADINA System software. The numerical model takes into account the friction coefficient with variable values depending on the temperature. Numerical simulations of the process made it possible to determine the temperature fields in the weld cross-section. For the assumed process parameters: rotational speed of 14,000 rpm, friction time of 1.5 s and friction force of 600 N, the peak temperature occurred in the middle of the friction surface at the end of the friction phase and amounted to 1050 C. The results of the temperature analysis are one of the most important parameters for the implementation of subsequent calculations, such as the calculation of structural changes, hardness, residual stresses and deformations.