Abstract <p>Numerical analysis by the finite element method of welding deformations in a thin-walled structure is considered. A comparison was made with the results of an experiment in which semiautomatic arc welding in a carbon dioxide environment with a consumable electrode was used to obtain a T-joint from standard carbon steel St3sp. In the numerical solution of thermoelastoplastic problems in the Marc system, the Goldak double ellipsoidal heat source model was used, taking into account large deformations and nonlinearity of the steel properties depending on temperature. In connection with fluctuations in voltage, current, and, accordingly, arc power during the welding process, the influence of different functional dependencies of the arc thermal power on time on the residual deformation of the structure was analyzed during the simulation. As residual deformations, deformations leading to nonflatness are analyzed as the most significant ones. The developed design model with welding process parameters provides the possibility of accurately forecasting residual deformations and adjustments to the technological process for reduction of geometric deviations.</p>