A non-destructive method for the residual stresses evaluation in welded joints is here proposed. Conventional residual stress determination techniques are based on the destruction of the original material stress state that causes a relaxation, hence is not possible to use those methods on working components. The proposed method provides for an active lock-in infrared thermography analysis by means of a FLIR thermal camera and an exciting laser head for the evaluation of the thermal diffusivity. In general, it is possible to relate the thermal diffusivity to the diffusion length, which is the ability of a thermal wave with a known wavelength to penetrate inside the sample, using the phase contrast trend as a function of the distance from the laser spot.