This experimental work is focused on the residual stresses induced by multi-pass welding of thick components. Two samples were produced, inspired by the nozzle-vessel geometry, with Submerged Arc Welding buttwelding, performed according to the standard welding procedures employed by Belleli Energy CPE. The components were characterized by different sizes and groove positions. Measurements of residual stresses were carried out by hole drilling according to ASTM E837-13a, Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method, in different positions of the samples. The measurements were performed on welded, and Dehydrogenation Heat Treatment (350°C, 4h), and Intermediate Stress Relieving treatments. The obtained results allowed a discussion of the influence of the component size on the residual stresses and the effectiveness of an intermediate heat treatment for reducing the stress state.