The standard practice recommended for high pressure vessels, having heavy walls, requires the implementation of weld joint preparation with narrow gap technique; this generally calls for a ‘two beads per layer’ sequence alongside the use of the submerged arc welding process. This process provides a high quality and uniformed weld joint whilst also reducing the residual stresses after welding. In refinery equipment that are subjected to high pressures and are exposed to hydrogen environment, high strength materials such as 2 1/4 Cr 1 Mo 1/4 V are commonly used. A recent study conducted on this material, and the process of submerged arc welding with narrow gap technique ‘two beads per layer,’ had identified a potential issue in complying with ASME Code specified creep resistance properties. In another setting, with regards to the properties of toughness in weld joints, other possible inconsistencies, in the narrow gap weld joint, between the weld centerline and center bead, were found. In order to overcome the deficiencies stated above, an innovative welding technology is presented in this paper which is based on the preparation of a narrower groove than the commonly used narrow gap technique. Such groove has been designed to implement the ‘single bead per layer’ approach. This paper illustrates that the use of this new technique results in improved quality of weld seams as applied in heavy wall high pressure vessels used in creep regime. The welding process considered is that of tandem submerged arc welding with two wires. The mechanical characteristics and results obtained by comparing the two techniques ‘two beads per layer’, and the new innovative one ‘single bead per layer’ will be evidenced and discussed.

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.