A primary plus secondary local PWHT method for mitigating weld residual stresses in pressure vessels

Abstract Local heat treatment is a widely used to control the welding residual stress for ultra-large pressure vessels due to the upsizing of pressure vessels such as the super-large diameter and overlength. Aiming to effectively reduce the harmful residual stress on the inner surface, a new method called the primary-auxiliary local post-welding heat treatment (PA-PWHT) is proposed in this paper and verified by experiment and finite element analysis. The mechanism for reducing the welding residual stress on the inner wall was also provided. The results show that the thermal deformation during the traditional local PWHT will lead to a secondary tensile stress on the inner surface of the welds, which make the residual stress distribution on the inner face deteriorates after local PWHT. The PA-PWHT method can produce small tensile stress and even compressive stress regardless of the stress distribution on the inner surface or the outer surface. For the typical closed welds studied in this paper, the axial and hoop residual stresses on the inner surface are reduced by 120% and 105%, respectively. It is verified that the proposed method is effective and feasible to implement, and able to suppress the risk of stress corrosion cracking (SCC) of pressure vessels from the perspective of residual stress.