Measure of residual stresses evolution during multipass welding with neutron diffraction technique

Residual Stresses (RS) are a consequence of welding operation. RS promote cold cracking, stress-corrosion cracking, or fatigue cracks growth phenomena of pressure vessels. RS can be treated with post weld heat treatment (PWHT). However, such heat treatments may not be applied on-site for welding repair of large pressure vessels. “Temper bead welding” (TBW) techniques have been developed as substitution to PWHT in construction codes because of their abilities to temper or to refine the heat affected zone (HAZ). Then TBW techniques reduce cracking risks related to the initial presence of brittle or hard metallurgical phases in the HAZ. However, the influence of TBW on residual stresses has not yet been clearly established or observed.

Residual stresses have been measured on specimens with up to five weld beads deposited side by side following a TBW technique. Neutron diffraction measurements were carried out at ILL on Salsa beamline, Grenoble (France). The measurements have been carried out in the middle plane perpendicular to the longitudinal welding line for the five specimens. The specimens are 300x100x30 mm³ plates of P355 NH steel. The results show that the longitudinal stress is tensile under the weld bead, and this area expends with the addition of a new weld bead. For the five plates, the peak of transverse stress is half of the maximal longitudinal stress and the curve exhibits a M shape.