Welding processes are usually associated with significant thermal gradients and may generate high tensile residual stresses in and near the bond line. These stresses reduce the material resistance to fatigue crack initiation and propagation. In order to improve the strength of weldments, heat treatment is used at the final stage of manufacturing. Heat treatment modifies the material microstructure and properties through diffusion and creep/relaxation. Stress-relieving heat treatments are used for welded structures to improve ductility, relieve internal stresses, refine the microstructure and improve cold working properties. The aim of this study is to investigate the residual stress relief in LFW joints during heat treatment in the presence of tensile load in situ on the neutron diffraction instrument.