Welding is of major importance for the nuclear industry as it affects safety, structural integrity and the life span of the nuclear power plant. The uneven thermal distribution and metallurgical misfits that develop during welding procedures introduce residual stresses, which can drive damage in safetycritical nuclear components. Post-weld heat treatment (PWHT) is known to reduce residual stresses, however, determining the PWHT parameters, such as hold temperature and soak time, needs to be standardized and not be based on judgement. Five 100mm thick electron beam welds were manufactured from SA508 Grade 3 Class 2 low alloy steel, four underwent four different PWHT schedules with differing hold temperatures and soak times, and one was retained in the as-welded (AW) state. The residual stresses in the as-welded condition were measured using incremental deep hole drilling and the contour method, while residual stresses after PWHT were measured using the contour method alone. All specimens exhibited the characteristic M-shaped stress distribution associated with solid-state phase transformation in the weld fusion zone and HAZ. The peak tensile stresses developed in untransformed parent material either side of the weld, and reached 650MPa in the as-welded state. Post-weld heat treatment resulted in significant relaxation, to between 100 and 200MPa.