During nuclear reactor pressure vessel manufacturing, conventional arc welding processes are often employed involving multiple passes to join thick sections. Such thick-section welding may cause variable through-thickness metallurgical and mechanical properties due to variable heat input, particularly in the heat-affected zone (HAZ) and weld metal. Structural steels for pressure vessel components can be selected based on properties such as the Luders plateau and yield/tensile (YS/UTS) ratio in order to assure extended service lives. In the present work, through-thickness tensile properties of narrow-gap (NG) welds performed on 78-mm-thick SA533 plate welded according to ASME IX with 58 passes of gas tungsten arc welding (NG-GTAW) and 38 passes of narrow-gap submerged arc welding (NG-SAW) are examined and compared. The Luders plateau strength, length and velocity were determined along with yield, YS/UTS ratio, ultimate tensile strength and elongation. These properties were analysed in connection with metallurgical and microstructural properties for both welding approaches and established the correlation among them in both cases with the mechanical properties and Luder behaviour. Multiple thermal cycles have tempered and refined the microstructure and hardness in weld and HAZ, which pretends to be a heat treatment of weldment and promotes the formation of Luders plateau. The YS/UTS ratio and Luders plateau behaviour has been evidenced to be influenced by large numbers of thermal cycles. Additionally, the heat input is also vital concern as it is observed that the Luders plateau stress is more in GTAW process than the SAW. However, Luders plateau length is significant in SAW than GTAW process. The implication of these variations on structural integrity aspects and the correlation of mechanical-microstructural properties have been justified in the present work.