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In this investigation, friction stir welding (FSW) was employed for joining HSLA steel plates (5 mm thick) of grade DMR249A in the development of light weight ship structures. This investigation aims to study the effect pre-heating temperature (PHT) on microstructure and mechanical properties of FSWed DMR249A steel-joints. The PHT of W99 tool was varied from 100 to 250°C. The optical microscopy (OM), scanning electron microscopy (SEM), and an elemental analysis of stir zone (SZ) of DMR249A steel-joint was carried out. The tensile properties, hardness and impact toughness of DMR249A steel-joints were evaluated and compared to BM and joints without PHT. Results disclosed that the DMR249A steel-joints made at PHT 100°C exhibited superior tensile properties and impact toughness compared to other joints. It is attributed to the evolution of finer acicular ferritic and upper bainitic microstructure with no debris of tool in SZ.