The aim of this work is to characterise the residual stresses in the longitudinal, transverse ans short transverse directions for 11 welds in 6.5mm thick HSLA-65 (ferritic) and 304 stainless steel (austenitic) plates. This work will for the first time enable an understanding to be obtained about;(a) The influence of tool size, tool material and traverse speed in single and double pass friction stir welds on the residual stress distribution across the weld and phase transformations in the weld nugget.(b) The differences between the residual stress distributions in transformable (HSLA-65) and non-transformable (304SS) steels welded under similar conditions.There is great interest in joining steels for shipbuilding by friction stir welding since it offers lower distortion than conventional fusion welds and thus the possibility of joining thinner plates of high strength steel.