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Residual stresses in 9mm thick ASTM A387 Grade 91 steel plates, joined using constant power (8kW) low and high heat input laser welding processes, are characterised using neutron diffraction. The measured longitudinal and normal components of residual stress show a bimodal distribution across the welded joint with a low tensile or compressive trough at the weld centre flanked by high magnitude tensile peaks in parent metal adjacent to the heat affected zone boundaries. The width of the central trough and spread of the outboard tensile zones are significantly greater for the high heat input weld. In both cases, the stress distributions can be explained by the strains associated with the austenite to martensite solid-state transformation as the joint cools after welding.