In this work, key hole mode plasma arc welding of 6 mm thick DP600 steel plates have been carried out and the effect of post-weld heat treatment is presented. The microstructures of the fusion zone and heat affected zones are studied. The tensile strength of the as-welded joint is found to decrease by 8%, while the post-weld heat treated weld joint and heat treated base metal showed a 15% reduction compared to as-received DP600. The failure occurred in the far heat affected zone, where softening occurred due to martensite tempering. The impact properties at room temperature were similar for all samples, whereas at −29 °C, the base metal impact property did not decrease while that of weld metal decreased considerably. Bend tests indicated good ductility and formability. © 2019 Elsevier B.V.