Microstructural characteristics of keyhole plasma arc-welded DP600 steel were analyzed using optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. The fusion zone (FZ) was observed to consist of allotriomorphic, Widmanstätten, and acicular ferrite along with bainite and martensite leading to enhancement in the hardness compared with the base metal. The coarse-grain HAZ consisted of bainite and martensite, while the fine-grain HAZ consisted of bainitic ferrite and tempered martensite. The sub-critical HAZ was found to consist of tempered martensite with reduced density of dislocations and carbide precipitation. This softening resulted in yield point phenomena and failure in the sub-critical HAZ during the transverse tensile test of welds. The non-isothermal tempering resulted in retarded cementite precipitation due to inadequate time for diffusion of carbon, which led to the lowest hardness of 168 HV0.5 in the sub-critical HAZ. The different features in the FZ and HAZ and their correlation with mechanical properties are discussed. © 2019, The Minerals, Metals & Materials Society and ASM International.