AbstractLaser beam welding of 13 mm plates of HY 80 steel resulted in different microstructures in the fusion zone depending on the welding process. Autogenous welds resulted in an untempered martensitic structure, whereas, with the introduction of a hot wire filler metal, the microstructures depended on the wire feed. A slow feed resulted in a mixed martensite–bainite microstructure and a fast wire feed resulted in a microstructure that is primarily acicular ferrite. The chemistry of the as deposited weld metals was different in each case. The thermal profiles for the autogenous weldment were calculated based on a two parameter model of laser welding that utilises the fusion zone boundary as a measure of‘best fit’. Based on the cooling rate of the autogenous fusion zone, cooling rates for the two welds with hot wire filler metal were rationalised. Using these cooling rates, the chemistry of the weld metal and a model of microstructural development of the weld metal, a computer calculation was made which gave volume fractions of the phases similar to those observed experimentally.MST/1845