MICROSTRUCTURES IN HOT-WIRE LASER-BEAM WELDING OF HY 80 STEEL

Laser beam welding of 13 mm plates of HY80 steel resulted in different microstructures in the fusion zone depending on the welding process. Autogenous welds resulted in an untempered martensitic structure, wher eas, with the introduction of a hot wire filler metal, the microstruct ures depended on the wire feed. A slow feed resulted in a mixed marten site-bainite microstructure and a fast wire feed resulted in a microst ructure that is primarily acicular ferrite. The chemistry of the as de posited weld metals was different in each case. The thermal profiles f or the autogenous weldment were calculated based on a two parameter mo del of laser welding that utilises the fusion zone boundary as a measu re of 'best fit'. Based on the cooling rate of the autogenous fusion z one, cooling rates for the two welds with hot wire filler metal were r ationalised. Using these cooling rates, the chemistry of the weld meta l and a model of microstructural development of the weld metal, a comp uter calculation was made which gave volume fractions of the phases si milar to those observed experimentally.