A model has been developed which is capable of describing the thermal cycles occurring in multipass welds. The method involves a calculation of the size and shape of the single bead-on-plate weld. The isotherms beneath the weld centre-line are estimated using an analytical solution for the problem of heat-flow into a thick plate. The volume of metal deposited per unit length of weld, i.e. the reinforcement, is then assumed to adopt the shape of a spherical cap on the surface of the plate. For multipass welds, the model applies the heat-flow equations as each bead is deposited. The model has been calibrated for a 2 1/4 Crl Mo multipass weld of a type commonly used for joining steam-pipes in a modern steam-generating power plant. The method has been used to investigate theoretically the effect of interpass temperature, welding current and Ac3 temperature on the fraction of weld microstructure which becomes reaustenitised during fabrication of the weldment. Where possible, the results are rationalised by comparison with experimental observations.

*Acta Metallurgica et Materialia,* Vol. 42, 1994, 3663-3678.

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