WELD METAL MICROSTRUCTURE PREDICTION FROM FUNDAMENTALS OF TRANSPORT PHENOMENA AND PHASE-TRANSFORMATION THEORY

Modelling the evolution of weld metal microstructure requires knowledg e of cooling rates at various locations in the fusion zone. In the rec ent past, significant advances have been made in the calculation of tr ansient three-dimensional temperature fields, considering convective h eat transfer and fluid flow in the weld pool. However, very little eff ort has been made to use these accurate cooling rates to understand fu sion zone microstructures. The present paper demonstrates the advantag es of microstructure calculations using fundamentals of transport phen omena and phase transformation theory. The velocity and temperature fi elds, the shape and size of the fusion zone, and the cooling rates at different locations were calculated by solution of the equations for c onservation of mass, momentum, and energy in three dimensions. The tim e-temperature transformation (TTT) diagrams were calculated for a seri es of steels with varying carbon and manganese contents using a phase transformation model. The TTT diagrams and the computed cooling rates were then used to obtain the continuous cooling transformation (CCT) d iagrams and the microstructures. The computed volume fractons of the v arious microstructural constituents were then compared with the experi mental results. Good agreement between the computed and the experiment al results indicates significant promise for predicting weld microstru cture from the fundamental principles of transport phenomena and phase transformation theory.