Weld metal microstructure prediction from fundamentals of transport phenomena and phase transformation theory

Abstract

Modelling the evolution of weld metal microstructure requires knowledge of cooling rates at various locations in the fusion zone. In the recent past, significant advances have been made in the calculation of transient three-dimensional temperature fields, considering convective heat transfer and fluid flow in the weld pool. However, very little effort has been made to use these accurate cooling rates to understand fusion zone microstructures. The present paper demonstrates the advantages of microstructure calculations using fundamentals of transport phenomena and phase transformation theory. The velocity and temperature fields, the shape and size of the fusion zone, and the cooling rates at different locations were calculated by solution of the equations for conservation of mass, momentum, and energy in three dimensions. The time-temperature transformation (TTT) diagrams were calculated for a series 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) diagrams and the microstructures. The computed volume fractions of the various microstructural constituents were then compared with the experimental results. Good agreement between the computed and the experimental results indicates significant promise for predicting weld microstructure from the fundamental principles of transport phenomena and phase transformation theory.