A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part 1. the model

The present investigation is concerned with modeling of the microstructure evolution in duplex stainless steels under thermal conditions applicable to welding. The important reactions that have been modeled are the dissolutio n of austenite during heating, subsequent grain growth in the delta ferrite regime, and finally, the decomposition of the delta ferrite to austenite d uring cooling. As a starting point, a differential formulation of the under lying diffusion problem is presented, based on the internal-state variable approach. These solutions are later manipulated and expressed in terms of t he Scheil integral in the cases where the evolution equation is separable o r can be made separable by a simple change of variables. The models have th en been applied to describe the heat-affected zone microstructure evolution during both thick-plate and thin-plate welding of three commercial duplex stainless steel grades: 2205, 2304, and 2507. The results may conveniently be presented in the form of novel process diagrams, which display contours of constant delta: ferrite grain size along with information about dissolut ion and reprecipitation of austenite for different combinations of weld inp ut energy and peak temperature. These diagrams are well suited for quantita tive readings and illustrate, in a condensed manner, the competition betwee n the different variables that lead to structural changes during welding of duplex stainless steels.