Modelling the Evolution of Microstructure in Steel Weld Metal

Physical models for the development of microstructure have the potential of revealing new phenomena and properties. They can also help identify the controlling variables. The ability to model weld metal microstructure relies on a deep understanding of the phase transformation theory governing the changes which occur as the weld solidifies and cools to ambient temperature. Considerable progress has been made with the help of thermodynamic and kinetic theory which accounts for the variety of alloying additions, non–equilibrium cooling conditions and other many other variables necessary to fully specify the welded component. These aspects are reviewed with the aim of presenting a reasonably detailed account of the methods involved, and of some important, outstanding difficulties. It is now well established that extremely small concentrations of certain elements can significantly influence the transformation behaviour of weld metals. Some of these elements are identical to those used in the manufacture of wrought microalloyed steels, whereas others enter the fusion zone as an unavoidable consequence of the welding process. The theory available to cope with such effects is as yet inadequate. Methods for incorporating the influence of trace elements such as oxygen, aluminium, boron, nitrogen, titanium and the rare earth elements into schemes for the prediction of microstructure are discussed. The very high sensitivity of modern microalloyed steels to carbon concentration is also assessed. Some basic ideas on how the approximate relationships between weld microstructure and mechanical properties can be included in computer models are discussed.