Influence of microstructure and composition on mechanical properties of some AISI 300 series weld metals

A brief overview is given of the·current understanding of the solidification microstructure, the effect of aging and precipitation processes, and the factors controlling the mechanical and high temperature properties of some AISI 300 series weld metals. The effects of alloy chemistry on the distribution and morphology of the carbide and intermetallic phases in the weld metal are discussed. It is shown that the precipitation behaviour depends strongly on the carbon, silicon, chromium, and molybdenum content of the weld metal and the rate of dissolution of the δ-ferrite. The precipitation reactions are considered to be of primary importance in controlling the mechanical properties of the weld metals, and can result in significant reduction in weld metal ductility and toughness. Much of the deterioration in properties can be directly attributed to the formation of secondary phases such as M23C6 and the intermetallic σ and χ phases. A much more complex situation is found to prevail with respect to creep properties and the importance of factors such as δ-ferrite content, composition, coating type, residual stress levels, welding process, inclusion distribution, and precipitation type and distribution on the high temperature strength and ductility are considered. A number of suggestions are made which may improve weld metal properties and the long term performance of austenitic weldments in service.