Microstructural features, mechanical properties and high temperature failures of ferritic to ferritic dissimilar welds

Dissimilar metal welds (DMWs) between ferritic steel grades are found extensively in the construction of thermal power plants. The potential combinations and approaches for joining dissimilar ferritic steels are nearly limitless. For DMWs, the difference in alloy composition (specifically chromium and carbide-forming elements) provides the main driving force for carbon diffusion during welding, post-weld heat treatment and long-term service at elevated temperatures. Since the high temperature creep strength of local, carbon-denuded zones can be dramatically reduced from that of the parent or filler material, the service performance of ferritic DMWs can be severely reduced. This article reviews experimental observations on microstructural evolution in dissimilar ferritic welds, activities to describe the observed phenomena by modelling and simulation and discusses the performance of these welds at high temperature. Lastly, a well-engineered approach to the design of ferritic DMWs is discussed in the context of thermal power plants which are subject to damage by creep. Abbreviations: HAZ: heat affected zone; PWHT: post weld heat treatment; GMAW: gas-metal arc welding; SMAW: shielded-metal arc welding; GTAW: gas tungsten arc welding; SAW: submerged arc welding; DMW: ferritic dissimilar metal weld; CGHAZ: coarse-grained heat affected zone; FGHAZ: fine-grained heat affected zone; CDZ: carbon-denuded zone; CEZ: carbon-enriched zone; CSEF: creep strength enhanced ferritic.