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Abstract
To obtain so-called super-steels providing a good combination of high strength and high toughness, efforts are being made to develop ultrafine-grained steels involving intense grain size refinement with retention of a mild steel-equivalent low-alloy composition in order to lessen their environmental impact and reduce total life cycle costs.1–5 Within this context, it is imperative to develop suitable welding and joining technologies for structural applications of ultrafine-grained steels; a number of previous studies have addressed their welding characteristics.6–8 Developed ultrafine-grained steels, however, face a number of important problems such as grain coarsening, HAZ softening and microstructural changes due to the thermal effect of the welding process itself. Such microstructural changes are predicted to exert a significant effect on joint characteristics, particularly deterioration of welded joint performance in ultrafinegrained steels as steels subjected to a high degree of grain size control. Microstructural changes in the joints of ultrafine-grained steels, however, have not yet been satisfactorily investigated.