| Summary: | Approved for public release; distribution is unlimited === The use of Ultra-Low Carbon Bainitic (ULCB) steels for weld wire applications is an area of current interest to the U. S. Navy and is being jointly studied by the Naval Postgraduate School and the Naval Surface Warfare Center, Annapolis, MD. The focus of the present work is to determine the effect of macrostructure, microstructure, and the size, distribution and chemical composition of the non-metallic inclusions on the strength and impact toughness of multipass Gas Metal Arc (GMA) and Gas Tungsten Arc (GTA) welds. Eight sample multipass GMA and GTA weldments using ULCB weld wire were studied by optical, scanning electron and transmission electron microscopy (SEM and TEM). The microstructures of the weld metals were dominantly bainitic except for the recrystallized regions of the GTA welds which had become ferritic. The macrostructure of the GMA weldments was dominated by columnar grains. SEM and optical fractography suggested that this macrostructure is responsible for the corresponding poor toughness in these weldments. In all weldments the non-metallic inclusions were found to be very small (on average % 0.5 microns) with a somewhat higher volume fraction in the GMA vice GTA weldments. Based on previous work, the small average size of these inclusions are surmised to have had very little effect on toughness
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