Improvement of microstructure and mechanical properties for underwater wet 16Mn/304L dissimilar steel welded joints assisted by presetting butter layer

Underwater wet flux-cored arc welding (FCAW) is a simple and economic method to join dissimilar steel structures widely used in offshore applications. The available research on underwater wet welding of dissimilar steel joints is inadequate. In this paper, a butter layer was applied to eliminate the formation of hardened martensite at heat affected zone of 16Mn side and to reduce the stress concentration at the fusion boundary. The influence of ERNiCrMo-3 butter layer on the microstructure, elemental distribution, and mechanical performance of underwater wet 16Mn/304 L dissimilar steel welded joints was investigated. The grain boundary type and grain growth orientation across the fusion boundary were particularly investigated. The use of the ERNiCrMo-3 butter layer helped to eliminate the formation of hardened martensite resulting from the rapid cooling rate. Type I boundary and type II grain boundary were high angle grain boundaries, which was found at the Ni-based weld metal/ferritic steel (butter layer/ferritic steel) interfaces. The impact toughness of HAZ at 16Mn side was improved by the use of butter layer because of the absence of martensite. The butter layer was accompanied by an increase in ultimate tensile strength in the orders of ~80 MPa. The expected research results can enrich the underwater welding/repair technology of dissimilar steel structures and guide the future design of dissimilar steel welded joints.