DBTT Reduction Enabled by SiO2-Bearing Flux in Submerged Arc Welds

The ductile-brittle transition temperature (DBTT) plays a critical role in determining the integrity of welded structures under common low-temperature service conditions. Fluxes are proven effective agents that significantly alter weld compositions and resulting microstructures. In this study, we systematically investigated the impact of programmed SiO2-containing fluxes upon EH36 shipbuilding steel welds via multi-scale microstructural characterization, instrumented Charpy impact testing, and fractographic analysis. We found that incremental SiO2 addition from 5 to 40 mass-% drastically enhanced the volume fraction of acicular ferrite from 0.386 to 0.747. As the SiO2 content increased, the DBTT decreased significantly from –58°C to –97°C but slightly rose to –85°C. Fractographic and secondary-crack analyses indicated that cracks preferentially propagated along grain boundary ferrite and ferrite side plates, whereas crack paths through acicular ferrite were more tortuous. We demonstrated that enhanced toughness is primarily governed by two complementary mechanisms: (1) microstructural refinement through enhanced acicular ferrite fraction, impeding microcrack propagation; (2) minimized presence of large inclusions within coarse ferrite grains, mitigating the risk of premature cleavage fracture initiation. Such findings highlight that superior low-temperature toughness necessitates balanced grain refinement and precise control over inclusions. Overall, we showcased a cost-effective strategy geared toward achieving optimized weld microstructures and mechanical performances merely by adjusting welding fluxes, which could also shed light on the development of high-performance welding consumables for alternative low-temperature structural steel.