Role of trace boron on microstructural evolution and mechanical performance of arc welded steel joints

Abstract

This work explores how small additions of boron influence the microstructure and mechanical behavior of arc-welded mild steel joints. Welds produced without boron were compared with those containing 45 ppm and 47 ppm boron. Mechanical performance was evaluated using tensile, hardness, and Charpy impact testing, while microstructural features were examined through FESEM, EBSD, and XRD analyses. The results show that even trace amounts of boron can noticeably improve weld performance. The joint containing 47 ppm boron exhibited the highest strength and impact resistance, with a moderate increase in tensile strength and a significant improvement in absorbed energy compared to the boron-free weld. These improvements are closely linked to the formation of finer microstructures, particularly acicular ferrite, along with an increase in high-angle grain boundaries. Fracture surface analysis revealed a clear shift from brittle behavior in the boron-free weld to a more ductile failure mode in boron-modified joints. Finite element simulations further supported the experimental findings by indicating a more uniform stress distribution in the presence of boron. Although the difference between 45 and 47 ppm is small, the study provides useful insight into the role of boron near its upper practical limit in welding applications, while also acknowledging the challenges associated with controlling such low concentrations.