Effect of boron treatment on the microstructure and toughness of Ti-containing steel weld metals

Ti-containing steel weld metals with boron addition contents of 0-85 ppm were prepared, and their microstructural characteristics as well as the impact toughness were investigated. The results show that in these microstructures, compared to the weld metal without boron, the addition of 22-39 ppm boron results in a remarkable increase in the amount of acicular ferrite at the expense of grain boundary ferrite, idiomorphic ferrite and side-plate ferrite. However, with a further increase in the boron content up to 61-85 ppm, the bainitic ferrite is formed, accompanied with a drop in the amount of acicular ferrite. In the acicular ferrite, the size of martensite-austenite (M/A) islands is much smaller, and the amount is much lower than those found in the bainitic ferrite. In the case of the weld metals primarily composed of acicular ferrite, during the fracture of the impact specimens, the crack propagation path is more bent in comparison with the weld metals with large amounts of grain boundary ferrite, idiomorphic ferrite, side-plate ferrite or bainitic ferrite, which that the presence of acicular ferrite improves the toughness of the weld metals. The coarse martensite-austenite islands readily induce micro-cracks at the interface between martensite-austenite islands and ferrite matrix, deteriorating the toughness. The weld metals with B contents of 22-39 ppm exhibit outstanding impact toughness because of high amount of acicular ferrite, accompanied with fine martensite-austenite islands.