Effect of boron addition on microstructure and toughness of Ti-containing weld metals

Abstract

Ti-containing weld metals with boron (B) contents of 0-85 ppm were prepared, and the microstructural characteristics and impact toughness of weld metals were investigated. The results show that compared with the weld metal without B, in the weld metal with 22 ppm B, the fraction of acicular ferrite (AF) is increased from 26 to 76 %, accompanied by a remarkable decrease in the contents of pro eutectoid ferrite (PF), grain boundary ferrite (GBF) and sideplate ferrite (SPF). In the 39 ppm B weld metal, the microstructure basically consists of AF. However, a further increase in the B content up to 61 ppm decreases the fraction of AF to 66 % due to the formation of bainitic ferrite (BF). For the weld metal with 85 ppm B, the microstructure is wholly composed of BF. The size of martensite-austenite (M/A) islands distributed between AF plates is much smaller, and the amount is much lower than this between BF plates. In the case of the weld metals primarily composed of AF, during fracture of impact specimens, crack propagation deflects much more frequently in comparison to the weld metals with large amounts of PF, GBF and SPF, or BF, which improves the toughness of weld metals. The coarse M/A islands readily induce microcracks at the interface between M/A islands and the ferrite matrix, deteriorating the toughness. The weld metals with B contents of 22 and 39 ppm exhibit outstanding impact toughness because of a high fraction of AF accompanied by fine M/A islands.