Effects of Ti, B and O on Weld Structure and Impact Toughness of High Heat Input Flux Cored Wire

Four weld metals respectively named LOLT (Low oxygen low titanium), LOHT (Low oxygen high titanium), HOLT (High oxygen low titanium), HOHT (High oxygen high titanium) with varying Ti and O additions were prepared by laboratory high heat input welding test. The effects of Ti, B and O on microstructure and impact toughness of these weld metals were investigated. The results show that with 205 kJ/cm high heat input, the contents of O and Ti in LOLT were appropriate, and a large number of cross-distributed acicular ferrite were formed, which improved the impact toughness; with the increasing Ti addition, granular bainite was formed in LOHT, which deteriorated its impact toughness. Compared with LOLT, the number of inclusions in HOLT was increased due to the higher O addition. Meanwhile, coarse grain boundary ferrite (GBF) was formed at prior austenite grain boundary because the interaction of B to the excess O prevented its segregation at austenite grain boundary, which resulted in the decrease of weld metal impact toughness. However, as Ti addition increased again, the number of inclusions in HOHT did not change much but GBF was not formed, which improved the impact toughness.