Microstructure and Fracture Toughness of Fe–Nb Dissimilar Welded Joints

Abstract

The relation between the microstructure and mechanical properties of the Fe–Nb dissimilar joint were investigated using nanoindentation. The weld metal consists mainly of Fe2Nb, α-Fe + Fe2Nb, Nb (s,s) and Fe7Nb6 phases. Radial cracks initiate from the corners of the impressions on the Fe2Nb phase (~20.5 GPa) when subjected to a peak load of 300 mN, whereas the fine lamellar structures (α-Fe + Fe2Nb) with an average hardness of 6.5 GPa are free from cracks. The calculated fracture toughness of the Fe2Nb intermetallics is 1.41 ± 0.53 MPam1/2. A simplified scenario of weld formation together with the thermal cycle is proposed to elaborate the way local phase determined the mechanical properties.

Impact of incorporating FeNbC into weld flux on the abrasive wear resistance of coatings produced by SAW in a microalloyed steel

“…The coating of the F15 specimen presented an austenitic matrix, with Fe 2 Nb dendritic blocks (Laves phase), also identified by Qiaoling et al [27] and Vob et al [28]. In addition to the dendritic structure, the microstructure is composed of the NbC phase in the form of a so-called 'Chinese writing' structure, as suggested by Kan et al [29], as a consequence of a constitutional undercooling caused by the rapid cooling rate and a depletion of Nb and C dissolved in the liquid along the interface of the solidification of primary NbC particle.…”

“…The microstructure is, hence, characterized by an austenitic matrix in a dendritic structure composed basically of Laves (Fe 2 Nb) phases. According to literature [27,28], this Fe 2 Nb dendritic structure can be formed with up to 9% of Nb. Figure 2(d) shows the microstructure of the FZ of the F15 sample (commercial flux + 15% FeNbC), and significant microstructure changes occur from this percentage, evidencing a larger interaction of the FeNbC with the metal solidification front.…”