Investigation of secondary phases and tensile strength of nitrogen-containing Alloy 718 weldment

Abstract

The influence of nitrogen content on the precipitation of secondary phases and the tensile strength of Alloy 718 during gas tungsten arc welding was investigated. Various types of precipitates were characterized using scanning electron microscopy and transmission electron microscopy. The results showed that in the fusion zone, the volume fraction of Nb-rich phases such as Laves, (Nb,Ti)C, and δ phases, as well as Ti-rich phases such as (Ti,Nb)CN and (Ti,Nb)N, increased with increase in the nitrogen content due to the microsegregation of Nb and Ti within interdendritic areas. Nitrogen was also found to decrease the size of γ″ particles within γ dendrites. For precipitates in the partially melted zone, constitutional liquation was observed for both (Nb,Ti)C and (Ti,Nb)N particles. Based on the results of tensile tests, the weld containing 0.015wt% nitrogen exhibited the highest ultimate tensile strength (UTS), whereas more addition of nitrogen led to a decrease in both the UTS and yield strength due to the increased content of brittle Laves phases and decreased size of γ″.

Keywords

nitrogen / secondary phases / gas tungsten arc welding / tensile strength / Alloy 718

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