The Formation of γ′-Denuded Zone and the Effect on the Properties of Inconel 740H Welded Joint After Creep at 750°C

Abstract

The formation of γ′-denuded zone and effect on the properties of Inconel 740H (IN 740H) welded joint after creep at 750°C for more than 10⁴ h were systematically investigated by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the creep rupture strength of IN 740H welded joint at 750°C for 10⁵ h was extrapolated to be 97.8 MPa. The formation of coarsened elongated γ′ phase in base metal (BM) attributes to the discontinuous coarsening (DC) of continuously precipitated γ′ phase, while that in weld metal (WM) results from DC of continuously as well as discontinuously precipitated γ′ phase. The formation of γ′-denuded zone mainly results from DC of γ′ phase and precipitation of large M₂₃C₆ carbides at grain boundaries. Grain boundary migration plays an important role in the formation of γ′-denuded zones. The precipitation strengthening of γ′ phase plays a vital role in influencing the hardness of BM. The formation of γ′-denuded zone not only deteriorates the creep rupture strength but also decreases the hardness. Easier formation of γ′-denuded zone in WM leads to the change of creep rupture location from BM to WM after long-term creep.