Thermal Exposure Effect on the Microstructural and Mechanical Properties of a Laser-Welded Inconel 617 Joint in an Air Environment

Abstract

A laser-welded Inconel 617 superalloy was thermally exposed at 750 °C for up to 7000 h. It was found that carbides (M23C6, M6C-type Ti(C,N)), topological close-packed (TCP) μ phase and γ' phase (Ni3Al) were formed in the laser welds of Inconel 617 during thermal exposure. The spherical γ' phase in laser-welded Inconel 617 coarsens due to Ostwald ripening, while it shows better thermal stability under long-term exposure at 750 °C. The relationship between the carbides (the globular M6C, the short rod-like M6C and the block-shaped M23C6 carbide) and the matrix γ is a cube-on-cube orientation, while the relationship between the long rod-like M6C carbide and matrix γ is a noncoherent orientation. An increase in the volume fraction and average diameter of the M23C6 and M6C carbides is controlled by the diffusion of C, Cr and Mo atoms. The precipitation of the block-shaped Mo-rich μ phase is promoted in the carbon-depleted region around the M6C carbide by the segregation of Co and Mo. The mass precipitation of the γ' phase, M6C carbide and M23C6 carbide results in an increase in the hardness and a significant drop in the impact toughness of the exposed welded metals after 7000 h of exposure.