The effect of post-weld heat treatment temperature on the microstructure of Inconel 625 deposited metal

Abstract

The effect of post-weld heat treatment (PWHT) temperatures on the microstructure of Inconel 625 deposited metal (DM) was examined using an optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The transformation mechanism of the γ″→δ phase and the grain growth kinetics of the γ′ phase during PWHT were revealed. The results indicate that the microstructure of as-welded DM is composed of columnar grains of different sizes, of which the average grain size is approximately 160μm. Certain precipitates, such as the dispersed γ′ phase, blocky MC-type carbide and irregular shape Laves phase, precipitate in the microstructure of the as-welded DM. Compared with as-welded DM, the microstructure of DM after PWHT at 650°C for 4h shows minimal variation. With an increase in PWHT temperature, a large number of body-centered tetragonal γ″ phases precipitate at interdendrite regions in the microstructure of DM after PWHT at 750°C for 4h. When the PWHT temperature increases to 850°C, the metastable γ″ phase directly transforms into a stable δ phase in shear mode, which exhibits a similar chemical composition but a different crystal structure than the γ″ phase. At 950°C, the γ″ phase and the δ phase disappear, whereas certain M6C-type carbides precipitate at the grain boundaries. Alloying elements such as Nb, Mo, Si, Al and Fe in the microstructure of as-welded DM exhibit segregation behavior. Due to an increasing PWHT temperature, the segregation behavior constantly weakens with minimal evolution to the temperature of 750°C. Above this temperature, partition coefficients tend toward 1, and composition heterogeneity disappears at 950°C. During PWHT, the γ′ phase continuously coarsens with an increase in PWHT temperature. The dynamic analysis shows that the coarsening behavior of the γ′ phase corresponds with the formula: d¯3-d¯03=A·e-Q/RT/T·t with an activation energy of 253kJ/mol.