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

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 gamma '' -> delta phase and the grain growth kinetics of the gamma' 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 160 mu m. Certain precipitates, such as the dispersed gamma' 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 degrees C for 4 h shows minimal variation. With an increase in PWHT temperature, a large number of body-centered tetragonal gamma '' phases precipitate at interdendrite regions in the microstructure of DM after PWHT at 750 degrees C for 4 h. When the PWHT temperature increases to 850 degrees C, the metastable gamma '' phase directly transforms into a stable delta phase in shear mode, which exhibits a similar chemical composition but a different crystal structure than the gamma '' phase. At 950 degrees C, the gamma '' phase and the d 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 degrees C. Above this temperature, partition coefficients tend toward 1, and composition heterogeneity disappears at 950 degrees C. During PWHT, the gamma' phase continuously coarsens with an increase in PWHT temperature. The dynamic analysis shows that the coarsening behavior of the gamma' phase corresponds with the formula: (d) over bar (3) - (d) over bar (3)(0) - A . e(-Q/RT)/T . t with an activation energy of 253 kJ/mol. (C) 2014 Elsevier B.V. All rights reserved.