The primary (MC) and secondary (M 23C 6) carbides, and primary and secondary γ′ particles are observed in the base metal.įigure 4. Optical micrographs of the pre-weld heat-treated IN939 alloy: (a, b) γ−γ′ eutectic (c) η phases and primary γ′ particles (d) primary MC carbides.įigure 5. FESEM images of the γ′ particles in the pre-weld heat-treated specimen welded with Hastelloy X alloy as a filler metal in (a) the interface between the weld pool and the base metal, (b) a distance of about 200 μm from the weld line, and (c) the base metal. Cracks are marked by arrows.įigure 3. Optical micrograph (a) and FESEM image (b) of the microstructure of as-received IN939 alloy. Application of a suitable solid solution filler metal could partially reduce the liquation cracking in the HAZ of IN939 alloy.įigure 1. Prepared and welded coupons under the bead weld configuration.įigure 2. Optical micrographs of cracking in the heat-affected zone (HAZ) of the samples welded with (a) Hastelloy X and (b) IN718 alloys as filler metals. The microstructure of the weld fusion zones showed the presence of fine spheroidal γ′ particles with sizes of about 0.2 μm after the post-weld heat treatment, which increased the hardness of the weld pools to about HV 350 and 380 for the Hastelloy X and IN718 filler metals, respectively. The constitutional melting of the secondary, eutectic, and Zr-rich phases promoted the liquation cracking in the HAZ. Microstructural studies using optical and field-emission scanning electron microscopy revealed that the IN939 alloy was susceptible to liquation cracking in the HAZ. The pre-weld heat treatment reduced the hardness of the base metal to about HV 310. On the pre-weld heat-treated samples, upon solving the secondary γ′ particles in the matrix, primary γ′ particles in the base metal grew to “ogdoadically diced cubes” of about 2 μm in side lengths. A solid solution and age-hardenable filler metals were further studied. The main aim of this study was to investigate liquation cracking in the heat-affected zone (HAZ) of the IN939 superalloy upon tungsten inert gas welding.
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