Solidification Behavior And Microstructural Evolution Of A High-chromium Nickel-base Alloy Weld Deposited Metal

The solidification behavior and microstructural evolution during post welding heat treatment of a high-chromium nickel-base alloy weld deposited metal was examined with thermodynamic calculation, optical microscope?OM?, scanning electron microscopy?SEM? and transmission electron microscopy?TEM?. The heat treatments consisted of 650? for 4 hours?650?-4h?, 750?-4h, 850?-2h, 4h, 6h and 8h, and 950?-4h. The results indicated that the microstructure of the weld deposited metal was composed of dendrites. Microsegregation potential of major alloying elements in the weld deposited metal was investigated. The microconstituents formed during solidification were identified to be TiC and ?/Laves eutectic constituent. Most ?/Laves eutectic constituents were located at the dendrite interstices due to the microsegregation of Nb, Mo and Si. The volume per cent of ?/Laves constituent was predicted by using Scheil equation and there was a reasonable agreement between the predicted and measured value. Two possible solidification paths were proposed in the ?-Nb-C pseudoternary solidification diagram to analyse the solidification sequence of the new high-chromium nickel-base alloy. The ?/Laves eutectic constituents would dissolve when the temperature was up to 950?, while TiC would still exsit in ? matrix. The g ? phase precipitated at interdendrite regions in the weld deposited metal after heat treatment at 650 ? for 4h. When the temperature increased to 850?, the metastable g ? phase directly dissolved into the ? matrix. At 650?, the Cr₂₃C₆ carbides precipitated, and it was coherent with ? matrix. The morphology of Cr₂₃C₆ carbides transformed as the temperature raised, it turned from thin film shape to rod at 750?, and became discrete granular particles at 850?. The acicular Cr₂₃C₆ carbides and ? phase precipitated after heat treatment at 850? for 2h, and the acicular Cr₂₃C₆ carbides continuously coarsened with an increase in the heat treatment time. When the temperature increased to 950?, the acicular Cr₂₃C₆ carbides disappeared, whereas the ? phase coarsened.