Microstructures and intermediate temperature brittleness of newly developed Ni-Fe based weld metal for ultra-supercritical power plants

Temperature dependent tensile behavior of Ni-Fe based weld metal by gas tungsten arc welding (GTAW) was evaluated in the range of 350-820 degrees C at a strain rate of 0.035/min. Intergranular fracture and intermediate temperature brittleness (ITB) took place at around 750 degrees C. The microstructure and the fracture surface morphology observation by optical microscopy, scanning and transmission electron microscopy showed that the ITB to a large extent depended on the grain boundary sliding (GBS) and the gamma prime (gamma') precipitation during the elevated temperature tensile deformation. X-ray diffraction (XRD) confirmed that the primary phases formed during the last stage of solidification were mainly TiN nitrides, MC carbides, and Laves phases in the form of Laves/gamma eutectics. Quantitative statistics of the Laves/gamma eutectics and the MX(MC, TiN) phases were processed. These primary phases had a pinning effect on the migration of grain boundaries and accordingly made the grain boundaries tortuous. The tortuous grain boundaries were expected to inhibit the GBS, relieving ITB. Transmission electron microscopy confirmed that fine Laves particles precipitated along the grain boundary in the weld metal with higher Mo content during the deformation process, and these Laves particles were also supposed to be one factor for inhibiting the GBS.