| Abstrakt: |
XH67 is a high-nickel superalloy used in high-pressure, oxygen-rich environment of rocket engine components, in view of its excellent ignition resistance over conventional superalloys. This alloy has complex chemistry and is strengthened by a combination of solid-solution strengthening (by Cr, Mo, W, and Fe), dispersion hardening (carbides of Cr and Ti in γ matrix), and precipitation hardening (γ′—Ni3(Ti, Al)) in solution-treated and aged condition. During the course of manufacturing process of rocket components, this material is subjected to multiple thermo-mechanical cycles during multi-stage forming operations followed by high-temperature annealing cycles, multi-pass welding, post-weld heat treatment, and during sintering of ignition-resistant metal-ceramic coating, which lead to alteration of its initial grain size. In this study, XH67 alloy with two different grain sizes of ~ 184 and ~ 79 μm has been subjected to simulation of HAZ conditions at various peak temperatures in the range of 1000 °C to 1250 °C with one to four number of weld passes using a Gleeble-3800 thermo-mechanical simulator to assess the heat-affected zone (HAZ) cracking susceptibility. Optical microscopy revealed the presence of cracks in both coarse and fine-grained materials after HAZ simulation for certain combinations of peak temperature and number of passes. Scanning electron microscopy along with energy-dispersive spectroscopy revealed the locations of cracks to be rich in Ti, Cr, and C, suggesting formation of eutectics. During HAZ simulation, the coarse-grained material could withstand multiple passes (4 cycles) up to a temperature of 1050 °C without formation of any defects, whereas the fine-grained material withstood up to 1150 °C without the formation of defects, even after 4 passes. Mechanical properties of defect-free specimens after HAZ simulation were evaluated, and fractography of tested specimens revealed that the coarse grain samples failed with brittle intergranular fracture, whereas the fine grain samples failed through ductile failure. The results of this study that help in mitigating the HAZ-cracking susceptibility and identifying the number of passes before defects are generated, during multi-pass welding of complex nickel-based superalloys with a given grain size, through simulation studies. [ABSTRACT FROM AUTHOR] |
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