Investigation of intrinsic correlation between microstructure evolution and mechanical properties for nickel-based weld metal

Abstract

The microstructure evolution was characterized in detail for nickel-based weld metal with multi-layers manufactured by narrow gap arc welding technology after thermal exposure at 750 °C up to 12,000 h. The coarsening kinetics of precipitations and its intrinsic correlation with mechanical properties were investigated and revealed. As exposure time lasted, the hardness of weld metal increased. The toughness decreased dramatically and the ripening rate of γ´ phases dropped with a slow rate of 0.0047 nm/h, finally kept stable rate when exposure time exceeded 8000 h. The coarsening rate constant of 3.86 × 10−29 m3/s for γ′ phases indicated lower coarsening kinetics and higher ripening resistance, which contributed to the good thermal stability for the weld metal. A mass of needle-like M6C carbides was found among interdendritic regions and on the grain boundaries, which became coarser with increasing exposure time due to the segregation of element Mo. Another finding was that spherical γ′ phases grew at the expense of the smaller ones and achieved stabilization at long-term exposure time. As the main factor for reducing toughness, the growth of needle-like shape M6C could be inhibited by the γ′ phases around them via preventing Mo and Cr as M6C-forming elements diffusing, giving rise to the stable toughness.