Microstructural evolution of 800H alloy after long-time creep rupture test at 675 °C

Abstract

This study investigated the microstructural evolution of 800H alloy tubes and their welding through creep rupture test at 675 °C with different applied stresses. Vickers hardness testing was used to determine microhardness and optical microscopy, scanning electron microscopy, and transmission electron microscopy were used to examine the microstructure of the tubes. The microhardness of the base metal, heat-affected zones, and welding seams of the tubes after testing were all significantly higher than the as-received 800H tubes due to the combination of Cr23C6 precipitates, dislocation multiplication, and work-hardening effect. The base metal of all tubes had a coarse and equiaxed austenitic structure with large Ti-N-rich precipitates, parallel needle-like Cr23C6 precipitates within twins, and discontinuously dispersed chain-like Cr23C6 precipitates along grain boundaries. As testing progressed, spherical Cr23C6 were precipitated within grains then coarsened, and the intervals between parallel needle-like Cr23C6 within twins were increased due to their low thermodynamic stability and synergistic effect of time and stress. The heat-affected zones exhibited a low correlation with test time, with all samples displaying dense spherical and needle-like Cr23C6, whereas the welding seams showed insignificant microstructural evolution. γ΄ (Ni3(Al, Ti)) precipitates were observed along the sides of the Cr23C6, further coarsening of γ΄ was observed as the creep rupture test progressed.