| Abstract Scope |
We investigated the microstructure distribution of large ingots through longitudinal section dissection, which revealed that columnar crystals of ingot were exceptionally developed at the surface, with length of approximately 10 cm, and no significant fine-grain layer was observed on the surface. Instead, a distinct fine-grain band was found at the mid-radius of the ingot. It was discovered that the columnar crystal structure on the ingot surface was highly prone to retaining thermal stress during cooling as simulating the solidification and annealing processes, which can initiate microcracks during subsequent hot deformation process. Analysis of microstructure composition revealed significant Ni element segregation from the surface to the center. Hot tensile test showed that surface columnar crystal structure exhibited the poorest grain deformation coordination during hot working plastic deformation due to its uniform orientation. Additionally, this region had the highest residual stress from solidification and cooling, resulting in the worst hot thermoplastic. |