Microstructure evolution of nickel-based superalloy with periodic thermal parameters during rotary tube piercing process

Due to the interaction of compression and torsion deformation, the grain refinement mechanism of rotary tube piercing (RTP) process is still ambiguous. Therefore, the microstructure evolution of a nickel-based superalloy was investigated by combining numerical simulations with experiments. By establishing a thermo-mechanical coupling finite element model (FEM), the evolution of temperature, strain, and strain rate for RTP process was studied quantitatively. The corresponding experiments were conducted on the independent-developed piercing mill, and the influences of reduction rate and roll speed on microstructure were discussed by the control variable method. The results reveal that the periodic shear strain components play a dominant role in the grain refinement process. Under single pass RTP process, the average grain size is refined from 130 to 25 mu m. The microstructure is divided into three typical states: the coarse grains before entering the deformation region, the mixed grains in the preparation region, and the fine equiaxed grains in the uniform wall thickness region and rounding region. The reduction rate has significant effects on the dynamic recrystallization (DRX) process, and the uniformity of microstructure is determined by the roll speed.