Hot deformation behavior and processing optimization of a Co–Ni secondary hardening steel: constitutive modeling, processing maps and microstructural analysis

The hot deformation behavior of a Co–Ni secondary hardening steel was investigated using Gleeble-3800 compression tests at 900–1150 °C and strain rates of 0.05–5 s –1 . A strain-compensated Arrhenius-type constitutive model based on the hyperbolic sine law was established using friction- and temperature-corrected flow stress data. The activation energy was determined to be 447 kJ·mol –1 , indicating high deformation resistance during hot deformation. The model showed high fitting and predictive accuracy within the investigated deformation domain. Processing maps based on the dynamic materials model reveal that high power dissipation efficiency domains were mainly located at 1000–1150 °C, whereas flow instability occurs under low-temperature and high-strain-rate conditions. EBSD analysis shows a transition from elongated deformed grains to more equiaxed recrystallized microstructures with increasing temperature and decreasing strain rate, accompanied by a reduction in KAM values. The dominant softening mechanism evolved from work hardening to dynamic recovery and dynamic recrystallization. An optimal processing window of 1000–1050 °C was identified over the entire investigated strain-rate range of 0.05–5 s –1 , where flow instability can be avoided while stable deformation and refined microstructures can be simultaneously achieved.