Effect of short-time annealing on stability of cryorolled nickel

Scanning electron microscopy, X-ray diffraction analysis and hardness measurements were used to study changes in the structure and strength of pure nickel subjected to furnace annealing after isothermal cryogenic rolling at liquid nitrogen temperature in the range of 30 – 90 % reductions. The aim of the present study was to evaluate and discuss the controlling factors of the stability under short-term annealing of the structure of Ni cryorolled to different strains. It was found, that a half-hour annealing in the temperature range of 250-500°C of cryorolled metal with dislocation-cellular work-hardened structure activated its static recovery and recrystallization. The intensity of these processes was the stronger the more the metal was work-hardened and the higher was the heating temperature. As a result, annealing of samples in all rolled states with a maximum temperature led to their softening down to the corresponding level of the fully annealed state. The highest thermal stability lasting up to the temperature of 400°C was demonstrated by the metal subjected to minimum straining. The structure with a minimum grain size of about 6 – 7 μm was processed in 90 % cryorolled Ni after annealing almost with 400°C. The nature of the metal structure transformations under annealing and its higher thermal stability in less rolled condition is considered.