Subject of the present investigation is the elastic-plastic rotating annular disk having a rigid casing. The plane state of stress is considered for two different boundary conditions: a) inner surface with constraint: allowing no motion in the radial direction, b) free inner surface without radial stress. The analysis is based on Tresca's yield condition, its associated flow rule, and linear strain hardening material behaviour. For both cases, closed form solutions are obtained and numerical results are presented. It is shown, that the influence of hardening parameter on the elastic-plastic interface radii is very small, for fized boundary conditions.