Microstructural observations were carried out on 10Cr-1Mo-1W-VNbN steels artificially deteriorated by ageing, creep and creep interrupted tests under various conditions. Martensitic lath structure was collapsed during high temperature exposure regardless of stress. The quantitative analysis of lath structure was performed by measuring lath width. The mean lath width increased with time and the increasing trend corresponded to both the creep strain and softening. The theoretical formulation of the softening was designed based on the kinetics of the dislocation structure. The formula gave quantitative correlation among the creep stress, temperature, time and hardness. The creep damage at any given condition was estimated by this formula, and the estimated result was in good agreement with the experimental value of the hardness at any creep time fraction t/tr.
