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Several continuum damage mechanics (CDM) modelling approaches for predicting creep deformation of tempered ferritic steels have been developed in the literature, which have evolved from efforts to extend the operability of power plant components. Few of these models, however, focus on damage assessment of ex-service states of power plant steels through the extraction of damage parameters. Furthermore, few CDM approaches leverage the high density of creep curve data available through full-field strain measurement techniques such as digital image correlation (DIC). This work uses multiple creep curves obtained from DIC computed strain data at several stresses and temperatures from individual specimens of X20CrMoV12-1 (X20) piping steel. These curves serve as input data to a modified Oruganti continuum damage mechanics (CDM) model whereby microstructural-specific damage parameters can be extracted. Good agreement is noted between CDM-extracted parameters and microstructural, creep cavity density and hardness damage indicators. Damage parameters based on subgrain growth are particularly sensitive to the ex-service state of the X20 steel. The proposed CDM approach using DIC computed creep curves is shown to be a material efficient alternative to traditional damage assessment methods of ex-service material. [ABSTRACT FROM AUTHOR]