Hardness model for creep-life assessment of high-strength martensitic steels

The development of creep-life assessment technology for creep-strength enhanced ferritic steels such as Grades 91 and 92 is strongly demanded by power-plant operators. However the degradation and failure mechanisms of these high-strength steels with martensitic structure have not yet been well clarified due to the complicated interaction among dislocation structures, precipitates, solute atoms and stress/strain conditions. The simple hardness measurement technique has been extensively applied to assess the material conditions and to detect creep deterioration. In this study based on the hardness changes measured on the specimens creep-tested and thermally aged at various conditions, a concept of strain-induced softening and stress-induced softening in the martensitic steels is proposed and discussed. The creep-softening mechanisms in the martensitic structure which is composed of lath matrix, lath boundaries, block boundaries, packet boundaries, prior-austenite grain boundaries, precipitates and dislocations under the uni-axial or localized multi-axial stress/strain conditions are to be considered to establish a hardness model for creep-life assessment.