実機で長時間使用された2.25Cr-1Mo鋼のクリープ挙動解析

抄録

For the remaining creep life assessment, it is essential to evaluate the degree of structural degradation. In the present study, accelerated creep tests have been employed to assess the degree of structural deterioration of a 2.25 Cr-1Mo steel which was service-exposed for 11 years as a power plant component. The experimental results showed that strain rate over a wide rage of strain is expressed by the equation; lnε =lnε₀+Sε, where ε₀ is the imaginary initial creep rate and S is the strain rate acceleration factor. The magnitudes of ε₀ and S are given by the following equations; ε₀=A₀σ₀ⁿexp(-Q₀/RT). and S=A_sσ₀^-n_sexp(Q_s/RT), where A₀ is the mechanically defined structure factor, σ₀ is the initial stress, n is the stress exponent (9.5), Q₀ is the activation energy (429 kJ/mol), Q_s is 24kJ/mol, A_s is a constant, and n_s is 0.98. Comparing these results with those of prestrained materials, it was found that the magnitudes of A₀, A_s and n_s are dependent upon the structural deterioration while those of n, Q₀ and Q_s are independent of thermo-mechanical history. This means that the rate controlling mechanism is independent of structural deterioration. These historydependent parameters allow us the quantitative assessment of the structural deterioration of service-exposed materials.