Abstract

The Larson–Miller constant (C) of martensitic steel, which is used as the Larson–Miller parameter, is considerably larger than the typically used value of 20 for many types of heat-resistant steels. To provide better understanding regarding this fact, an analytical formulation for the Larson–Miller constant is developed using a model based on interactions between moveable dislocations and elastic singularities in a system. To verify the proposed equation, eight types of Grade 91, 92, and 122 steels are used, whose maximum rupture life exceeds 1E5 h. Creep data are classified into 257 groups by temperature, stress, and strain or time. C values are obtained by applying multiple-regression analyses to time parameters that obey the exponential law, assuming a thermally activated process. C and C_cal are calculated for each data group based on an exponential law and a proposed equation, respectively. The statistical values for C and C_cal are as follows: ̄C=32.41, C_min=7.87, C_max=64.88, and (C_cal⁄C)=99.3%. Although (∆C)̄=(C_cal-C) ̅=0.02 is extremely low, the standard deviation of ∆C is large, i.e., 1.27. Results confirmed that the proposed equation can estimate wide-ranging C values, although the equation is expected to be improved. A major component of C for C>15 is an increase in the entropy change caused by elastic interactions between moveable dislocations and elastic singularities in a system.

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