Creep behavior analysis of 25Cr-20Ni stainless steels with a special reference to tertiary creep

Abstract

For two kinds of 25Cr-20Ni stainless steels, A and B with and without a small amount of Nb and N, creep behavior has been studied in a stress and temperature range from 141 to 343 MPa and from 923 to 973 K with a special reference to tertiary creep. The average creep life of the A steel was about 100 times longer than that of the B steel. The logarithm of strain rate (ε) in a tertiary stage was well expressed by the following equation: ln ε = ln ε0+Ω·ε, where ε0 is the imaginary initial strain rate, ε is the true strain and Ω is the strain rate acceleration factor. The apparent activation energy for the initial creep rate was 330 kJ/mol in the A steel, while that of the B steel was 274 kJ/mol in a power law creep region and 478 kJ/mol in a region of power law breakdown (PLB). The activation energy for the B steel below PLB is close to the that for self-diffusion. When compensating for the temperature dependence of the Young's modulus and the omega value, it was found that the apparent activation energy for the A steel was reduced to the activation energy for diffusion of chromium atom in a gamma steel. The stress exponent of the B steel was about 12 above PLB and 5.1 in a power law creep region. Notwithstanding that the creep condition for the A steel was in a power law creep region, its stress exponent was 8.3 larger than that of the B steel corresponding to the same creep conditions. This was ascribed to the presence of fine precipitates in the A steel. When stress was increased abruptly during deformation, instantaneous plastic strain was observed in the B steel but not in the A steel. These results as well as TEM observation suggest that dislocations in the A steel drag the atmosphere composed of Cr-N clusters behind them.