Comparison of low stress creep properties of ferritic and austenitic creep resistant steels

Creep tests at stresses below 30 MPa and temperatures from 923 to 1073 K were conducted on an austenitic AISI-316H creep resistant steel. The helicoid spring specimen technique was used with the average test duration of ∼2000 h. The results were compared to the results obtained on modified 9% Cr ferritic P-91 type creep resistant steel by the same experimental technique under comparable conditions. The shapes of creep curves for both steels are rather similar, with a deep decrease in creep rate during primary stage followed by a secondary stage with constant strain rate. Viscous creep (i.e. creep having the stress exponent close to one) was demonstrated in both steels under the loading conditions described above. The secondary creep rate for AISI-316H is about two times lower than that of P-91 under the same testing conditions. The difference can be easily explained by the difference in mean grain size of both materials, supposing that the diffusional creep is the prevailing deformation mechanism. Unfortunately, it is not possible to identify creep mechanism clearly due to no possibility to obtain specimens with different grain size and the same microstructure. Further, the creep mechanism identification is complicated by the fact, that the measured apparent activation energy is almost the same for both materials. This result is in contradiction to the diffusion data published in the literature, where a considerably lower value of activation enthalpy in γ-phase than in α-phase have been reported for both volume and grain boundary diffusion.