Transition from power-law to viscous creep behaviour of p-91 type heat-resistant steel

Short-term creep tests were performed on a 9% Cr (P-91 type) steel at temperatures from 873 to 923 K and at stresses below 100 MPa by means of the helicoid spring specimens technique. The steady state creep rates correspond to viscous behaviour under the above conditions, characterized by the apparent stress exponent close to 1. Since the stress exponent at higher stresses is about 10, the change in the deformation mechanism at lower stresses is evident. The deformation mechanisms map resulting from the presented data shows that the service loading conditions respond to the viscous creep. Extrapolation from power-law creep regime to low stresses can cause serious underestimation of predicted deformation rates. The primary stage can be described by the Liʹs equation. Preliminary annealing at 873 K for 1.8 × 107 s reduces the primary strain, but it has no effect on the steady state creep rate.