TRANSITION FROM POWER-LAW TO VISCOUS CREEP-BEHAVIOR 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 rat es correspond to viscous behaviour under the above conditions, charact erized by the apparent stress exponent close to 1. Since the stress ex ponent at higher stresses is about 10, the change in the deformation m echanism at lower stresses is evident. The deformation mechanisms map resulting from the presented data shows that the service loading condi tions 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 Li's equ ation. Preliminary annealing at 873 K for 1.8 x 10(7) s reduces the pr imary strain, but it has no effect on the steady state creep rate. (C) 1997 Elsevier Science S.A.