Confirmation of low stress creep regime in 9% chromium steel by stress change creep experiments

High sensitive helicoid spring specimen creep technique was modified to allow stress changes during the low stress, low strain rate experiments. The results obtained on the P-91 type modified martensitic 9% Cr steel support the previous observations of the change of the creep deformation mechanism at low stresses. The stress exponent derived from the stress change experiments approaches a value of about 4.5, considerably higher than that derived from the constant stress experiments. On the other hand, it is much lower than the value of 12, reported for the power-law creep regime of the same steel at stresses above 100 MPa. The creep behavior at low stresses is still not fully understood. ansient stages were observed after each stress change including back flow after the stress decrease. The relaxation periods are relatively long, notably after the stress decrease. The stress redistribution is suggested as a main source of the observed transient effects.