Uni-axial and multi-axial creep behaviour of P91-type steel under constant load

The uni-axial and multi-axial creep behaviour of P91-type steel under constant load was studied. The conventional, uni-axial, constant-load creep tests were performed at initial stresses ranging from 120 MPa to 240 MPa, and at temperatures from 625 to 675 °C, while the multi-axial, small-punch, constant-load creep tests were performed at loads ranging from 350 N to 550 N, and at temperatures from 650 to 690 °C. Both types of test can be considered as short-term creep tests because the maximum time-to-rupture was less than 500 h. Since it is well known that the creep behaviour of P91-type steel cannot be satisfactorily described by a simple, Arrhenius-type, power-law constitutive model, an improved stress-dependent, energy-barrier model was used for a description of the uni-axial as well as the multi-axial creep behaviour of the P91-type steel. It was found that the obtained values of the apparent activation energies Qc during the uni-axial and multi-axial creep tests were very close, and in both types of test they were considerably higher than the activation energy for the lattice diffusion in iron.