Effect of loading history on the threshold stress in the creep deformation of an austenitic stainless steel

The steady state creep rate of AISI 316L(N) SS at 923 K can be described by the threshold stress compensated power law relationship. The threshold stress for creep deformation over the stress range 142-230 MPa has been evaluated graphically as 60 ± 10 MPa from the conventional creep tests. A threshold stress of about 110 MPa was computed from two stress decrease tests which is much higher than that obtained from conventional creep tests. The higher apparent threshold stress has been rationalized by invoking the effect of dislocation cell structure on the constant A in the steady state creep rate equation. A stress change test wherein the stress was progressively increased yielded a threshold stress identical to that obtained from the conventional creep tests. This implies that the steady state structure at a particular stress is path independent so long as the preceding stresses are lower. The dislocation substructure indicates that the creep deformation proceeds by the dislocation network mechanism. The threshold stress for this mechanism represents the stress for the activation of dislocation links constituting the Frank network.