Determination of the creep transition stress in dispersion-hardened alloys by a stress-relaxation method

Abstract

AbstractDispersion-hardened alloys have secondary creep rates which show a transition from a low to a high stress dependence at a particular value of applied stress, σp. This transition stress occurs at the crossover in controlling creep mechanisms and therefore defines a lower limit for extrapolation of creep rates from high stresses. Previous methods of identifying σp for a particular system have involved creep testing or electron metallography. The work described in this paper proposes that σp may be measured by a simple stress-relaxation test. A method is evaluated and two steels, 20%Cr-25%Ni-Nb and nitrided 20%Cr-25%Ni-Ti, with previously characterized σp values are used to test the model. The agreement between σp values from relaxation tests and creep tests is good. It is concluded that the results indicate the validity of the stress-relaxation technique and show how such a short-term mechanical test obviates the need for protracted creep testing or electron metallography.