Change in creep curve shapes and applicability of θ projection method of Ni-22Cr-18Fe-9Mo alloy

Change in creep curve shapes and applicability of θ projection method of a Ni-22Cr-18Fe-9Mo alloy developed for an intermediate heat exchanger of high temperature gas-cooled reactors have been investigated with the following results: 1) Creep curves at 800degC for this alloy, in solution treated and in aged conditions, are tertiary creep dominated ones where most of the life is an accelerating creep stage although a primary creep stage is slightly recognized in some cases. 2) Creep curves at 1000degC for this alloy in a solution treated condition are irregular ones which have a region with a low creep rate preceding the steady-state creep rate and accelerating creep rate. This low creep rate stage diminishes after ageing at 1000degC. Creep curves at 1000degC for the aged alloy are normal type ones. 3) Creep rates at 800 and 1000degC increase after ageing at 1000degC. This effect is small under low stresses at 1000degC. Rupture strain of the aged alloy increases because carbides formed on grain boundaries by ageing suppress formation and growth of voids and cracks. 4) Normal type creep obtained at 1000degC for aged Ni-22Cr-18Fe-9Mo alloy are fitted using the θ projection method with four θ parameters. Stress dependence of θ parameters can be expressed in terms of simple equations. 5) The θ projection method with four θ parameters cannot be applied to the remaining creep curves where most of the life is occupied by a tertiary creep stage. Therefore, the θ projection method consisting of only the tertiary creep component with two θ parameters was applied. The creep curves can be fitted using this method. (author)