Interrelationship between creep deformation and creep rupture in 2¼Cr-1Mo steel

Abstract

Creep deformation and rupture have been studied in 2¼Cr-IMo steel over the stress range 60-210 MN m⁻² at 565°C. Creep damage accumulates by the initiation and growth of extensive cavitation at the prior austenite grain boundaries. Cavity formation predominates during the initial transient and individual cavities appear to nucleate on grain boundary carbides. Quantitative analysis of the cavitation kinetics in relation to the creep deformation processes suggests that cavity growth is directly related to deformation occurring at the grain boundaries. It is inferred that the vacancy diffusion mechanism is inhibited and growth is limited by the local creep process occurring at the grain boundaries. At low stresses the latter is controlled by intra-granular recovery creep such that the overall rate of cavitation is proportional to the macroscopic strain rate. However, results obtained under biaxial shear conditions imply that the rate of cavitation is additionally dependent upon the normal boundary stress or maximum principal stress. At very high stresses evidence indicates a transition in rupture process to one of ductile shear. The implications of these results are discussed with regard to high-temperature plant integrity.