Creep crack growth in ductile, creep-resistant steels

TLDR: In this paper, the deformation behavior of CT specimens was analyzed in detail, and it was demonstrated that the Ct parameter correlates crack growth rates during the transients, whereas C* becomes the appropriate load parameter during steady-state creep.

Abstract: Creep crack growth in two of the commonly used creep-resistant ferritic steels was investigated. Both steels were tested in the as-processed condition and after many years of service in electric power plants. The test temerature was 540°C (in one case also 500°C and 565°C) and test durations ranged from a few days to a year. In 1/2Cr-1/2Mo-1/4V steel, crack growth occurred intergranularly by grain boundary cavitation, while 21/4Cr-1Mo steel also exhibited transgranular growth. The deformation behavior of CT specimens was analyzed in detail. It was found that for typically 20 percent of the lifetime, the instantaneous elastic-plastic strains and, more importantly, primary creep determine the deformation response. The evolution of the crack length parallels that of the load-line deflection: Initially, the crack grows relatively fast, decelerates until a steady growth rate is reached, and finally accelerates due to the increasing stress intensity at the longer crack. This behavior can be described reasonably accurately by models for creep crack growth taking into account the elastic-plastic transient and primary creep. It is demonstrated that the Ct parameter correlates crack growth rates during the transients, whereas C* becomes the appropriate load parameter during steady-state creep. However, C* fails to describe crack growth in single-edge notched tension specimens with shallow cracks. This is tentatively ascribed to excessive crack-tip blunting and other geometry changes. Unloading and reloading frequently leads to accelerated crack growth.