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1987
Journal Article
Titel
Creep crack growth in ductile, creep-resistant steels
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 temperature was 540 degrees C and test durations ranged from a few days to a year. In 1/2Cr-1/2Mo-1/4V steel, crack growth occured intergranularly by grain boundary cavitation, while 2 1/4Cr-1Mo steel also exhibited transgranular growth. The deformation behavior of CT specimens was analyzed in detail. It was found that for typically 20% of the lifetime, the instantaneous elastic-plastic strains and, 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 integral becomes the appropriate load parameter during steady-state creep. However, C integral 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. (IWM)