Creep Crack Growth of Interrupted High and Low Creep Ductility Grade 91 Steel

This research compares creep crack growth behavior of two heats of the creep strength enhanced ferritic (CSEF) steel, grade 91. The two heats represent extremes of creep damage susceptibility for this steel with one heat exhibiting low creep ductility while the other heat exhibits high creep ductility. Crack growth was monitored with direct current potential drop and a high optical magnification lens. Load line displacement was measured throughout the duration of the tests. Compact tension specimens machined from the high and low creep ductility heats were loaded at an identical initial stress intensity factor (Kmax,i) and tested for more than 2,000 hrs. Each test specimen was tested to a load line displacement or crack extension of approximately 1 mm at which time the test was interrupted, sectioned, mounted, and analyzed using optical and scanning electron microscopy to assess the present oxidation, micro-cracking, creep damage, and void density. Supporting metallurgical evidence and obtained test data through the duration of each crack growth test was scrutinized to critically evaluate crack growth relationships linked to creep crack growth parameter (C*) and stress distribution and creep damage distribution in the vicinity of the crack.