LONG-TERM CREEP CRACK BEHAVIOR OF TYPICAL POWER-PLANT STEELS

The creep crack behaviour of the steels was investigated in a wide loa ding range up to a test duration of 40 000 h and down to a creep crack growth rate of 2 . 10(-5) mm/h with specimens of different shape and size. For steels of type 1%Cr-1%Mo-0.6%Ni-0.3%V, 1%Cr-0.9%Mo-0.7%Ni-03 .%V, 12%Cr-1%Mo-0.3%V-0.22%C and 12%Cr-1%Mo-0.3%V-0.20%C tested at 550 -degrees-C, the creep crack growth rate could be described by the para meter C2 with significantly smaller scatter bands than by the paramet er C1 or the stress intensity factor K(I). For steel 12%Cr-2%Ni-1%Mo tested at 450-degrees-C, parameter K(I) leads to the smallest scatter band. The creep crack initiation can be described in a two-criteria di agram based on nominal stress and stress intensity factor. However the method is assumed to be over-conservative in case of increasing speci men size. As a result of several aperiodic creep fatigue crack tests, precracking under fatigue conditions gave a weak increase of the creep crack growth rate whereas by precracking under creep conditions the f atigue crack rate was strongly decreased.