Investigation On Crack Growth Behavior Of USC Turbine HP/IP Rotor Steel

Different kinds of defects exist in high temperature engineering components, such as flaws and inclusions, which may induce crack initiation and crack propagation at long-term service conditions. For improving reliability of these components and developing more accurate life prediction, it is important to understand initiation and growth behavior of cracks at high temperatures. Steam turbine rotor works at high temperature creep condition, so it's necessary to research creep crack growth behavior. Additionally, since a more flexible service mode of power plants causes start up and shut down events as well as variable loading conditions, crack behavior at creep-fatigue interaction condition must be considered for life assessment of these components.The crack growth behaviors of ultra supercritical (USC) steam turbine rotor steel 1Cr10Mo1W1NiVNbN have been investigated under creep and creep fatigue conditions at 600℃. The tests were carried out by means of compact tension (CT) specimens and the crack length was measured by D.C. electrical potential technique. Creep (creep-fatigue) crack growth rate was evaluated by stress intensity factor K. The effects of different initial stresses and different hold times at maximum load on crack initiation and growth behavior have been also studied. In order to study the creep crack growth modes of different conditions, the fractography of tested specimens were observed by means of OM and SEM.As the load increases, the crack initiation time becomes shorter, and the initial creep crack growth rate becomes higher, the time needed for crack to grow to the same length becomes shorter too. The crack growth rate is well correlated by the stress intensity factor K and creep fracture mechanisms depending on the stress condition. Creep voids were found at low stress condition, and intergranular fractures were found as the main crack growth mode. At high stress condition, the creep crack growth mainly characterizes with transgranular mode.Because of fatigue influence, the initiation time of creep-fatigue crack becomes shorter as a shorter hold time. Transgranular and intergranular fracture modes were both observed at creep fatigue condition. As the hold time increases, the time needed for crack to growth to the same length becomes longer. Additionally creep damage features such as creep voids were found and the proportion of intergranular feature was increased.