Industrial Gas Turbines have traditionally been designed for base load operation. However, modern applications require flexibility as well as availability. These requirements lead to components in the hot section experiencing both cyclic and hold time loading, at high temperatures. These loading profiles inevitably lead to damage from both fatigue, due to engine cycles, and creep, due to dwells at load, as well as the interaction of these two damage mechanisms over the duration of the service interval. It is these interactions which can lead to higher average creep rates and more damage than expected.
This paper presents the results of a study into the influence of cyclic loading on the average creep rate for a proprietary single crystal nickel based alloy, which is based on the chemistry of INCONEL® 792, a relatively high chromium, gamma/gamma-prime strengthened superalloy. Creep tests have been conducted isothermally with reload cycles of varying duration (or dwell) that result in an unexpected reduction in creep life and an increase in overall creep rate when compared to the continuous isothermal creep tests performed at constant stress without reloading. A hypothesis is presented which attributes the increase in overall creep rate to the influence of a recovery potential stress. The dwell period of each reload cycle is critical to calculating the recovery potential and subsequent creep rate. Results show that tests with relatively short dwell periods exhibited lower lives and higher creep rates than tests with fewer cycles and longer dwell periods. The implication of these findings are significant when considering actual operation, where variations in engine cycles and dwell periods, could influence the accumulation of creep damage. Therefore, an initial approach is presented which accounts for cycles as part of the creep damage calculation.