Aspects of creep-fatigue in gas turbine hot parts

Abstract: The goal is reliable creep-fatigue lifing methodology for industrial gas turbines.A literature study on high temperature materials used in gas turbines reveals that:• Most testing/modelling work has focused on characterisation and modelling of material response in isothermal standard tests, i.e. creep, tensile and LCF tests. This is not applicable to creep-fatigue lifing of industrial gas turbines. The difference between test and component conditions implies that models calibrated with standard data would be used far outside their domain of confidence.• A limited amount of testing/modelling work has included TMF testing. TMF test conditions are reasonably close to component conditions in aero turbines but the results are still not applicable for industrial gas turbines which, unlike TMF tests and aero gas turbines, are run at full load and therefore at maximum temperature, for extended times, typically in the order of 40000h.• A small amount of tests on aged material can be found in literature. It provides valuable butinsufficient insight into the effects of long temperature exposure.The conclusion is that methodology improvement requires more appropriate testing. Testing/modelling work based on this has been done. Results so far:• Constitutive and life models have been developed for the carbide precipitating material Haynes 230. We have been limited to 'what can be done' in existing test rigs but the resulting methodology constitutes a definite step forward.• Constitutive models for the gamma prime strengthened material IN792 have been evaluated and calibrated using creep and 'step relaxation' data. This is a step forward compared to use of standard data but we have yet to perform tests on aged material and anisothermal verification tests to finalise the modelling.• We are developing a prototype for a new class of TMF rigs. Heating, cooling and insulation is used to reproduce local conditions at critical points in components under controlled laboratory conditions. The rigs will be designed for long term testing at a reasonable cost. The drawback is that the test conditions, e.g. strain ranges, must be calculated by FE simulation rather than measured.• Tests on virgin and aged specimen are ongoing to characterise the embrittling effect of long term exposure as a basis for a rational ranking of materials, i.e. on the basis of tests on appropriately aged rather than virgin material.Most of the results of the testing/modelling work should be of generic value for the material classesrepresented by Haynes 230 and IN792.