Microstructural Evolution in High Temperature Creep and Thermally Aged HA230

Haynes Alloy 230 is a sheet material used for combustor components in a number of small industrial gas turbines manufactured by Siemens. During normal operating service the material is subjected to high temperatures and cyclic mechanical and thermal stresses, which can lead to degradation of the microstructure and mechanical properties of the alloy, and hence limit component design life. As a result of this a long-term programme has been initiated to investigate the effects of thermal and creep exposure on the microstructure of this material using advanced FEGSEM and analytical TEM techniques with the objectives of: - determining the effects of turbine operating factors on the microstructural evolution of the alloy during service exposure; - identification of alloy phases which could potentially act as indicators of the average exposure temperatures experienced for specific service periods; - development of a microstructurally based model to enable the assessment of in-service operating temperatures as an aid to evaluation of the remnant life of HA230 combustor components. Originally this alloy was specifically designed to have excellent long-term thermal stability and resistance to the precipitation of damaging phases. However, whilst this appears to be true for the case of thermal exposure, there is growing evidence from the studies conducted to date that in addition to M6C and intergranular precipitation of M23C6 resulting from thermal exposure, other types of phases may also precipitate in the alloy due to time dependent plastic deformation during long-term creep and/or thermo-mechanical fatigue exposure leading to reductions in both ductility and high temperature strength. This paper describes initial studies on the effects of long-term high temperature exposure on hardness and microstructural changes of creep rupture tested and thermally exposed samples of HA230 being carried out as part of the current COST 538 technology programme.