Effects of Long-Term High Temperature Exposure on the Microstructure of Haynes Alloy 230

Haynes Alloy 230 is a nickel-based 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. This PhD thesis describes studies of the effects of long-term high temperature exposure on hardness and microstructural changes in creep rupture tested and thermally exposed samples of HA230. Advanced FEGSEM was used for documentation of the microstructural evolution in the temperature range of 500-1170°C following exposure up to 30000 hours. For identification of alloy phases, which could potentially act as indicators of the average exposure temperatures experienced for specific service periods, XRD and analytical TEM techniques were used. Primary carbides M12C and secondary carbides, M12C and M23C6, were identified. No TCP phases were observed within the microstructure of the tested samples, but the secondary M23C6 forms large ‘pool-like’ precipitates’ above 990°C which are very hard and brittle and could be detrimental for the material (e.g. reductions in ductility, high temperature strength). A hardness and microstructurally based model was used for the assessment of an ex-service HA230 transition duct and could be an aid for evaluation of the remnant life of HA230 combustor components. The tests were carried out as part of the COST 538 technology programme between 2004 and 2009.