High Temperature Microstructural Degradation of Haynes Alloy 230

Haynes Alloy 230TM is a solid solution strengthened nickelbased material used for combustion components in industrial gas turbines. In this application a primary limit to the component design life is the response of the material to high temperature exposure with and without mechanical loading. Over long periods of operation in service, this leads to degradation of the microstructure of the alloy and consequently to its mechanical properties. A detailed understanding of the processes associated with in-service the microstructural degradation of the alloy and its effects on the mechanical properties of the material is therefore of great importance in the drive for increased component life and reliable extended plant operation. In order to investigate the effects of thermal and long-term creep exposure on the degradation behaviour of Haynes Alloy 230 sheet material during service, detailed studies of the microstructural changes taking place in this material have been made using advanced analytical FEGSEM, EDX and XRD techniques. The objective of the programme is to quantify the microstructural changes and phase precipitation reactions occurring as a result of service exposure, based upon the use of laboratory controlled thermally aged and creep tested samples. Haynes Alloy 230 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 unstressed thermal exposure, there is growing evidence from the studies to date that, in addition to the primary M6C and the precipitation of M23C6 resulting from thermal exposure, other phases can precipitate in the alloy, under the influence of timedependent plastic deformation during long-term creep exposure, which can lead to reductions in both ductility and high temperature strength. This paper describes some detailed studies on the effects of long-term high temperature exposure on the hardness and microstructural changes in creep rupture tested and thermally exposed samples of Haynes Alloy 230.