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

Haynes Alloy 230 was specifically designed to have excellent long-term thermal stability and resistance to the precipitation of damaging phases. This paper describes in detail studies on the effects of long-term high temperature exposure on the hardness, microstructural changes and tensile properties of thermally exposed samples of Haynes Alloy 230. The samples from the 2mm thick sheet material have been investigated using X-Ray diffraction and advanced electron microscopy techniques (FEGSEM, TEM etc.). The evolution of the precipitating phases was monitored across a wide range of temperatures (from 500°C to 1170°C) and durations (from 24 hours up to 30000 hours) and several key phases have been identified. In addition to the primary W-rich carbide and the precipitation of Cr-rich M23C6, a new brittle phase/carbide was observed within the microstructure at the highest exposure temperatures (above 930°C). The microstructurally based model assists in the assessment of in-service operating temperatures as a means of evaluating the remaining operational life of the components.