Thermal Stability of a Ni-Cr-W-Mo Alloy - Long-Term Exposures

Abstract

HAYNES® 230® alloy, based on the Ni-Cr-W-Mo system, is extensively used in gas turbines and other applications at high temperatures. It is recognized that the microstructure of many alloys under long-term service conditions can change. Microstructural changes may affect mechanical properties which in turn may determine component design life in service. Given the importance and widespread usage of 230 alloy, a number of studies on its thermal stability have been conducted. This study will focus on the effect of long-term thermal exposures at temperatures ranging from 649 to 871°C (1200 to 1600°F) on the microstructure and properties of 230 alloy. Room and elevated temperature tensile, and room temperature Charpy impact toughness properties of thermally exposed material are determined and correlated to observed changes in the microstructure. Microstructural analyses were performed using extraction/X-ray diffraction together with optical and scanning electron microscopy. The mode of fracture was ascertained by fractography of the thermally-exposed and broken test samples. The paper presents previously unpublished results for long-term exposures with durations ranging up to 50,000 hours (~ 6 years).