Thermal stability of a Stellite/steel hardfacing interface during long-term aging

Abstract Disbonding and delamination of Stellite hardfacing used in high-temperature valves have caused serious challenges in the power generation industry in recent years. To determine the microstructural factors that may contribute to the failures, Stellite 21 and Stellite 6 hardfacing alloys were deposited onto an F91 steel substrate using plasma-transferred arc welding. A series of aging experiments simulating service temperatures were conducted at temperatures between 550 and 650 °C for exposure times ranging from 1008 to 8760 h. The F91/S21 interface was found to be unstable during aging; a hard layer was formed, consisting mainly of three phases. This layer follows a parabolic rate of growth; its thickness increases as the square root of aging time. The activation energy was determined, which might be associated with carbon diffusion. Complementary characterization methods showed that the three phases in the hard layer are: a BCC-like intermetallic compound, Fe x Co y ; a Cr-rich M 23 C 6 -type carbide; and an (Fe,Co)(Cr,Mo)-type σ phase. The area fraction and the equivalent diameter of the M 23 C 6 carbides increase with aging temperature and time, while the area fraction of the σ phase decreases with temperature. Nanoindentation maps showed that both the M 23 C 6 carbides and the σ phase in the interfacial layer are significantly harder than the other phases. The σ phase contributes more in the increase of the interfacial layer's hardness at lower temperatures than that at higher temperatures. These hard phases may affect high-temperature valve integrity.