Abstract: The Stellite 6 alloy coating on COSTE steel valve in a steam turbine fell off after 6 years in service. Stellite6alloy was deposited on COSTE steel by electrode arc welding, and the failure reason of Stellite6 coating was studied. The results show that COSTE steel has diluted Stellite 6 alloy during surfacing, and the Fe element gathers near the interface of the Stellite6. During service, the Fe element diffuses extensively from the steel substrate to the Stellite6 coating. Excessive Fe content promotes the α-Co(Cr, Fe, W) solid solution in Stellite6 coating to generate brittle BCC α'-FeCo(Cr, W) phase and σ-Cr Co(Fe, W) tetragonal phase. The brittle phase structure eventually leads to the failure of Stellite6 coating under long-time working load and thermal stress action.