Graphitization Effects on High Temperature Ductwork

North America has an aging fleet of coal plants which contain substantial ductwork systems. This ductwork is exposed to harsh conditions associated with plant operations and environmental conditions. These harsh conditions increase risk of metallurgical deterioration over time. This paper will discuss the effects of high temperature operations and the resulting graphitization effects within ductwork. Graphitization is the breakdown of the material's metallurgical microstructure, thus forming a weakened failure path. There have been documented failures in coal-fired power plants due to graphitization which have caused plant shutdowns and injuries to personnel. Currently, there is no industry consensus document providing information on the formation of graphitization and how to combat this problem. Multiple individual reports and technical papers on graphitization and its effects have been written in the past 50 years. The formation of a single source document outlining guides and standards would be beneficial to the power industry. The primary goal of this report will be to condense the relevant industry information on this topic into a summary discussing the formation characteristics of graphitization. Additionally, the report will provide technical information on the graphitization process, including critical components, design considerations, and inspection and remediation methods to correct possible existing conditions. Through research of top industry documents, it has been concluded that the effects of graphitization need to be closely considered and monitored as temperatures exceed 800.F. Temperatures below 800.F may experience graphitization, but at such a marginal rate that it can be neglected over the design service life. By providing a single location outlining the formation characteristics of graphitization, engineers will benefit from standardized procedures for the inspection, analysis and design of existing and new hot-side ductwork components, thus yielding improved consistency and product quality.