Secondary combustion and tube operating conditions promote graphitization in carbon steels. Heat-affected zones and area with residual stresses from the tube straightening process are susceptible to graphitization. The typical microstructure of carbon steel is ferrite and pearlite. In carbon or carbon-molybdenum steels, the iron carbides are unstable when exposed to temperatures beyond 800oF for a prolonged period and therefore decompose into ferrite and graphite. These graphite particles reduce the strength of the steel and make the material susceptible to brittle failures. These materials are typically used in primary superheaters where the temperatures are expected to be below 800oF. However, it is likely that secondary combustion pushes the temperature in those regions beyond the sweet spots, causing premature failures. 

Primary Superheater

Primary Reheater 

A few options exist to determine the extent of graphitization. Metallurgical analysis can fully characterize the distribution and morphology of graphite nodules. Mechanical bend testing and fracture toughness testing can indicate if the material is embrittled from graphite formation. Upgrading to low-alloy steels such as Grade 11 or 22 eliminates the susceptibility to graphitization.