Chain Graphitization/Superheater Tube

A radius bend section of a primary superheater tube had failed at the outlet header and was received for failure analysis. The tube was specified as 2.0” OD x 0.203” MWT, SA-209 T1 C-Mo steel. It had been in service for 33 years.

The failure was thick-edged, and helical/spiral in appearance. The tube extrados was taken as 12:00 orientation. Therefore, the failure is at the 8:00.

 

The cracks had initiated from the OD and ID as shown. No significant tube wastage was observed at the failure location.

 
 

More nodular chain graphitization adjacent to the primary fracture surface is shown. The process of “chain graphitization” occurs in areas of high residual stresses when carbon-molybdenum steels are operated above their normal operating temperatures for extended periods of time. This is known to occur in weld heat-affected zones and in areas where roll straightening during tube manufacturing has left spiral bands of concentrated strain. Presumably, strains of this nature were left in the tube during manufacturing and account for the spiral appearance of this kind of failure. The presence of graphite nodules and chains in the T1 material is an indication of long-term exposure to temperatures above 850 °F. Above such temperatures C-Mo (T1) steels are prone to decomposition of the iron carbides within the microstructure to form stable ferrite and graphite. The graphite nodules may eventually combine, as has happened with this tube, resulting in chain alignment and allowed catastrophic, brittle failure.

 
 
      Some graphite nodules were visible along the fracture face. The failure was brittle; no plastic deformation was observed at any part of the crack.
         
     

The oxide scale thickness was measured as 3 mils. Based on the scale thickness and years of service, the tube temperature was estimated at 917 oF. This temperature is above the recommended maximum for T1 material, 900 oF.

         
     

Away from the failure, the mid-wall microstructure also revealed graphitization.

 

The tube failed due to chain graphitization. The microstructure away from the failure also exhibited graphitization. Minimal wall thinning was observed and no measured wall thickness was below MWT of 0.203”. The hardness was below the range for new SA-209 T1A, 74-79 RB, indicating that some softening had occurred due to carbide spheroidization and graphitization.