Failure Analysis Case History No. 011

Horizontal, carbon steel furnace tubes in a Rich Oil Fractionator furnace failed causing a fire.

Laboratory analysis showed that the carbon steel tube sample had experienced long term overheating under a heavy coke layer. I.D. pitting was observed under the coke. The pits were believed to be due to metal dusting, based on a dark etching, carburized layer below the pits. Observation of the microstructure showed that the dark etching layer was100% pearlite. The undegraded pearlite proved that metal temperatures had exceeded the AC₃ transformation temperature at some time. Long term metal temperatures were estimated at approximately 1000F, based on the presence of graphite nodules in the completely decarburized ferrite matrix.