(33f) FAILURE OBSERVED AT CROSSOVER PIPING OF PYROLYSIS FURNACE AND PATH FORWARD

304H Stainless Steel has long been known as the material of choice for furnace convection sections. However, exposure to elevated temperatures for long durations can lead to higher temperature degradation. In this case study, we represent the unique failure observed at the crossover piping, 304H metallurgy, at small bore connections after four and half years of service. This systematic failure was observed on 100+ small-bore connections at the same process location on three furnaces. The location of cracks on individual small-bore connections was between the 5 o’clock position and the 7 o’clock position. The crossover piping was operating in the range of 1300°F to 1350°F. The failure analysis has indicated presence of thermal as well as mechanical stresses, however, without sensitized microstructure. Cracks were observed in all three directions: longitudinal, circumferential, and radial. At the toe of the weld, cracks were stepwise, however were not interconnected. Thermal fatigue is identified as the principal root cause. In addition, unexplained chromium dilution in the weld was observed. The probable cause of the chromium dilution is “Self-healing”. This presentation is intended for inspectors, engineers and managers explaining the complex failure mode, root cause assessment and effective resolution.