DURING A RECENT turn-around of a Thermo- for catalytic cracking unit, several nozzles on the reactor fractured during the removal of the thermocouple wells. A slight build-up of coke between the thermocouple well and the nozzle hindered easy removal of the wells. When force was applied to the thermocouple well to free it, the nozzle fractured at the point shown in Figure 1. The 234-inch outside diameter, ¾-inch wall nozzles were of a 11-13 percent chromium alloy of a composition similar to the reactor cladding.
The appearance of the nozzle at the point of fracture is shown in Figure 2. The metal in the nozzle at the point of failure had been subjected to a temperature of from 850 to 1,000 F for approximately 100,000 hours. These temperatures embrace the 885 F temperature at which embrittlement has been reported1 in ferritic stainless steels with a chromium content...
