Leakage Reason of Thermocouple Bushing in Reactive Gas Cooler in Acrylonitrile Plant

The thermocouple bushing of reaction gas cooler in an acrylonitrile plant leaked during use. The causes of the leakage were analyzed by means of macroscopic observation, chemical composition analysis, microstructure observation, microhardness test, scanning electron microscopy and energy dispersive spectroscopy. The results showed that the cracking mode of the thermocouple bushing was stress corrosion cracking. The main reason of its leakage was the difference of surface state and microstructure between heat affected zone and base metal zone of thermocouple bushing produced stress, which led to the formation of stress corrosion cracks in the heat affected zone preferent ially. In the environment where hydrogen sulfide and chloride coexisted. The cracks propagated along the grain boundary to the base metal, which led to cracking at the weld joint between the thermocouple bushing and the flange boss, and finally led to the leakage of thethermocouple bushing during use.