Abstract: A sheathed thermocouple used in a coke oven in a domestic steel plant became invalid after installed, and used for 5 days, which could not show the temperature readings any more. The failure reasons of the thermocouple were investigated by means of macroscopic appearance observation, fracture analysis by the scanning electron microscope (SEM), microstructure analysis by optical microscope (OM), composition analysis by electro probe microanalyzer (EPMA), respectively. The results show that the failure of the thermocouple is mainly attributed to the preferential oxidation of the nickel-chromium end. Improvement measures and suggestions: select the right type of thermocouple reasonably when using, eliminate the oxide skin on the surface of K-type thermocouple and the intervention of reducing medium during processing, and adopt a smaller length diameter ratio.

Key words: thermocouple for coke oven, Ni-Cr/Ni-Si thermocouple, preferential oxidation, green corrosion