A crack was observed on the tube of radiation section of ethylene cracking furnace during operation. The material of the tube is KHR35CT-HiSi. This paper details the investigation into the failure and highlights the most probable cause of the failure based on available documents and experimental analysis, visual examination, chemical composition analysis, metallographic examination, scanning electron microscope analysis, energy spectrum analysis, X-ray diffraction analysis, fracture morphology observation was performed. Comprehensive data access, macro examination and the results of analysis, the failure of the furnace pipe is due to brittle fracture. The main reason for embrittlement of furnace tube is serious carburization of the pipe inside. Carburization can lead to two consequences. The first is to make the pipe become brittle; the second is the difference of the thermal expansion coefficient between carburization layer and non-carburization layer, which makes the pipe produce a large thermal stress, that can produce a micro crack in the Transitional position of the carburization layer and the non–carburization layer. The above two points are confirmed by experiments. The brittle fracture of tube is produced by the combined action of the micro crack and carburization.

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