Rapid detection technology of the carburization of Fe–Cr–Ni high-temperature heat-resistant alloys in the ethylene cracking furnace and the hydrogen conversion furnace is in great need to prevent industrial accident. This paper implemented the GMRI-I carburizing detector to test the degree of carburization of 25Cr35NiNb+ microalloy inlet tube and 35Cr45NiNb+ microalloy outlet tube of the ethylene cracking furnace after service as well as the reinforced joint made of UNS N08810 from the hydrogen-producing reformer. In conjunction with the results of the low-power acid corrosion test, a rapid detection method for the degree of carburization was proposed. Based on the experimental results of the carburizing detector and the laboratory test data, the carburization degree test curve of the above three alloys based on the magnetic field strength has been established. The detection accuracy was circa. 5% and the error was ±10%, which is convenient for guiding the use and replacement of Fe–Cr–Ni high-temperature heat-resistant alloy materials for ethylene cracking furnace and hydrogen conversion furnace, thereby ensuring the safe and stable operation of the device.