Abstract:To analyze the carburizing resistance of Fe-Cr-Ni alloy cracking furnace tube, solid carburizing agent with particle size of 1.5–3 mm was used to conduct carburizing test on two kinds of traditional furnace tubes (25Cr35NiNb+MA and 35Cr45NiNb+MA) as well as two kinds of aluminum-added alloy furnace tubes with prolonged coke cleaning period (27Cr44Ni5W3Al+MA and 29Cr44Ni4Al+MA). The test was carried out at 1075 ℃ for 50–200 h. The composition, microstructure and properties of the furnace tubes after carburizing test were analyzed by optical emission spectrometer, scanning electron microscope, X-ray diffractometer and Vickers hardness tester, and the carburizing kinetics and microstructure transformation rules were studied. The results show that the thickness of the carburized layer of four kinds of materials is increased with the prolongation of carburizing time. After carburizing at 1075 ℃ for 200 h, the thicknesses of the carburized layers of the furnace tubes are about 2.0, 1.8, 1.0 and 0.7 mm, and the average carburizing rates are about 0.01, 0.009, 0.005 and 0.0035 mm/h. The carburizing resistance of the two aluminum-added alloy tubes is better than that of the traditional tubes. The microstructure in the aged zone of 27Cr44Ni5W3Al+MA furnace tube is composed of austenite, blocky M23C6 and fishbone-like M7C3. The "M" in carbides contains elements such as Cr and W. In the carburized zone, blocky M23C6 is transformed into blocky M7C3 and WC, and fishbone-like M7C3 coarsens. The microstructure in the aged zone of 29Cr44Ni4Al+MA furnace tube is composed of austenite, blocky M23C6 and NbTiC. The "M" in carbides is mainly Cr element. In carburized zone, blocky M23C6 is transformed into blocky M7C3, and NbTiC changes from blocky to granular. The inner walls of two aluminum-added alloy furnace tubes form a dense and stable Al2O3 film. Compared with the Cr2O3 film on the inner walls of traditional furnace tubes, the Al2O3 film can more effectively block the penetration of carbon atoms into the substrate, thereby enhancing the anti-carburization performance.