Cr5Mo furnace tubes have served in a coking furnace nearly 15 years. For the purpose of risk reduction and to assure a safe long term stable operation of the furnace, these tubes have been investigated by measns of macrographic observation, chemical composition analysis, mechanical property analysis, hardness and metallurgical structure analysis, etc. The experiment results shows that after a long period serve at high temperature, the hardness of the tube has decreased in 15 years. However, the hardness of inner wall is higher than the others, from which we can infer that the inner wall of the tubes is carburized. Taking GB9948-2006 as a reference standard, the yield stress of the furnace tube in normal temperature has decreased by 16%, and the yield stress in high temperature is also failed to fit the standard. Metallographic analysis shows that the grain fineness is 6 grade and the nodulizing grade is 3.5 grade, pearlite spheroidization is getting severer and it will reduce the creep limit and creep rupture strength. Based on the result of high temperature endurance test, isotherm method and L-M extrapolation are adopted to predict the remaining life of furnace tubes, assessment shows that these furnace tubes are able to serve for another cycle under the operating temperature of 650°C and the stress of 18MPa.

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