Abstract: Microstructure and hardness changes at different positions on axial section and at certain depths from outer surface by grinding HP40Nb furnace tube after service for different times were compared, and then the feasibility of cladding metallographic technique and on-site hardness test in evaluating the damage of furnace tube in service was analyzed. The results show that after service of the furnace tube at high temperature for a long time, the microstructure of different positions on axial section, especially carbide morphology, was significantly different, and even holes appeared because of overheating, which led to the early failure. The hardness at different positions on the axial section varied greatly, and the closer to the wall thickness center, the higher the hardness, which was consistent with the changes of microstructure. The surface of furnace tube was ground, and the hardness at certain depth was measured; when the measured hardness continued to be stable, the microstructure at this depth could characterize most of the microstructure of the furnace tube. This verified the feasibility of cladding metallography and on-site hardness test for evaluating the microstructure demage of HP40Nb furnace tubes in service.