Oxidation and carburizing behaviors of HP40Nb tube after 100,000h service in pyrolysis furnace

The behaviors of the oxidation and carburizing of the HP40Nb furnace tube after 100,000 h service in high temperature and strong carburizing environment were analyzed in this study. The results show that there are three zones including surface oxide layer, intercrystalline oxidation zone and carburizing zone from inwall to center of the tube. The oxidation behavior of the furnace tube consists of outer oxide layer and internal oxidation. Because of standard free energies of the reactions of Cr and Si with per mole of O₂ varies with the diffusion depth of oxygen, initial stage of the internal oxidation Cr is reflected first and SiO₂ is closer to the interior of the alloy. Excess activated carbon atoms are precipitated in the form of Cr₂₃C₆ in the central region of the alloy, forming the carburizing zone. The severe depletion of Cr in the intercrystalline oxidation zone leads to an increase of the critical carbon concentration of Cr carbide in this region, resulting in only a small amount of NbC distribution in this region. The intercrystalline carbide is coarsened into the form of coarse chain and block.