Computational and experimental failure analysis of continuous-annealing furnace radiant tubes exposed to excessive temperature

Radiant tubes of a continuous-annealing furnace at Mobarakeh Steel Company failed after a fraction of their rated service life. The tubes were manufactured from INCONEL alloy 601 superalloy. In this study, a failure analysis of the radiant tubes was performed by careful visual inspection of the failed tubes, scanning electron microscopy observation of crack region samples, energy-dispersive X-ray spectroscopy and X-ray diffractometer analysis of the tube metals and oxide scales. The temperature distribution for steady-state heat transfer and the structural stresses induced by the weight of the tube material were also studied in this paper. The finite element method (FEM) was employed to compute the effect of increasing temperature on tube service life and to define the critical regions. perimental results showed that the mode of tube failure was a combination of creep damage and high-temperature oxidation attack which led to cracking. Significant growth of carbide precipitates was also observed in the failed zones; these precipitates result in both the drastic reduction of material ductility and the propagation of creep cracking. The simulation results showed that the damaged region in radiant tubes is susceptible to high-temperature creep. Tube failure occurred as a result of bending in the tube length, which may be responsible for subsequent hot-spot formations and high-temperature oxidation. Furthermore, the tube failure verified experimental results. Moreover, a simulation indicated that modification in radian tube installation is necessary because utilization of supports in specific locations can extend creep life.