Creep damage of high alloyed reformer tubes

Reformer furnaces are widely used in petrochemical industry to obtain hydrogen from hydrocarbons through an endothermic catalytic reaction. This production takes place in radiant tubes, designed for a nominal life of 100,000. h at temperatures up to 980. °C and internal pressures of 10-40. bar. Their life is primarily limited by creep damage, driven by a combination of high service temperatures and hoop stresses that can evolve into catastrophic failure. Creep life exhaustion is evidenced by progressive diameter deformation and changes in the alloy microstructure. Recently, a nondestructive technique, based on tubes diameter measurements in situ by Laser Optic Tube Inspection System (LOTIS), has appeared very useful to describe quantitatively creep damage and promising to be utilized for residual life evaluations as well.In this chapter, the authors report their experience about creep damage of radiant tubes in a reforming furnace after long-time service at 900-950. °C. These tubes, made of HP-40 grade modified with Nb and Ti additions, were inspected in situ by LOTIS during the planned plant shutdown and put out of service when they show an excessive diameter expansion. Our investigations were addressed to assess a reliable criterion on the correlation between creep deformation and tubes residual life. Metallurgical degradation was investigated on samples cut from decommissioned tubes in order to verify any links among diameter deformation, microstructure, and creep behavior. Metallographic samples were observed by optical and scanning electron microscopy; energy dispersive X-ray microanalysis was performed to measure locally chemical composition; mechanical properties were evaluated by tensile and creep tests. Creep data are utilized to develop a method for predicting residual life of tubes after long-time service. So the experimental results are analyzed and discussed with the purpose of optimizing a procedure to ascertain creep damage and establish a criterion for tubes replacement.