Aging Behavior of Nb-Modified HP Steel Tubes after Extended High-Temperature Service in Steam Reformer Furnaces: Microstructural Classification and Characterization

Abstract Steam reformer furnaces for large-scale hydrogen production in the petrochemical industry consist of a set of catalyst tubes made from state-of-the-art centrifugally cast heat-resistant modified HP-Nb or HP-NbTi stainless steel. These tubes are designed for a typical lifespan exceeding 100,000 hours, usually limited by creep damage. Currently, continuous service times can exceed 200,000 hours due to improvements in manufacturing methods, alloying additions, and confidence in nondestructive testing. Long-term in-service exposure to temperatures up to 1000 °C and internal pressures of 1-5 MPa presents severe operating conditions that lead to microstructural evolution. The relationship between creep and aging is indirect, but periodic assessment of microstructural evolution during operation provides important information for residual creep life assessment. This study proposes a general microstructural classification criterion for Nb-modified and Nb-Ti micro-alloyed HP alloys based on six characteristic aging states derived from systematic microstructural observations of tubes from different furnaces, manufacturers, and service histories, with aging times ranging from 70,000 to 135,000 h. Aging states are closely related to service temperatures, with limited dependence on operating time. After approximately 5000 hours of operation, the aging states are defined and remain in that configuration until the end of the tube’s service life. During scheduled shutdowns, the proposed criterion can be used as a basis for a comprehensive industrial procedure for nondestructive examination of extraction replica by optical microscopy. Complementary characterization of interdendritic phases and secondary precipitates was carried out using scanning and transmission electron microscopy, respectively. Long-term creep data obtained under different stress levels at 900 °C, 950 °C, and 980 °C further demonstrate a strong correlation between creep behavior and aging state, reinforcing the applicability of the proposed classification for life cycle assessment methodologies.