A Study on the Service Degradation and Failure Mechanisms of Austenitic Heat-Resistant Steel Tubes in the Radiation Section of an Ethylene Cracker Furnace Based on a Multi-Evidence Diagnostic Framework

The radiation-section furnace tubes of ethylene crackers are critical pressure-bearing components, and their failure results from multiple coupled factors, which single evidence cannot fully explain. This paper proposes a multi-evidence diagnostic framework integrating operating conditions, macroscopic damage, geometry, composition/carburization, microstructural degradation, performance, and fracture. Using bulged HB-101 and fractured HB-106 tubes from SL-series crackers, combined with physicochemical testing and condition analysis, the degradation-to-failure mechanisms are systematically revealed. Localized overheating is identified as the most probable root operational cause, supported by operational records, infrared thermography, and microstructural evidence. Flame deflection likely raised the local metal temperature to approximately 1100 °C or above in the bulged zone, accelerating carburization. Alternative contributing causes are discussed. Carburization and carbide coarsening constitute the main degradation pathway, with grain-boundary carbides transforming from network-chain to coarse blocky forms, reducing the high-temperature creep strength and impact toughness. Base-metal creep bulging results from combined overheating, carburization, and creep, while weld brittle cracking arises from welding defects, carburization embrittlement, and thermal stress. This framework enables full-chain attribution from operational causes to failure, supporting failure analysis and the life-cycle safety management of cracking furnace tubes.