Mechanical properties improvement of heat resistant 35Cr-45Ni alloy used in reformer tubes after aging in service temperature (1050 ̊C) by solution annealing

One of the most current heat resistant nickel alloys is 35Cr-45Ni alloy, which is used in Ethylene furnace reformer tubes at 1050°C. This alloy is operated simultaneously in high temperature, oxidation, and carburization and creep conditions. After long-term of service, carbides making alloying elements, not only causes the formation of hard carbides; but also causes growthing them. This microstructure has a low-toughness along with creep cavities and cracks which is formed during service; therefore the tube repair-welding will be impossible. In this study the role of solution annealing on mechanical properties of long-term used tubes was investigated.The samples of aged used tubes were annealed under various conditions. The temperatures was 1100°C and 1200°C during 2, 4, 6 and 8 hours. Then mechanical tests and structural studies were applied on annealed and aged tube. The structural examination revealed that after annealing, the continuous carbides lose their cohesion in grain boundaries and potential cracked parts disappeard. The result of mechanical test also showed that solution annealing at 1200°C during 6 or 8 hours increases the strength by 70% and the flexibility more than twice. Consequently, makes the alloy properties similar to the cast alloy properties.