Stabilizing the austenitic microstructure: Nickel promotes the formation and maintenance of a face-centered cubic (FCC) austenitic structure in the alloy, even at elevated temperatures. This structure is inherently more stable than the body-centered cubic (BCC) ferrite structure, preventing phase transformation and structural embrittlement during long-term high-temperature service.
Enhancing high-temperature mechanical properties: It improves the alloy's creep resistance and rupture strength at high temperatures. By solid-solution strengthening the austenitic matrix, nickel hinders the movement of dislocations within the crystal lattice, thereby slowing down the creep deformation process and enabling the alloy to maintain structural integrity under sustained high-temperature and stress conditions.
Improving resistance to reducing atmospheres: Nickel provides excellent resistance to corrosion in reducing environments (e.g., hydrogen-containing or carbon monoxide-containing atmospheres), which is critical for applications in petrochemical cracking furnaces and hydrogenation reactors.
Reducing thermal expansion mismatch: It helps optimize the alloy's thermal expansion coefficient, minimizing thermal stress and fatigue damage caused by repeated heating and cooling cycles.
Forming a protective oxide film: At high temperatures, chromium reacts with oxygen in the atmosphere to form a dense, adherent, and self-healing chromium oxide (Cr₂O₃) film on the alloy's surface. This film acts as a barrier, effectively preventing oxygen from penetrating the matrix and inhibiting further oxidation and carburization of the alloy at temperatures up to 1100°C.
Enhancing resistance to oxidizing and corrosive media: Chromium improves the alloy's tolerance to oxidizing environments such as high-temperature air, flue gas, and nitric acid solutions. It also enhances resistance to sulfidation in sulfur-containing atmospheres, which is essential for applications in power plant boilers and waste incinerators.
Assisting in solid-solution strengthening: Similar to nickel, chromium participates in the solid-solution strengthening of the austenitic matrix, further improving the alloy's tensile strength and hardness at both room and high temperatures without significantly compromising its toughness.
Improving scaling resistance: The Cr₂O₃ film reduces the formation of loose, non-adherent oxide scales, which can cause material loss and heat transfer inefficiency in high-temperature equipment.
