High temperature springs may suffer various failures during service, with the most common being stress relaxation, high-temperature creep, oxidation corrosion, and fatigue fracture.
Stress relaxation is characterized by a gradual reduction in the spring's free length and preload, which can lead to equipment failure. For instance, looseness in valve springs may cause poor valve sealing. This phenomenon is closely related to material properties, temperature, and initial stress. It can be delayed by increasing the aging temperature or adopting materials with better anti-relaxation performance (e.g., nickel-based alloys).
High-temperature creep refers to the spring's slow deformation under constant load, usually occurring when the temperature exceeds half of the material's recrystallization temperature. Creep can cause permanent geometric changes and even fracture of the spring. During design, reference should be made to the material's creep limit data, and the working stress should be restricted.
Oxidation and corrosion are the main forms of surface damage. At high temperatures, oxide scales form on the spring surface; peeling of these scales will thin the cross-section and cause local stress concentration. In corrosive media (e.g., sulfur, chlorine), intergranular corrosion or stress corrosion cracking may occur. Preventive measures include selecting more corrosion-resistant alloys or applying protective coatings (e.g., aluminum oxide or ceramic coatings) on the surface.
Fatigue fracture usually originates from surface defects or stress concentration areas. At high temperatures, the material's fatigue strength decreases and crack propagation accelerates. Improving fatigue life can be achieved by reducing surface scratches, controlling the decarburization layer depth, and introducing surface compressive stress through shot peening during manufacturing.
