Heat treatment is a key process that endows high-temperature springs with ultimate performance. Different material systems correspond to different heat treatment regimes, all aimed at obtaining stable structures and suitable comprehensive mechanical properties.
For precipitation hardening stainless steel, such as 17-7PH, the typical heat treatment process includes solution treatment and aging treatment. Solid solution treatment is to heat the spring to around 1050 ℃ to fully dissolve the alloy elements, and then rapidly cool it (usually air-cooled or water-cooled) to obtain a softer austenite or martensite structure, which is convenient for subsequent forming. After forming, perform aging treatment, such as holding at 480 ℃ for a certain period of time, to precipitate small strengthening phases, significantly improving strength and elasticity. The control of aging temperature and time needs to be precise. Excessive temperature or time can lead to over aging and a decrease in strength.
For nickel based alloys such as Inconel X-750, the solution treatment temperature is usually 1150 ℃, followed by two-stage aging. The precipitation of the strengthening phase γ 'in this type of alloy is temperature sensitive, so the temperature control accuracy and uniformity of the heat treatment furnace are required to be high. If the furnace load is too large, the temperature difference inside the furnace will cause inconsistent performance of the same batch of springs.
Austenitic stainless steel springs are usually not subjected to precipitation strengthening, but are softened by solution treatment and then subjected to cold drawing strengthening, followed by stress relief annealing at lower temperatures (such as 400-450 ℃) to eliminate processing stress and stabilize dimensions. The surface oxide skin after heat treatment needs to be cleaned by acid washing or sandblasting, and attention should be paid to avoiding the risk of hydrogen embrittlement during the cleaning process.
