Stress Relief of Coil Springs using Fluidized Bed

After manufacturing coil springs, internal stress exists within the polycrystalline structure of spring steels—such as chrome-silicon and chrome-vanadium alloys. These stresses can lead to defects and may impact the working lifespan of springs. Internal stress must be relieved to maximize the elastic properties of the chrome-silicon and chrome-vanadium alloys. Stress relief is, therefore, a critical step in the manufacturing process, which is accomplished by treatment of the springs at 800 °F, typically using large belt furnaces and high-velocity convection ovens.

The fluidized bed heat treatment system is an alternative for stress relief of small- and medium-sized coil springs. Springs are suspended in a parts basket and deposited into a fluidized bed at 800 °F, consisting of fine aluminum oxide particles gently mixed by an upward air flow. With its high heat transfer coefficient of approximately 70 BTU/hr • ft2 • °F, fluidized bed relieves the stress in coil springs in less than 15 minutes; whereas, ovens require over 30 minutes. Bed temperature is accurately controlled using either electric heaters or gas burners, with thermal uniformity throughout the bed within 3 °F. Fluidized beds allow for cooling at conclusion of the thermal treatment as well.

Fluidized bed, with its advantages of uniformity and quick turnaround time, render it the best option for the rapid and efficient stress relief processing of coil springs and heat treatment of other metal components.