A wound spring can lose its spring tension due to anelastic behavior, which causes the spring to unwind or change its shape over time.
To avoid this springs are placed in an oven at 315 - 375 ºC (600 - 707 ºF) for 2 hours for spring aging. This will allow the spring to change shape or unwind. This unwinding or changing shape can be accommodated during the design of the spring and be compensated. Once the springs are treated to spring aging, they do not usually change shape.