Relaxation at 1300°F of Spring-Temper, triple-heat-treated springs

(Stresses corrected for curvature; modulus corrected for temperature.) Loaded at 20 ksi.

Although all these relaxation data were derived from tests on helical springs, they would be applicable to flat springs under the same conditions. In these tests, relaxation is the reduction of load necessary to maintain the spring at a constant height.

In fabricating springs for low-temperature service, they may be cold-pressed after precipitation treating to moderately increase load-carrying capacity. This process adds additional cold work. Cold-pressing of springs for high-temperature service is not beneficial; it increases the cold work and hence relaxation is increased. Heat loading or prestressing at temperature is beneficial to helical springs for high-temperature applications. Prestressing is done by clamping the spring under a load that is about 10% higher than the maximum working load, putting the assembly in a furnace at a temperature of about 100°F higher than the maximum service temperature, and holding for 1 hr.

Springs must be protected from sagging during triple-heat treatment, especially the 2100°F solution treatment.

This is done by placing the spring on an arbor which makes a snug fit with its inside diameter.

Another criterion in the design of flat springs is the fatigue strength of the material. Fatigue strength can be used in conjunction with design stresses to establish the actual limiting stress.

For instance, figure below shows the room-temperature fatigue strength of Spring-Temper strip precipitation-treated 1300°F/20 hr. If a spring is required to have a life of 2,000,000 cycles, the design stress of 120 ksi shown in Table 30 must be reduced to 70 ksi from figure below. (Although the precipitation treatments are slightly different, the data may be interpreted on this basis with sufficient accuracy.) Fatigue data for other conditions shown earlier under “Mechanical Properties” may be used in the same way.