Stress relaxation of nickel-based superalloy helical springs at high temperatures

The creep resistance of materials in spring applications is generally acknowledged to be well below that observed in other applications. Helical springs formed from three candidate nickel-based superalloys, Nimonic 90, René 41 and Haynes 282, have been tested under compression in order to gain some insight into this phenomenon. Stress relaxation tests conducted at 600-700 °C found that, under constant displacement, the degradation of the spring force is one to three orders of magnitude faster than would be predicted from creep data from extruded samples under equivalent tensile loading. An analytical model for torsional creep in helical springs is derived from a modified version of the Dyson creep model. The effects of various microstructural features on the deformation rate are considered. Effects such as the coarsening of the precipitate-strengthening gamma-prime phase, tertiary creep due to dislocation multiplication, damage evolution and hardening due to transfer of the stress to the particles from the matrix are concluded to make negligible contributions. It is predicted that the poor performance of the springs is due to the very high population of geometrically necessary dislocations that result from the bending and twisting of the wire into a helical coil. It is expected that these dislocations are resistant to conventional heat treatments, resulting in a persistent residual stress field and a large number of dislocations to facilitate the creep process. In some cases, the stress relaxation is found to be so fast that the precipitate hardening of the alloy is too slow to prevent significant initial degradation of the spring.