Cyclic creep and cyclic deformation of high-strength spring steels and theevaluation of the sag effect: Part I. Cyclic plastic deformation behavior

The cyclic creep and cyclic plastic deformation behavior of two commercial suspension spring steels of high hardness levels, namely, SAE 9259 and SAE 5160, were studied under different testing conditions of cyclic peak stress and cyclic stress ratio. The experimental results indicate that both the c yclic stress ratio and cyclic peak stress have strong, but complicated, eff ects on the cyclic creep and cyclic plastic deformation behavior of these m aterials. It has also been found that the addition of silicon can increase the resistance of these steels to cyclic creep and cyclic plastic deformati on, although the extent of this increase is also related to other cyclic de formation conditions. A transition in the relationship between the total pl astic strain range and the cyclic stress ratio (R) has been detected at app roximately R = 0.5. The mechanism of such a transition is explained by the operation of cross-slip during the unloading process of cycling. Moreover, a cyclic softening behavior of these spring steels in the quench-tempered c ondition was also detected and is attributed to the activation and reorgani zation of obstacle dislocations introduced into the steels during the proce ss of martensitic transformation. More importantly, this study has indicate d that parameters such as the cyclic creep strain, the cyclic creep rate in the secondary creep stage, and the total cyclic plastic strain range can b etter reflect, and should be used to depict and characterize, the sag behav ior of spring steels as well as other materials. Finally, the effect of sil icon on sag behavior, in comparison with the results from the Bauschinger-e ffect test, has also been discussed through the influence of Si on carbide formation and distribution.