Strain aging effects on the cyclic behavior of austenitic stainless steels

The strain cyclic behavior of three austenitic stainless steels (AISI 304H, 316H and 316L) has been investigated at temperatures ranging from room temperature to 1023 K. Cyclic hardening curves obtained by plotting the peak stress per cycle against the number of cycles showed a similar trend for the three types of steels. At low temperatures a rapid hardening is followed by a softening until a saturation peak stress is attained. At temperatures higher than 523 K softening is gradually replaced by a more pronounced cyclic hardening and an increase in saturation peak stress. A maximum in this value is obtained at approximately 823 K for a total strain of 0.01 and a strain rate of 2x10-3 s-1. When the saturation peak stress is plotted against temperature for three different strain rates (2x10-3, 2x10-4, 2x10-5 s-1) two peaks are present in the region where strain aging effects are probably to occur. This region is characterized by a negative strain rate dependence and an enhanced hardness of the material. Cell formation and dislocation-impurity atoms interactions are responsible for the hardness observed. Transmission Electron Microscopy observations corroborate the proposed model. (orig.)