Microstructural developments in type 316L stainless steel during low-cycle fatigue at 350–550°C and their effects on cyclic strength and life

The low-cycle-fatigue behaviour and the related dislocation substructures of type 316L stainless steel in both the solution annealed and aged states have been investigated. Fully reversed, strain controlled continuous fatigue tests were performed on the material over strain ranges of ±0·25, ±0·3125, ±0·375, and ± 0·5% at a constant strain rate of 10⁻³ s⁻¹ and at temperatures of 350, 450, and 550°C. It is shown that cyclic strengthening increases with temperature and short-term aging. This is attributed to the interaction between dislocations and carbide precipitates, which grow during fatigue as well as during aging. A model which predicts fatigue-associated growth is developed and shows reasonable agreement with experimental results. The results also demonstrate that the dislocation structures observed in the annealed specimens at all three temperatures are characterized by cells. Aging produces smaller and better-defined cells at 350°C, but causes thick and dense dislocation walls (as opposed to cells) to form at 450 and 550°C. In both conditions a reduction of fatigue life occurs as temperature increases. Although aging has a detrimental effect on life at 450 and 550°C, it has a beneficial effect at 350°C.

MST/395