The effect of nitrogen on dynamic strain ageing (DSA) behaviors of type 316L stainless steels containing different nitrogen contents (0.01 and 0.1%) was studied in tension varying the strain rate (1×$10^{-2}$/sec ~ 2×$10^{-4}$/sec) and the test temperature (R.T. ~ 750℃). The temperature range for DSA was shifted to higher temperature with increasing nitrogen content. The critical strain, $ε_c$, for the onset of serration increased with nitrogen content at 500℃ and was almost the same at 600℃. Types A and B serrations were observed at 600℃ and the transition strain from serration type A to B increased up to 0.1% nitrogen content and then saturated. The activation energies for DSA were 23.4~26.2 kcal/mol at the onset and 65.0~76.6 kcal/mol at the end of serration. The activation energy for DSA was slightly increased with nitrogen addition. The activation energy at the onset of serration was related to vacancy diffusion and the activation energy at the end of serration was attributed to chromium diffusion to dislocations. Chromium diffusivity was measured as function of nitrogen content by microanalysis of chromium depleted zones using analytical electron microscopy. Diffusion coefficients of chromium were decreased with adding nitrogen. DSA was retarded with the increase of nitrogen content because nitrogen reduced the chromium diffusion to dislocations due to the strong interaction between nitrogen and chromium.
The effect of nitrogen on high temperature low cycle fatigue (LCF) properties was studied using type 316L stainless steel with different nitrogen contents (0.04~0.15%). Strain-controlled LCF tests were conducted in the temperature range of R.T. ~ 600℃ and air atmosphere. The waveform of LCF was a symmetrical triangular with a total strain amplitude of 0.8%~2.0% and a constant strain rate of $2×10^{-3}$/sec was employed for all tests. Cyclic stress responses of the alloys exhibited a gradual cyclic softening at R.T., but a cyclic hardening at an early stage of fatigue life at 300~600℃. The hardening at high temperature was attributed to DSA. The occurrence of DSA was identified from the following results; strain rate sensitivity was negative, the hardening stress (maximum cyclic stress - first cyclic stress) increased with temperature, dislocation structure changed from cell to planar, and the temperature range for DSA was identical to the temperature range of cyclic hardening. Nitrogen addition decreased the hardening stress because nitrogen retarded DSA. The dislocation structures were changed from cell to planar structure with nitrogen addition by short range order (SRO). Fatigue life increased with adding nitrogen at high temperature but showed the maximum at 0.1% nitrogen content, which was attributed to the interaction of DSA and SRO.
LCF tests were conducted up to 40% of fatigue life to investigate the effect of nitrogen on crack nucleation behaviors during high temperature LCF test. The preferred sites for crack nucleation were grain boundaries but some cracks nucleated at twin boundaries and slip bands. Crack population on the surface of specimens decreased with nitrogen addition but maximum crack depth was not changed with nitrogen content at high temperature. So, the nucleation stage of crack is independent of nitrogen content at high temperature LCF test. The increase of high temperature LCF life with adding nitrogen may be explained by the fact that nitrogen decreases crack propagation rate rather than crack nucleation.
