Study on low cycle fatigue behaviors of Type 316 stainless steel in 310 .deg. C water environment

The low cycle fatigue tests of type 316 stainless steel in 310 .deg. C low oxygen water were conducted to investigate the fatigue behaviors of the stainless steel. They depend on the environmental and loading conditions. The test strain rates and amplitudes were 0.4, 0.04, 0.008 %./s, and 1, 0.8, 0.6, 0.4 % respectively. The environmental condition was a temperature of 310 .deg. C, pressure of 15 MPa, conductivity of less than 0.1 μS/cm, dissolved oxygen level of< 1 ppb, pH level of 6.4. At cyclic stress response, the type 316 stainless steel showed primary hardening at all strain amplitude and rate conditions. The secondary hardening were only appeared at low strain amplitude. By the hysteresis loop analysis, the primary hardening was known to be happened by dynamic strain aging. Negative strain rate sensitivity was also appeared in cyclic stress response and caused by the increase of dislocation density and the changes of dislocation structure. Well developed striations were shown on the fatigue surface. Initial shape of the fatigue crack showed the film rupture slip dissolution symptoms, while the hydrogen induced cracking phenomena were observed at the later position of the fatigue surface. ε-N curve were well fitted with reference data from other laboratories. The data showed shorter fatigue lives in 310 .deg. C water environment than those of room temperature air condition