A Study Of Micro-mechanisms And Macro-Constitutive Modeling Of The Cyclic Plasticity Of Austenitic Stainless Steels

Austenitic stainless steels have become a preferred engineering material for large-scale industrial mechanical structures, such as the heat exchanger in chemical engineering plants and the cooling piping system in nuclear power plants, which is due to their strong corrosion resistance, good malleability as well as the relative high strength. In these service situations, the fluctuation of pressurized cool-hot fluids causes the period changes of coupled temperature and force fields, which pushes the limits of the cyclic mechanical reliability of austenitic stainless steels, and draws great attention of engineering materials scientists.In the present study, the cyclic plastic behavior of two austenitic stainless steels-304 L and 316 LN steels were experimentally investigated at room temperature and elevated temperatures. The results revealed that:(1) the 304 L austenitic stainless steel exhibited remarkable secondary cyclic hardening during strain cycling at room temperature;(2) the 316 LN austenitic stainless steel displayed significant cyclic hardening during the isothermal strain cycling in the temperature range of 623-823 K, and exhibited additional cyclic hardening under the coupled temperature cycling of 423-823 K and mechanical strain cycling;(3) the cyclic stress response of 316 LN austenitic stainless steel was observed to show positive proportional relation with strain rate at room temperature but lack of strain rate sensitivity at elevated temperatures;(4) significant ratcheting strain was observed during the stress cycling of 316 LN austenitic stainless steel at room temperature, whereas a trend of fast shaking-down was seen at elevated temperatures. These cyclic plastic characteristics of austenitic stainless steels are believed to be attributed to the martensitic phase transformation and dynamic strain aging(DSA). Real-time in situ neutron diffraction measurements were performed during the strain cycling of 304 L austenitic stainless steel at room temperature, and the results revealed that the martensite phase bore a much higher stress than the austenite phase, but the average stress state kept almost constant in both phases, thus leading to the conclusion that the martensite amount plays the key role in the remarkable secondary cyclic hardening of the steel. Manifestations of DSA taking effect were summarized by analyzing the stress-strain behavior of the 316 LN austenitic stainless steel during monotonic tension, strain cycling and stress cycling tests, which demonstrated that DSA is most likely the cause for the significant cyclic hardening at elevated temperatures.Based on the above experimental results, macroscopic constitutive modeling were conducted to describe the cyclic plastic behavior of the 316 LN austenitic stainless steel:(1) with a modification of the hardening exponent mi in the Ohno-Wang kinematic hardening rule, a visco-plastic constitutive model that can describe the cyclic hardening and strain rate sensitivity of the material at room temperature was developed, which can better predict the ratcheting behavior of the material under various loading conditions at room temperature;(2) in the framework of the McDowell model, a thermo-visco-plastic constitutive model was proposed with the isotropic hardening discretely working on the evolution of the short-range back stress, which can not only well simulate the monotonic tension and strain cycling behavior of the material under different loading conditions at various temperatures, but also give good prediction of the stress-strain behavior of the material during the thermal-mechanical coupled cycling, uniaxial stress cycling and monotonic tension with cyclic loading history.