A Study On The Uniaxial Ratcheting-fatigue Behavior Of Austenitic Stainless Steel For Primary Coolant Circuit Auxiliary Pipes Of Pressure Water Reactor

The mechanical properties, cyclic hardening/softening and uniaxial ratcheting behaviors of Z2CND18.12N and Z2CN18.10austenitic stainless steels used in the auxiliary piping of the primary coolant circuit for pressurized water reactor were studied at room temperature. Special efforts were focused on the effects of pre-strain on the uniaxial ratcheting behaviors of Z2CN18.10steel under stress cycling with positive mean stress. The experimental results at room temperature show that:(1) Z2CND18.12N steel presents much higher yield stress, tensile strength and anti-ratcheting ability while Z2CN18.10shows better ductility;(2) both steels exhibits viscoplastic behavior and higher loading rate responds to the same plastic strain with higher level of stress response;(3) cyclic hardening followed by gradually cyclic softening till stablization is observed in1%fully reversed strain cyclic test for Z2CN18.10steel;(4) tensile pre-strain restrains ratcheting strain accumulation. Moreover, infuence of lower level of pre-strain on ratcheting strain gradually vanishes after a certain number of cycles;(5) lower level of compressive pre-strain is found to accelerate ratcheting strain accumulation while higher level of compressive pre-strain retards the accumulation;(6) tensile pre-straining is beneficial to ratcheting-fatigue life while compressive pre-straining is detrimental;(7) a modified fatigue model to address the effect of pre-straining is proposed to predict the fatigue lives of the stress cyclic tests with positive mean stress;(8) to improve its range of applicability, a pre-strain exponent n c is introduced to the modified fatigue model and more than ten groups of reference data are adopted to evaluate its reliability. Moreover, applicable condition for the modified model is put forward.Based on the Ohno-Wang â…¡ kinematic hardening rule, rate-independent model, viscoplastic model, isotropic hardening model and modified model were constructed to describe the ratcheting behavior under various pre-strain conditions. All the four models gave fairly good prediction on ratcheting strains for various tensile pre-strains. The isotropic hardening model and modified model predicted acceptable ratcheting strain though still showed sligh tendency of over prediction.