A Study Of Multiaxial Cyclic Deformation And Constitutive Modeling Of 316L/316LN Stainless Steels For Pressurized Water Reactor Pipes

The primary loop is the important shield for the pressure water reactor(PWR).The main pipe with large radius of the primary coolant circuit accounts for the cycle flow of freezing medium.The pipeline is designed for temperature of above 321?,and pressure of 17.1 MPa.The main pipe of the third generation nuclear plant such as AP1000 is made of forging austenitic stainless steel 316 L and 316 LN.Due to the effect of thermal transient,thermal stratification,thermal shock,thermal cycle and turbulence during the heat up and cool down of plant,there is an obvious and complex thermal loading effect in pipeline.Therefore,it is very important to research the cyclic deformation behavior and develop an appropriate constitutive model to describe.In this paper,a series of investigations for 316 L and 316 LN austenitic stainless steel about the uniaxial and multiaxial cyclic plastic behavior at room temperature and high temperature were conducted.The following results were revealed.(1)For 316 L stainless steel,during strain cycling at room temperature,the material showed cyclic hardening followed by cyclic softening.Besides,under the multiaxial loading path,there was an obvious nonproportional hardening;(2)The cyclic softening behavior of316 L stainless steel at room temperature was affected by loading history,which was called memorized nonproportional softening phenomenon.When cyclic loading switched to uniaxial path from multiaxial path,the material showed cyclic softening behavior immediately.The rate and degree of softening increased with the increasing of nonproportionality of loading history.Meanwhile,when cyclic loading switched to uniaxial path from multiaxial path,the softening is partly recoverable,which was to say,the decreasing level caused by softening was larger than that caused by the following cyclic hardening;(3)The stress response of 316 LN stainless steel had a positive ratedependence with loading rate at room temperature,while the dependence was weakened with the increasing of temperature;(4)for 316 LN stainless steel,in the temperature range of 293-823 K,under uniaxial strain cycling loading,the material exhibited obvious cyclic hardening behavior,which becomes more serious with temperature increasing.And it was indicated by both the literature and the experimental data that the dynamic strain aging(DSA)would cause obvious cyclic hardening at high temperature;(5)At the temperature range of 293-823 K,in the uniaxial asymmetric stress-controlled cyclic experiments,the 316 LN stainless steel showed ratcheting strain accumulation behavior.At room temperature the accumulation was obvious,while the ratcheting was easy to be quick-shakedown with temperature increasing,which had a relationship wih dynamic strain aging;(6)At the temperature range of 293-823 K,in the multiaxial asymmetric stress-controlled cyclic experiments,the 316 LN stainless steel showed ratcheting strain accumulation behavior.At room temperature the accumulation was obvious,while the ratcheting was easy to be quick-shakedown with temperature increasing.Besides,the ratcheting strain accumulation under multiaxial path was lower than that of uniaxial path in the same loading level;(7)For 316 LN stainless steel,which was aged at 623 K for 500-30000 hours,there was an obvious decreasing of mechanical properties at both room and elevated temperature,the manifestations of which were the decreasing of yield stress,ultimate stress and cyclic hardening level.From the observation of OM,SEM and EBSD,it revealed that after long time thermal aging,the grain size and precipitated phase increased,while the twin boundaries decreased.Due to the interaction effect of these reasons,the mechanical properties of material decreased.Based on the experimental results above,the macroscopic constitutive modeling for cyclic plastic behavior of 316 L and 316 LN stainless steel were proposed.(1)By introducing a modification of memorized nonproportionality and partly recoverable softening term into the Ohno Wang kinematic hardening rule and the Marquis isotropic hardening rule,a cyclic visco-plastic constitutive model,which could describe the material deformation at room temperature under uniaxial,multiaxial and alternative loading paths,was developed;(2)By introducing a hardening index into dynamic recoverable term of the Ohno Wang kinematic hardening rule,a unified visco-plastic constitutive model,which could describe the ratcheting behavior more appropriately at elevated temperature,was proposed;(3)By modifying the multiaxial parameter in CJ-K kinematic hardening rule,a visco-plastic model which could describe the multiaxial ratcheting behavior more appropriately at different temperatures;(4)By introducing an aging modified term into dynamic recoverable term of Ohno Wang kinematic hardening rule,a constitutive model which could appropriately describe the cyclic behavior of thermal aging material,was developed.