A Study On The Ratcheting Effect And Ratcheting-fatigue Behavior Of Z2CND18.12N Piping Material After Thermal Aging Process

The piping in nuclear power plants always expose in the high temperature environment during the fabrication or welding processes, which will lead to the thermal aging effect. The thermal aging effect will result in the precipitates at the grain boundaries, which is called “sensitization”, and will affect the mechanical properties of the material significantly. The piping systems in the pressurized water reactor（PWR） of the nuclear power plants are subjected to complex work conditions such as internal pressure, vibration and so on, which will bring out the ratcheting effect. The ratcheting effect could reduce the fatigue life of the material of the construction and affect the safety and the reliability of the piping systems. To guarantee the safe operation of the nuclear power plant piping systems, the Z2CND18.12 N austenitic stainless steel used for the assistant piping in the first loop of the PWR is studied on the thermal aging effect, ratcheting effect, ratcheting-fatigue life, cyclic constitutive model and etc. This study makes sense for the design and operation of the nuclear power plant.In this study, uniaxial tensile tests and uniaxial ratcheting tests were conducted using the multiaxial fatigue test machine on the Z2CND18.12 N stainless steel to obtain the basic mechanical properties at the room temperature. In the strain-controlled cyclic tests, this material showed cyclic hardening effect at first, then showed cyclic softening effect. In the uniaxial ratcheting tests, the ratcheting strains decreased with the increasing loading rates and increased with the increasing mean stresses and increasing stress amplitudes. The loading history and the stress ratio also influenced the ratcheting effect apparently.To study the thermal aging effect of the Z2CND18.12 N stainless steel, the specimens were aged for 1 hour, 10 hours, 50 hours, 100 hours and 500 hours at 500℃ and 700℃ respectively. The uniaxial tensile tests and the uniaxial ratcheting tests were conducted on the aged specimens at room temperature. In the uniaxial tensile study, there were little but consistent decreases of the Young’s modulus and the yield strength of the aged specimens compared with the original specimen. The ultimate tensile strength and the elongation of the aged specimens were unchanged. In the uniaxial ratcheting study, the ratcheting strains of the aged specimens increased with the increasing thermal aging time and the increasing thermal aging temperature. The ratcheting-fatigue lives decreased with the increasing thermal aging time and increasing thermal aging temperature. The thermal aging damage was introduced to predict the ratcheting-fatigue life, which showed good prediction results.The microstructures of the thermal aged specimens were observed with SEM and EDS. From the SEM observation, the evolution of precipitation with increasing thermal aging temperature and aging time was obtained. From the EDS observation, the compositions of the precipitates were analyzed. The precipitates including M₂₃C₆,η and x varied with the different thermal aging conditons.The OW II constitutive model was employed to simulate the uniaxial ratcheting behavior with isotropic hardening rule which brought out well simulation results of original specimen. Theim was modified to a power law associated with thermal aging time to predict the ratcheting strain of aged specimens. This modification also brought out good simulation results.