Influence of Ultra-low Temperature Thermal Fatigue on Microstructure and Mechanical Property of Z2CND18.12 Austenitic Stainless Steel

At present, ice plug technology is often used in the maintenance process of nuclear power plants to isolate part of the main pipeline and use refrigerant such as liquid nitrogen for ultra-low temperature treatment, so that the internal fluid is frozen to form ice plug to complete the maintenance of low water level valves. Generally, the design of pipeline materials for nuclear power plants is mainly aimed at the working conditions of high temperature nuclear power plants, and the design of ultra-low temperature environment of liquid nitrogen is less considered, which is easy to cause deterioration of pipeline performance and even lead to broken accidents. Therefore, it is necessary to study the ultra-low temperature thermal fatigue performance of pipelines. In order to investigate the influence of freeze-thaw-heating process on Z2CND18.12 austenitic stainless steel for primary circuit pipes during ice blockage operations, 20 cycles of ultra-low temperature thermal fatigue （−196 ℃→0 ℃→350 ℃→room temperature）simulation experiments were conducted. In this paper, Z2CND18.12 nitrogen-controlled austenitic stainless steel used in a nuclear power plant was selected. In the test, boric acid water was injected into the pipeline and frozen at liquid nitrogen (−196 ℃) to make the internal fluid form an ice plug, and the ice plug was maintained for 96 h. Then it was naturally thawed to room temperature, heated after draining the boric acid water, kept at 350 ℃ for 12 h, and cooled to room temperature again with the furnace. So far, a weekly ultra-low temperature thermal fatigue experiment was completed. The ultra-low temperature thermal fatigue test samples with cycles of 1, 3, 5, 10 and 20 were obtained by repeated hot and cold cycles. After the test was completed, the changes in macroscopic mechanical properties of the materials were evaluated by means of room temperature stretching, 350 ℃ stretching, room temperature impact and microhardness test. Microstructure observation techniques such as metallography, SEM, TEM and XRD were used to study the microstructure of the materials after ultra-low temperature thermal fatigue treatment. The result shows that after 20 cycles of ultra-low temperature thermal fatigue, the Z2CND18.12 austenitic stainless steel pipe still maintains the strength, plasticity, and toughness of the original sample. The microstructure dose not undergo phase transformation or precipitation, but the dislocation density increases and the grain size slightly decreases. It indicates that the microstructure of Z2CND18.12 austenitic stainless steel pipe is stable, with excellent mechanical properties, and can resist the repeated effects of deep cold and hot cycles.