Deformation Behaviors And Mechanism At Different Temperatures Of316LN Stainless Steel For Nuclear Power Plant

The development of nuclear power is dominated by pressurized-water reactor (PWR) in China, with the technologies being the second generation, close to the third generation. In all international nuclear power stations, safety has to go through strict examination, and due to the complexity of service environment for materials used in nuclear plant, their selection process is very rigid. Having the merits of anti-corrosion and comprehensive mechanical properties,316LN stainless steel is the preferred material for primary circuit pipes and other nuclear plant components in the third generation PWR. Through the survey of relevant literatures, the present work studied the following aspects of316LN stainless steel, in combination with the actual production needs:To start with, the forged316LN SS was used in the hot compress tests. According to the stress-strain curves, thermal activation energy was calculated, constitutive equation was established, and the hot processing maps were drawn. Hot processing maps intuitively indicate that hot working parameters should be chosen preferentially in the region with higher energy dissipation rate. The law of dynamic recrystallization in the compression process was obtained by analyzing the microstructure.Secondly, tensile tests of forged and rolled316LN SS were conducted at temperatures ranging from room temperature to1000℃. Stress-strain curves, harding rate-strain curves, microhardness, and microstructures near the tensile fracture were analyzed to investigate the tensile deformation characteristic and mechanisms at different temperatures.At temperature600℃or below, the hardening rate-strain curves of forged and rolled steels exhibit three stages that consist of the stable stage, the decline stage and the necking stage. The high dislocation density in forged steel speeds up dynamic recovery. Thus, the stable stage in the hardening-strain curve of forged steel has no obvious part of increasing. The yield strength of forged steel is higher than that of rolled material. High dislocation density of the initial microstructure accelerates its dynamic recovery, but it is not enough to eliminate the effect of the initial microstructure. At600℃or below, the deformation mechanisms of316LN steel are dominated by twinning and sliding. The higher the temperature, the easier of dislocation cross-slip.At700℃, the hardening rate-strain curves can be divided into two stages steels:the decline stage and the necking stage. At600-700℃, the diffusion rate of nitrogen and carbon atoms and dislocation movement rate are very close, resulting in the pinning effect on dislocation. And this also causes the dynamic strain aging and serrated flow in the hardening rate-strain curves. The serrated flow in forged steel was observed most obviously at700℃. At such temperature, lots of dislocations pile up in the rolled steel due to dislocations cross-slipp, while dislocation network appears in forged material. The change of dislocation movement mechanism results in the difference in tensile deformation hardening behavior and deformation microstructure at700℃.At temperatures above800℃, the curves also show three stages:the rapid decline stage, the stable stage, and the necking stage. Sub-grains and high density dislocations in forged steel contribute little to the tensile deformation behaviors at1000℃. The main softening mechanisms are polygonization and dynamic recrystallization. Large amounts of sub-grains and high density dislocations in forged steel bring more recrystallization nuclei, resulting in the shorter stage of rapid decline. Complete recrystallization occurs in rolled steel at900℃, while in forged steel at950℃.Finally, the pre-creep tests were conducted on forged and rolled316LN SS at350℃and600℃for500-2000hours under different stress. Through the properties testing and microstructure analysis of the samples after pre-creep tests, the law of structure and properties change was obtained.During the pre-creep process, the segregation of nitrogen atoms is sped up with increasing of temperature and lasting time. This segregation can increase the stacking fault energy and narrows the extended dislocations. After pre-creep at600℃for2000hours, short-range order chromium-nitrogen atoms radicals form and dislocation pairs were observed in TEM. Grain boundaries become broaden, and their hardness are significantly higher than that within the grains. The combination effect of nitrogen segregation and dislocation changes causes an obvious increase in yield strength of the rolled steel.Sub-grain and high dislocation density have a close relationship with the properties and microstructure in forged steel after pre-creep. The recovery of high dislocation density causes an obvious increase in microhardness of rolled steel after pre-creep for500hours. During pre-creep process, sub-grain boundaries decompose and migrate, and some even disappear. Dislocations form a network when they meet and interact each other. The approximate-quadrangular dislocation network appears in the specimen after lasting at350℃/20MPa for 500hours, while the approximate-hexagonal dislocation network forms after lasting at600℃/120MPa for500hours. As a relatively stable structure, dislocation network can prolong the creep life of the material. The diameter of these dislocation meshes gradually become smaller with increasing of lasting time.