Microstructural Evolution Of 316LN Stainless Steel For Primary Pipes Of AP1000 Pressurized Water Reactor

A nitrogen alloyed ultralow carbon 316LN stainless steel (SS) is selected for primary pipes of AP1000 pressurized water reactor (PWR) nuclear power plant on account of its excellent combination of mechanical properties and corrosion resistance, as well as good workability. Unlike the second generation PWR casting and welding primary pipes, the AP1000 primary pipes will be integral forged with nozzles, in order to fulfill the service life extension to 60 years. The manufacturing process and materials performance under service environment would be key factors to the safety of PWR. The chemical composition and microstructure are the essential influential factors in obtaining satisfactory properties. Therefore, it is of practical importance to investigate the microstructural variation of 316LN SS based on varying compositions.In this paper, the solidified microstructure of 316LN SS and its influence factors, the microstructural evolution under hot working conditions as well as the variation of microstructure and properties after aging at 400℃ were investigated. These research results will be helpful for understanding the microstructural evolution during manufacturing and prediction of the microstructure degradation during service. Moreover, these results will provide fundamental data for composition optimization of 316LN SS for primary pipes.It is found that the primary dendrite arm spacing of as-cast 316LN SS increases with increasing content of Cr, Mo, Ni, while it decreases as nitrogen increases. In addition, the formation of δ-ferrite is inhibited by increasing of N, Ni content and decreasing of Cr, Mo content. The effect of interstitial nitrogen on the formation of δ-ferrite is bigger than that of substitutional elements. The solidification mode of 316LN SS depends primarily on the chemical compositions, and the empirical Cr and Ni equivalents models of Hammar & Svensson work fairly well in predicting the solidification mode of 316LN SS. The solidified microstructure of 316LN SS will have three types of solidification mode, i.e. A mode. AF and FA mode, according to the prediction. The cooling rate has a little effect on the variation of solidification mode of 316LN in the range of casting cooling rate, except that a transformation of peritectic reaction to eutectic reaction was found with FA mode when the cooling rate is larger than 10℃/s. The morphology of δ-ferrite. the transformation process of δ-ferrite to austenite and the element distribution in both phases are all affected by increasing cooling rate.Dynamic recrystallization (DRX) occurs in 316LN SS under hot working conditions. A 316LN SS with AF mode has higher values of recrystallization temperature and εc/εp than other type 316LN SS.δ-ferrite acts as a particle which stimulates nucleation and therefore promotes dynamic recrystallization through providing nucleation sites. The nucleation site provided by primary precipitated skeletal δ-ferrite is more than that by secondary precipitated "island shape" δ-ferrite. Skeletal δ-ferrite also improves the distribution of harmful impure elements when promoting the dynamic recrystallization, and thus it increases the hot ductility. However, it should be treated dialectically as to the effect of δ-ferrite during hot working because more δ-ferrite can promote DRX on one hand but also increases the tendency of cracking on the other hand.δ-ferrite was dissolved and spherified during dynamic recrystallization and the subsequent static recrystallization process. The effect of strain on dissolving of δ-ferrite is bigger than that of temperature and holding time.After a long-term thermal aging at 400℃, the impact energy of 316LN SS decreases while the microhardness in grain boundaries increases with increasing of aging time. The tensile properties do not change significantly during aging but a characteristic of embrittlement in fracture morphology was observed as aging time increases. Stress cracking corrosion (SCC) does not occur in the aged 316LN SS when there is no chloridion in the simulated water environment of primary coolant circuit. However, the content of the oxides of chromium and molybdenum decreases and that of the oxide of nickel increases with increasing of aging time, which means the decline of oxidation film’s stability. No apparent precipitation occurs in 316LN SS after a long-term aging upto 10000h. A trace of Cr and Si rich precipitation was observed. This kind of precipitation is reckoned as Chi (χ) phase according to the results of thermo-dynamic calculation. During aging, Cr、Mo and N diffuse to grain boundaries and cause increasing of micro-hardness. This process can be promoted by the increasing content of Cr、Mo、Ni and N in the matrix.