Effect of Thermal Aging on Mechanical Properties and Intergranular Corrosion Resistance of 316LN

Nitrogen enhanced low carbon 316LN stainless steel, as a single-phase austenitic stainless steel, is selected as a candidate material of primary pipe for the third-generation pressurized water reactors. In the present work, the solution annealed 316LN was thermal aged at 400℃ for 400, 1000, 5000 and 10000 hours, respectively. The microstructures of the solution annealed and thermal aged steels were compared by optical microscope, scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The effect of thermal aging on mechanical properties of the steels was evaluated by using small punch test, nanoindentation and microhardness tester. The intergranular corrosion resistance of the steels was measured by using electrolytic etching in oxalic acid solution, and the effect of thermal aging on the corrosion resistance of various types of grain boundaries were characterized by using EBSD and atomic force microscope (AFM). The results show that thermal aging at 400℃ for 10000 h will not cause microstructural changes in the micron level of 316LN stainless steel. No change was observed in grain size, grain boundary morphology and characteristics of grain boundary distribution, and no new phase was formed during thermal aging. However, after thermal aging, the lattice parameter becomes smaller, which may be ascribed to that the interstitial atoms and dislocations originally dissolved in grains diffused and migrated towards the grain boundaries. This may result in changes of the mechanical properties of the 316LN, including the increase of strength and hardness, and the lower of plasticity and intergranular corrosion resistance. The intergranular corrosion susceptibilities of all types of grain boundaries increase with the extension of thermal aging time, but the susceptibility of coincidence site lattice (CSL) grain boundaries is always lower than that of random grain boundaries. Hence grain boundary engineering, which is a thermomechanical process and could produce high fraction of low-CSL grain boundaries in materials, could be applied to mitigate the intergranular corrosion of 316LN. © 2023 Chinese Journal of Materials Research. All rights reserved.