Abstract
The effect of boron on the microstructure and cryogenic mechanical properties in N50 austenitic stainless steel after aging treatment was revealed by a comparative investigation of 0B and 25B (25 ppm) N50 steels. The results showed there is a large amount of irregular block and rod-like Cr2N precipitated along grain boundaries with intra-granular precipitation of lamellar Cr2N in 0B steel; nevertheless, the nucleation and growth of Cr2N were significantly suppressed in 25B steel. It is related to the retarded diffusion of Cr, Mo, and V atoms caused by the pre-occupied B atoms during the precipitation of nitride. Theoretical calculations verify that the high B segregation at grain boundaries is caused by both equilibrium and non-equilibrium segregation. The weakened precipitation of nitride and the enhanced cohesion of grain boundaries by segregated B synergistically suppress the initiation and extension of intergranular cracks. The impact toughness of 25B–N50 steel at both 77 K and 4.2 K can reach 200 J, which is about 2.5 times higher than that of 0B–N50 steel.
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