Abstract
Nitrogen strengthened stainless steel named N50 fully meets the strength demand of developing novel alloys for more powerful fusion reactor applications in superconducting coil armor materials. The detailed evolution of microstructure and its resultant effect on tensile properties of N50 steel with two different C contents after aging at 650 °C were investigated. The results indicated that the cryogenic ductility of 0.04 wt % C steel decreased significantly with the extension of aging time, while the ductility of 0.01 wt % C steel remained stable. Bulk primary MX precipitates and Z phases are distributed in the matrix after solution annealing. After aging treatment, the flake M23C6 mostly occupies the grain boundaries of 0.04 wt % C steel, while the smaller cubical Z phases were the main intergranular precipitates in the 0.01 wt % C steel. The higher carbon content increases the nucleation driving force of M23C6 and contributes to its more precipitations after aging. The larger intergranular M23C6 in 0.04 wt % C steel leads to its brittle fracture features. While the stable intergranular Z phases in 0.01 wt % C steel obtained relatively excellent low-temperature plasticity. Serving as a superconducting coil armor material, the carbon content of N50 steel should be limited to acquire better cryogenic mechanical properties by eliminating the precipitation of intergranular M23C6.
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