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High temperature gas nitriding (HTGN) at 1050°C and tempering of a 17Cr-1Ni-0.5C-0.4V (CNV) steel were experimentally investigated. The phases appearing in the surface layer of the HTGN-treated steel were martensite and austenite with mostly Cr2N precipitates that were formed by permeated nitrogen, and a small amount of Cr23C6 and VN precipitates. The reverse migration of carbon hindered the diffusion of nitrogen when nitrogen permeated from the surface to the interior, which resulted in the accumulation of nitrogen on the outermost surface. The strong affinity between nitrogen and chromium atoms induced the diffusion of chromium from the interior to the surface, leading to the substitution of Cr23C6 for Cr2N. After tempering the HTGN-treated steel at 500°C, the dense precipitates of Cr2N and the increased martensite phase in the surface layer led to secondary hardening, which increased the hardness value up to 901 Hv.

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