Nature of YN Phase Formed with Low Temperature Plasma Nitriding of Austenitic Stainless Steels

The surface nitrided layer of austenitic stainless steels plasma-nitrided at a relatively low temperature of about 623 K in an H₂-rich plasma has been investigated in order to elucidate the nature of an f.c.t. nitride Y_N formed in this layer which has high corrosion resistance and a high hardness of about Hv 1500. The concentration profile of nitrogen in this layer showed a gradual decrease from a maximum value of about 9.5 mass% at the surface to a minimum value of about 6 mass% at the nitriding substrate interface.

Y_N nitride was formed only in the low-temperature plasma-nitrided layer of austenitic stainless steels of Fe–Cr–Ni, and not in the austenitic steels of Fe–high Ni or Fe–high Mn and in the ferritic stainless steels of Fe–Cr.

Low temperature nitrided specimens were vacuuum-annealed above 723 K in the case of 18Cr–8Ni and 18Cr–20Ni steels, α phase and CrN was formed accompanied with the disappearance of the newly developed Y_N phase. In the case of stable 18Cr–48Ni steel, α phase was not formed. The superior corrosion resistance of low temperature nitrided stainless steels deteriorates above 723 K owing to the disappearance of the Y_N phase and the formation of α phase and CrN.