1992 Volume 33 Issue 1 Pages 7-14
Pure iron was ion-nitrided in a mixed gas (80 vol% N2 + 20 vol% H2) under a reduced pressure of 665 Pa at 803, 923 and 1013 K and then quenched in water for the elucidation of the ion-nitriding behavior accompanied with phase transformations above the eutectoid transformation temperature of 863 K for the Fe–N system.
When nitrided at 803 K, the nitrided specimen surface consists of two layers as usual nitrided cases do; (i) a compound layer with a hardness of about Hv 700 which is mainly composed of γ′-Fe4N, and (ii) a diffusion layer of about Hv 250.
When nitrided at 923 K, the nitrided specimen surface consists of; (i) a compound layer of about Hv 750, mainly composed of γ′-Fe4N, under which are (ii) an inner layer of γ austenite of about Hv 400 and (iii) a diffusion layer.
When nitrided at 1013 K, the nitrided specimen surface consists of; (i) a thin compound layer, (ii) an inner hardened layer of γ and α′ martensite, of which the hardness increases from Hv 400 to Hv 700 toward the inside corresponding to the content of the α′ phase, and (iii) a diffusion layer.
To explain the formation of the relatively thick hardened layers of austenite at 923 K and martensite at 1013 K, the nitrogen concentration at each temperature and the degree of the stability of the γ phase when quenched in water after nitriding were examined.
