Plasma nitriding behavior of low carbon binary alloy steels

Effect of alloying elements, Cr, Ta, Ti and V with the content of 0.5, 1.0 and 1.5 mass% on plasma nitriding behavior at 823 K of nitriding temperature was examined in low carbon binary alloy steels. Deep diffusion layer with high hardness up to HV900 below the surface thin compound (γ′-Fe4N) layer was formed in these alloys. The hardness increased with increasing alloying content. Optical microscope and X-ray diffraction (XRD) analyses did not reveal the formation of any nitride of these alloying elements in the diffusion layer. Transmission electron microscope revealed the evidence of coherent pre-precipitation phase. In XRD analysis, broadening and shift of diffraction peak of α-Fe as a matrix of the diffusion layer were observed. The peak broadening and the peak shift had a good correlation with the hardness and the nitrogen content in the diffusion layer, respectively. Accordingly, these results suggested that the hardening in the diffusion layer was caused by the microstrain in the matrix induced by the formation of coherent pre-precipitation phase.