The relation between the ion-nitriding depth(d) in AISI 4140 and the Rayleigh wave velocity($V_R$) was studied by measurements of the critical angle arising on the interface between the water and surface of the ion-nitrided specimen.
The square of ion-nitriding depth(d) determined by both etching technique and microhardness test was proportional to the ion-nitriding time(T).
The Rayleigh critical angle was measured using the right critical angle(corner reflection method), and the Rayleigh wave length($λ_R$) was obtained by varying the frequency of ultrasonic wave for each ion-nitriding depth(d).
From the variation of the Rayleigh wave velocity($V_R$) with the ratio of ionnitriding depth(d) and the Rayleigh wave length($X_R$), the ion-nitriding depth(d) was estimated under the condition $0.3<\frac{d}{λ_{R}}<1$
When the scattering effect was significant arising from the complicated structure of the ion-nitrided surface layer, ion-nitriding depth(d) was estimated using $\frac{\partial{V_R}}{\partial(d/λ_{R})}$ depth.
