Feasibility and kinetics of nitriding of pure titanium and Ti–6Al–4V in molten salt bath of potassium nitrate

Abstract

Surface nitriding of pure Ti and Ti–6Al–4V alloy was performed in potassium nitrate molten salt bath at temperatures ranging from 600 to 700°C for 2–18 h. The formation of a compound nitride layer comprising of ϵ-Ti₂N and δ-TiN phases was identified with thicknesses of 1–12 μm for pure Ti and 1–6 μm for Ti–6Al–4V respectively. Ti–6Al–4V was found to develop an oxide phase upon nitriding for >8 h at 700°C. Maximum hardness values of 1470 HV for pure Ti and 1367 HV for Ti–6Al–4V were achieved. Under identical treatment conditions, the pure Ti sample was found to produce a thicker nitride layer than that of Ti–6Al–4V, proving that the presence of alloying elements hinders the growth of the nitride layer. Our study concludes that the present salt bath nitriding technique can produce a comparatively thick case depth with reasonably high hardness at relatively low temperature.