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 700uC for 2-18 h. The formation of a compound nitride layer comprising of e-Ti 2 N and d-TiN phases was identified with thicknesses of 1-12 mm for pure Ti and 1-6 mm for Ti-6Al-4V respectively. Ti-6Al-4V was found to develop an oxide phase upon nitriding for .8 h at 700uC. 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.

A novel low temperature and green salt bath nitriding of titanium alloy

“…From the Reaction 4, it can be known that, at the temperature of 550°C [14], CNOˉ can decompose into the active N atoms, along with the CNˉ, carbonate ( ) and carbon monoxide (CO); meanwhile, CO can decompose into the active carbon (C) atoms, along with the carbon dioxide (CO 2 ), as shown in Reaction 5. According to the Reactions 4 and 5, it can be found that when CNOˉ provides the active N atom, CNˉ would also be generated; furthermore, with the treatment time increasing, the content of CNˉ would be increased accordingly, which would directly threaten the health of operators and damage the environment.…”

“…After that, the nitriding agent was packed in a crucible, ensuring a sufficiently thick pack around the treated sample. Furthermore, to reduce the effect of nitriding temperature on the microstructural characteristics of the titanium matrix and ensure the growth of the nitride layer, after deeply analysing the current research about the salt bath nitriding of titanium alloy [14,15], the nitriding temperature and time are determined to be 550°C and 7 h (shown in Table 2), respectively. Then, the nitriding experiments were performed in a box-type resistance furnace; after the treatments, samples were taken out and cooled by air.…”

“…Meanwhile, compared with the carburising or boriding, in the process of nitriding of titanium alloy, the treatment temperature can be effectively reduced [8-10], especially for the salt bath nitriding using the mixture of potassium cyanate (KCNO) and sodium cyanate (NaCNO) as the nitriding agent [11-13], the nitriding temperature can be reduced to less than 600°C (owing to that the cyanate (CNOˉ) can decompose the active nitrogen (N) atoms at less than 600°C) [14]. Thus, the salt bath nitriding process can significantly reduce the effect of the treatment temperature on the microstructural characteristics of the titanium matrix and, more importantly, the surface hardness and wear resistance of the titanium alloy can be effectively improved [15].…”

“…Thus, the salt bath nitriding process can significantly reduce the effect of the treatment temperature on the microstructural characteristics of the titanium matrix and, more importantly, the surface hardness and wear resistance of the titanium alloy can be effectively improved [15]. In addition, compared with other nitriding processes such as plasma nitriding [8] and gas nitriding [16], salt bath nitriding is relatively inexpensive [14].…”